Agroclimatology for agronomy

1
AGROCLIMATOLOGYY (AGR442)
WACHEMO UNIVERSITY, COLLEGE OF
AGRICULTURE, DEPARTMENT OF PLANT
SCIENCE
AGROCLIMATOLOGYY (AGR442) FOR
MSc.PROGRAM IN AGROMOY
COPLAINED BY DANIEL MANORE
( ASSISSTANT PROFESSOR IN AGRONOMY)
danimanore@gmail.com
AUGUST 2013, HOSSANA, ETHIOPIA

2
COURSE OBJECTIVES
After going through this course, you should be able to:
Explain the principles of climatology and biogeography
Determine the climatic conditions of places and relating them to the dynamics of the earth’s
atmosphere
Explain the principle and laws of radiation
Define atmospheric moisture with particular reference to humidity and the
hydrological cycle
Explain the dynamics of pressure and wind systems
Describe the processes of condensation and precipitation
Explain the causes of seasonal variations in temperature, radiation, rainfall
and evapotranspiration
Identify equipments in a meteorological station in relation to theirpositioning and uses
State the characteristics of different climatic zones in the tropics
Explain the relationship between agriculture and climate withreference to crops, livestock, irrigation,
pest and disease
Discuss climate change issues in agriculture and the variousmethods of amelioration
PRINCIPLES, AIMS AND SCOPE OF CLIMATOLOGY
1.0INTRODUCTION
Climatology originated from two Greek words; Klima meaning zone or place and logia or climate
science which means the study of climate, scientific definition of climate means an average weather
condition of a place over a period of time. This modern field of study is regarded as a branch of the
atmospheric sciences and a sub-field of physical geography which is one of the earth sciences. This
unit will explain the principles, aims as well as the scope of climatology.
2.0 OBJECTIVES
At the end of this unit, you should be able to:
define climatology
explain the principles of climatology

3
examine various approaches to climatology
state the aims of climatology
identify the scope of climatology
MAIN CONTENT
Meaning and Principles of Climatology
Defining the Concept Climatology
Climatology is a science that deals with the study of the climates of different parts of the world. It is
concerned with the description and explanation of climatic regions of the world, its spatial and
temporal variations and influence on the environment and life on the earth’ssurface (Ayoade, 2011).
Climatology studies the long-term state of the atmosphere. It is fundamentally concerned with the
weather and climate of a given area. Climatology examines both the nature of micro, meso and
macro (global) climates and the natural and anthropogenicinfluences on them. Climate implies an
average or long – term record of weather conditions at a certain region for at least 30 years. It
conveys a generalization of all the recorded weather observations in a given location. Climatology is
a branch of atmospheric science concerned withdescribing and analyzing the causes and practical
effects of climatic variations. Climatology treats other atmospheric processes as meteorology and
also seek to identify slower-acting influences and long-term change including the circulation of the
oceans, the atmospheric gases and the measureable variations in the intensity of solar radiation.
Climate is the expected mean and variability of the weather conditions for a particular location,
season and time of the day. Climate is often described as the mean values of meteorological
variables such as temperature, precipitation, wind, humidity and cloud cover. A complex description
also includes the variability of these quantities and theirextreme values. The climate of a region often
has regular seasonal and diurnal variations, with the climate for January often being very different
from that of July at most locations. Climate also exhibits significant year to year variability and
longer-term changes on both a regional and global basis (Ayodele, 2011).
Principles of Climatology
The basic principles of the climatology include such sub-themes as; min-environment relationship,
plants and animal life as product of the prevailing climatic conditions, the prevailing climatic
condition in turn as a product of the amount of solar energy and its interaction with the earth’s
surfaces, and the climatic condition of a place as a great determinant of the atmosphere. Specific

4
principles of climatology include thus;
1. Air temperature received from a place depends on the amount and duration of incoming solar
radiation.
2. Air temperature is additionally moderated by the amount of water vapour in the atmosphere, the
degree of cloud cover, the nature ofthe surface of the earth’s surface, elevation above sea level and
degree and direction of air movement
3. Much of the incoming solar radiation is sent back to space and the troposphere through re-
radiation and reflection
4. Air is heated more by the process of re-radiation than by direct energy from the sun
5. Cold and hot temperature extremes are developed on land and notsea because the land is heated and
gives out energy much more easily than the sea
6. Temperatures are moderated by large bodies of water near the land
7. Coastal areas have lower summer temperature and higher winter temperature than those places at the
same distance from the equator excluding sea cost
8. Temperatures are warmest at the earth’s surface and lower as elevation increases
9. Air is heavier and pressure is higher close to the earth’s surface. Thus, cold air is denser than hot air.
10. Air pressure at a given location changes as surface heat or cold changes
11. Air moves from high pressure belt to low pressure belt. Thus, the greater the difference in air
pressure between places, the greater the wind
12. Heavy air stay close to the earth’s surface as it moves, thus, producing wind, forces an upward
movement of worm air. The velocity, or speed of the wind is in direct proportion to pressure
difference. If distance between high and low pressure zones are short, pressure gradients are steep
and wind velocities are great
13. Wind movement is slowed by the fractional effect caused by the earth’s surface. The effect is
strongest at the surface and decreases with high until no effect is recorded
14. Wind systems of the world set ocean currents in motion
15. Difference in density of water cause water movement. High density water exist in areas of high
pressure. Ocean water is low in density
16. Two air masses coming into contact creates the possibility of storm development

5
17. An intense tropical cyclone or hurricane begins in a low pressure zone over worm waters, usually in
the northern hemisphere
Approaches to Climatology
Climatology is approached in different ways. These include:
i) Pale Climatology
This approach seeks to reconstruct past climates by examining records such as ice cores, and tree
rings (dendroclimatology). Pale climatology seeks to explain climate variations for all parts of the
earth during any given geological period, beginning with the time of the earth’s formation. The
basic research data are drawn mainly from geology and pale botany; speculative attempts at
explanation have come largely fromastronomy, atmospheric physics, meteorology and geophysics.
Climate is the long-term expression of weather; in the modern world, climate is most noticeably
expressed in vegetation and soil types and associated features of land surfaces. To study ancient
climates, pale climatologists must be familiar with various disciplines of geology, such as
sedimentology, and paleontology, (scientific study of life of the geologicpast, involving analysis of
plants and animals fossils, preserved in rocks)and with climate dynamics which includes aspects of
geography, atmospheric and ocean physics.
ii) Paleotempestology
This is the second approach to climatology. This approach helps determine hurricane frequency over
millennia. The study of contemporary climates incorporates meteorological data accumulated over
many years, such as records of rainfall, temperature, andatmospheric composition. Knowledge of the
atmosphere and itsdynamics is also embodied in models, either mathematical or statistical, which
help by integrating different observations and testing how they fit together. Modeling is used for
presenting actual climatic phenomena andunderstanding of past, present and potential future climate.
iii) Historical climatology
This approach to climatology is the study of climate as it relates to human history which focuses only
on the last few thousands of years.
Aims of Climatology
The aims of climatology are to provide a comprehensive description of the earth’s climate over the
range of geographic scales and to have a better understanding of its features in terms of physical
principles, andto develop suitable models of the earth’s climate for production of futurechanges that

6
may result from natural and human influences. It also aims to develop sound understanding of how
climatic elements affect human occupations of the earth: This subsumes an understanding of how
climate influences the way people use the land, distribution of human population and activities,
distribution of plants and animals as well as soil types and characteristics.
Climatology is concerned with seasonal to inter-annual variability characteristic, climate extremes
and season ability not only analysis of climate pattern and statistics as it affects temperature,
precipitation, atmospheric moisture; atmospheric circulation and disturbances. Climatology also
addresses sits subject matter on many spatial scales, from micro through meso and synoptic to the
hemispheric and global systems. Furthermore, climatology works within a general systems
paradigm. The climate system theory states that, climate is the manifestation of the interaction
among major climate system components of the atmosphere is influenced by the balance between
large and logical factors, climate can be a determinant of a resource for and a hazard to human
activities and human activities have a significant potential to influence climate, gives the opportunity
for climatologist to constantly measure, record and analyze climatic data to provide information on
the changing effects of climate on the environment,agriculture and other human activities. (Egeh, &
Okoloye, 2008; Henderson-sellers, 1995; Donald, 1994).
Scope of Climatology
Climatological studies consist of the following:
1. Structure and composition of the atmosphere
2. Horizontal and vertical distribution of temperature
3. Vertical and horizontal distribution of pressure
4. Surface winds – corriolist effect, planetary and non-planetarywinds, and local winds
5. Global air convergence and divergence
6. Upper atmospheric circulation – Hadley, Ferial and polar cells
7. Humidity
8. Condensation and precipitation
9. Air masses
10. Fronts
11. Cyclones and related phenomena

7
12. Climatic classification, location and characteristics
13. Hydrological cycle
Climatology is the tool used to develop long-range forecasts. There are three principal areas to the
study of climatology; physical, descriptive and dynamic climatology. However, there are several
other subdivisions in literature which are subsumed under one or more of the principalareas in
climatology
i) Physical Climatology
This approach seeks to describe the variation in climate focusing on the physical processes
influencing climate and the processes producing the various kinds of physical climates such as
marine, desert and mountains.It also emphasizes the global energy and water balance regimes of the
earth and the atmosphere Physical climatology deals with explanationsof climate rather than with
presentations.
ii) Descriptive or Regional Climatology
Descriptive climatology is presented by verbal and graphic description without going into causes and
theory. This approach typically orients itself in terms of geographic regions; it is often referred to as
regional climatology. A description of various types of climates is made on the basis of analyzed
statistics from a particular location. A further attempt is made to describe the interaction of
weather and climatic elementsupon people and areas under consideration.
iii) Dynamic Climatology
Dynamic climatology attempts to relate the characteristics of the general circulation of the
atmosphere to the climate. Dynamic climatology is often used by the theoretical meteorologists to
address dynamics and effects of thermodynamics. Three other areas which Ayoade (2011) has
described as new in the study of climatology are:
i. Synoptic climatology- the study of the weather and climate over an area in relation to the pattern of
prevailing atmospheric circulation. It is essentially a new approach to regional climatology.
ii. Applied climatology- emphasizes atmospheric motions on various scales particularly the general
circulation of the atmosphere.
iii. Historical climatology- the study of the development of climate through time.

8
Three prefixes can be added to climatology to denote scale or magnitude. They are micro, meso and
macro, indicating small, medium and large scales respectively. These terms are also applied to
meteorology.
i) Micro-climatology
Microclimatology often studies small-scale contracts, such as conditionsbetween hilltop and valley
or between city and surrounding country. They may be of an extremely small scale, such as one side
of a hedge contrasted with the other, a ploughed furrow versus level soil or opposite leaf surfaces.
Climate in micro scale may be modified byrelatively simple human influences.
ii) Meso-climatology
This embraces a rather distinct middle ground between macro-climatology and microclimatology.
The areas are smaller than those of macro and are larger than those of micro, and they may or may
not be climatically representative of a general region.
iii) Macro-climatology
This is the study of large-scale climate of a large area or country. This type is not easily modified by
human efforts. However, continued pollution of the earth, its streams, rivers and atmosphere, can
eventually make these modifications easy. Geographers, hydrologists and oceanographers use
quantitative measures of climate to describe or analyze the influence of atmospheric movement.
Classification of climate has developed primarily in the field of geography. The basic role of the
atmosphere is an essential part of the study of hydrology. Both air and water measurements are
required to understand the energy exchangebetween air and ocean (heat budget) as examined in the
study of oceanography.
iv) Ecology
This aspect of science studies the mutual relationship between organisms and their environment.
This is briefly explained here due to the fact that environment and living organisms directly are
affected by weather and climate, including those changes in climate that are gradually being made by
action of man. The interference with nature by diverting and damming rivers, clearing its lands,
stripping its soils and scarring its landscape has produced changes in climate. These changes have
been on the micro and meso scales and possibly on the macro scaleor magnitude.
Unit 2. PRINCIPLES, AIMS AND SCOPE OFBIOGEOGRAPHY
2.0 INTRODUCTION
Biogeography examines the characteristics of the environment and influence of atmospheric

9
processes particularly climate on biotic and abiotic components of the terrestrial and aquatic
ecosystems. Generally, climatic elements have significant influence on the ecological parameters
studied in biography. For a better understanding of the effects of the dynamic characteristics of
climatic components on the ecosystem, there is need to study biogeography with emphasis on the
distribution of plants and animals using the scientific principles of environmental studies, interaction
complex of soil, plants and animals, and the interaction of plants and animals with climate. This unit
will explain the principles, aims and scope of biogeography.
Principles of Biogeography
Biogeography is the study of the distribution of plants and animals in relation to the complex
biological atmospheric and edaphic processes which control their activities and spread in space and
time. Its subject matter covers many life form of plants and animals which inhabit the
biosphere. Its field is the interface between many disciplines such as biology, geography, botany,
zoology, genetics, geology, climatology, pedology, geomorphology, etc.
Biogeography deals with the geographical aspects of the distribution of plants and animal life. The
geographical distribution of plants is referredto as phytogeography, while the distribution of animals
is referred to as zoogeography. Biogeography also provides explanation of the factors responsible for
the distribution of plants and animals using the scientific principles of environmental studies. From
this note, biogeography is defined as the study of the distribution of plants and animals including
microorganisms together with the geographical relationship with their environment.
Biogeography includes the study of all components of the physical environments that constitute the
habitat of various species and organisms. Biogeography focuses on the biological and geographical
components of the environment. Therefore, biogeography as a subject is both biological and
geographical. This is because it studies the spatial distribution of plants and animals and the
biological process taking place in nature. Furthermore, it tries to explain the biological factors of
distribution and the implication of the pattern of distribution. Biogeography studies the biotic
complex of the environment. Bioticcomplex is the interacting complex of soil, plants, animals and
theinteraction of plants and animals with climate (Bharatdwaj, 2006).
Aims of Biogeography
Biogeography is a science that uses methods of science to investigateand predict the relations
between the observed patterns of species and the controlling abiotic and biotic processes. It is
dependent therefore on objective empirical observations to generate research hypotheses that make
prediction.

10
Biogeography aims to provide detailed examination of the origin, distribution, structure and
functioning of the major terrestrial ecosystems and effects of humans on their ecological integrity.
Particularly, biogeography examines how the patterns of energy, water and nutrients through the
soil-vegetation – atmosphere continuum determine the biological functioning and diversity of the
majorterrestrial ecosystems.
Biogeography also aims to determine the impact of human activity on all scales with particular
emphasis on the evolution and expansion of agro ecosystems as the world population has continued
to increase. The other human impacts are directly related to human changes, global and local
biogeochemical cycles, particularly via air pollution and acid deposition and via increases in
atmospheric carbon dioxide and climate change. Both impact on the ecosystem.
Nature and Scope of Biogeography
Biogeography is the study of the biosphere which includes the consideration of the physical
environment, soil, animals and plants. Biogeography indicates both biological and geographical
science. Geographers study the distribution patterns of plants and animals of the biosphere in spatial
and temporal contexts and attempts to analyze the processes and factors which are responsible for
such spatial and temporal variations, the biologists limit themselves to the study of physiological,
morphological, behavioural and functional aspects of an individual organism. Although the
geographer studies distributional patterns of community of plants and animals also emphasizes two
more aspects viz:
i) intimate inter-relationship between the abiotic and biotic
components
ii) reciprocal relationship between man and biosphere
The scope of biogeography specifically includes the following:
1. phytogeography – the study of plants distribution
2. zoogeography – the study of animal’s distribution
3. The study of all components of the physical environment that constitute habitat for various
species of biological organisms.
This consists of land, water, air and energy. Specifically, thisinclude the following studies;
a) study of the interactions of organisms and their physicalenvironment
b) atmospheric factors in the distribution of plants and animals –gaseous composition, supply of light,

11
condensation and precipitation, temperature and generalatmospheric conditions
c) edaphic factors in the growth and distribution of plants and animals – this include those soil
properties which affectplants growth and distribution and conversely that of animals. The physical
and chemical properties of soil either promote or inhibit plants growth and distribution
4. biotic and anthropogenic factors in the growth and distribution ofplants and animals
5. plants and animals evolution and distribution
6. effect of man on plants and animal evolution and distribution
7. ecosystem and the food chain
8. motor biomes of the world
9. environmental degradation and conservation and animals in relation to the complex biological,
atmospheric and edaphic processes which control their activities and spread in space and time. Its
subject matter covers many life forms of plants and animals which inhabit the biosphere. Its field is
the interface or overlap between many disciplines such as biology, geography, geology, climatory,
pedology, geomorphology, botany, zoology, genetics, to mention but a few.
THE ELEMENTS AND CONTROL OF CLIMATE AND WEATHER AND THEDYNAMICS
OF THE EARTH’S ATMOSPHERE
INTRODUCTION
Differences in weather conditions exist on daily basis due to certain climatic factors that play
significant role on the planet earth. People often wonder why they experience hot and cold
weather at time and location. This unit explains the meaning of climate and weather, the different
elements of climate and weather and differentiates between the two concepts. The unit also
discusses how to collect various climaticdata and to prepare charts on them as well as how to
observe and measure weather and climate over a period of time using weather instruments. Also
discussed in this unit, are the characteristics of these elements.
2.0 OBJECTIVES
• define climate
• define weather

12
• differentiate between climate and weather
• list the elements of climate and weather and state what each isused for.
• identify the type of rainfall associated with your geographicalarea.
The Meaning of Climate And Weather
Climate can be defined as the average weather conditions of a place over a long period of time,
usually about or above 30 years. The elements of weather which control the climate can be
systematically observed, recorded and processed over a long period of time. The climatic conditions
of a location may be affected by certain factors whose effectsmay differ based on the location and
the factors present at the location. These factors include; Latitude or distance from the equator,
altitude or elevation, distance from the sea, prevailing winds, direction of mountain, amount of
rainfall, ocean currents, slope of the land andvegetation. The study of climate is called climatology
while specialistsin climatology are known as climatologists.
Weather is the condition of the atmosphere at a particular time over a certain or short period of time.
This is determined by various meteorological conditions. The daily and seasonal changes or
variationin weather influence human lives. The study of weather is known as meteorology while
those who study meteorology are known asmeteorologists. (Briggs & Smithson, 1985).
Difference between Climate and Weather
The following differences exist between weather and climate:
i. Weather is the condition of the atmosphere at any given time or a short period while climate is the
average weather condition of a location over a long period of time.
ii. The study of climate is known as climatology and those who study climate are known as
climatologists; while meteorology is the study of weather and those who study meteorology are
called meteorologists.
Elements of Climate and Weather
The following are the elements of weather and climate: Temperature, rainfall, atmospheric pressure,
humidity, wind, sunshine and clouds. Themain elements considered as very significant for now are
temperature and rainfall, the nature of winds and the degree of humidity.
Temperature
Temperature is a significant element of climate and weather, the sun is the ultimate source of energy

13
on the Earth’s surface. The energy existsin heat and light called solar radiation. Temperature is
described as the hot and cold conditions experienced in a particular location at a given period of
time. Temperature is highest at ground level compared to the atmosphere. This means that
temperature decreases with increase inheight. Usually of about 6.5o
C for every 1000 meters of
ascend abovethe sea level.
Temperature is usually measured in degree centigrade (o
C) using an instrument called thermometer.
It consists of a narrow glass tube containing some mercury of alcohol. There are two major types of
thermometers which record temperature under different conditions: maximum and minimum
thermometer. Maximum thermometer records the highest temperature attained during a day while
minimum thermometer records the lowest temperature reached during the day. Thermometers are
read at different time of the day and are kept alongside with other instruments in a place known as
“Stevenson screen” designed to protect the thermometers from the effects of sun andrain so as to get
accurate temperature readings of the day.
Temperature is usually represented on maps by lines drawn to joinlocations having the same amount
of temperatures known as ‘isotherm”. Oo
C and 32o
F are known to be the freezing point of
temperature in centigrade and Fahrenheit respectively. The boiling point for centigrade is 100o
C
while for Fahrenheit is 212o
F. Temperature can be converted from centigrade to Fahrenheit and
Fahrenheit to centigrade using the appropriate formula:
To obtain centigrade from FahrenheitC = o
F – 32
1.8
While to obtain Fahrenheit from centigradeF = (1.8 x o
C) + 32o
F
Calculating Temperature
There are formulae for calculating temperature applicable to the situation or condition of need.
1. Mean daily temperature = Max. Tempt + Min. Tempt
2
That is, maximum temperature and minimum temperature for aday.
2. Duirnal range of temperature: diurnal means daily and iscalculated as max. tempt – min. tempt
for that day

14
3. Annual temperature = Total temperature from January to
December for that year
4. Mean annual temperature is expressed by
= Temperature from January to December
12
5. An annual range of temperature = difference between
temperature of hottest month and coldest month
6. Monthly range of temperature = difference between temperatureof hottest and coldest daily tempt
for the month
Rainfall
Rainfall is an important element of climate which may result from the cooling of the air as it rises
higher in the lower atmosphere. Rain is described as a liquid state of precipitation which is derived
from large droplets of water – normally produced by the clouds. It is measured by an instrument
called raingauge. Raingauge consists of a metal container,a metal jar or glass bottle and metal funnel.
The instrument is kept in an open space far from buildings and shelter in order to obtain accurate
measurement by collecting rain water directly without obstruction and addition from roof tops and
trees after the rain has stopped.
Raingauge must be examined every day and records taken.
Funnel
Metal containerGround
Glass bottle or jar
Fig. 1: Raingauge
When using the raingauge, the instrument should be sunk into the ground such that 30cm of it is
above the ground level and firmly
positioned. Rainfall is usually measured in (mm) or (cm),a line used to join two places on a map
30cm

15
with same amount of rainfall is called “isohyets.” Formulae for calculating rainfall is the same with
that of temperature.
There are three types of rainfalls formed under different conditions with different features;
(i) Convection rainfall: is common in regions with high temperature i.e. the tropics, formed
after intensive heating of the earth surface, air is forced to rise thus carrying water vapour into the
upper atmosphere in a process called evaporation. The water vapour condensesinto cumulonimbus
clouds and later turn into droplets of water. Convectional rainfall has the following features:
Normally accompanied with lightning and thunderstorm
Torrential in nature
It occurs in equatorial and tropical monsoon regions
Usually occurs in the afternoon period of the day
It falls within short distances
(ii) Orographic Rainfall: This is sometimes called relief rainfall. This occurs whenever
moisture – laden air is forced to ascend an area with high-elevation. The air upon reaching the land
surface is compelledto move to the upper atmosphere where the air becomes cool and saturated.
Condensation at this point sets in thereby forming clouds and finally rain.
Orography rainfall has the following features:
It only occurs where there is mountain barrier to deflect theprevailing wind upward
Only the windward side (direction of the prevailing wind) receives or experiences a significant
amount of rainfall, while the leeward side is occupied by the descending dry wind which brings no
rainfall.
(iii) Frontal (cyclonic) Rainfall: This is associated with two different air masses of varying
temperature. The meeting of the tropical warm air (tropical maritime airmass) and the polar cold air
(tropical continental airmass) results in this type of rainfall.
The warmer moist air which is lighter in weight rises when it meets the heavier denser dry air along
the inter-tropical convergence zone (ITCZ) otherwise known as the ‘front’. The denser air will

16
undercut the lighter air and forces it to rise and when this occurs, a low pressure condition is
created so that the temperature of the warmer air decreases. The decrease in temperature of the
warmer air will give rise to condensation and clouds formation, when the cooling is below the dew
point, it resultsto rainfall.
Cyclonic rainfall is characterized by the following:
• It occurs between latitudes 50o
N – 70o
N and 50o
S and 70o
S of theequator
• It falls within a short distance and lass within a short period oftime but may be continuous in
nature within short intervals.
Atmospheric Pressure
Air is made up of a number of mixed gases and has weight. Atmosphericpressure is described as the
weight of the volume of air which extends from the ground surface to the outermost layers of the
atmosphere.
There is a decrease in atmospheric pressure with increase in height(altitude), temperature and the
rotation of the earth. Atmospheric pressure over a place does not remain constant or fixed for a very
long time due to both daily and seasonal variations. Pressure is measuredwith an instrument
called barometer. Places with same amount of pressure on a map are joined together by lines called
“isobars”.
Vacuum Glass tube
760mm
Mercury container
Pressure
of
the
atmosphere

17
Fig. 2: A barometer

18
Humidity
This is expressed as the dampness of the atmosphere due to the pressure of water vapour. It is
derived through evaporation and transpiration fromwater bodies and plants respectively.
There is a maximum amount of water vapour which the air can hold at a time and when reached, the
air is saturated. The humidity of the air to a greater degree depends on the temperature because a rise
in temperature leads to increase in the quantity of water vapour which it holds. The proportion of
water vapour in the atmosphere compare with the quantity which could be in the same portion of the
atmosphere, if such portion of the atmosphere were saturated is known as relative humidity (RH). It
is measured using an instrument called hygrometer, which consists of wet and dry bulbs
thermometer. The measurement of humidity is recorded inpercentage.
Wind
Wind is air in motion and has direction and speed. Wind developed as the air moves from area of
high pressure to area of relatively lower pressure. The air expands and rises when it get heated and
becomes lighter. The surrounding air which is denser in nature then moves to takethe place of the
ascending air. It is the horizontal movement of air that creates wind. Wind has permanent
characteristics of movement, from areas of higher pressure to areas of lower pressure. Wind vane is
used to measure the direction of wind while anemometer is used for measuring wind speed.
Sunshine
The amount of sunshine in a given location to a greater extent depends on the season, and seasons in
turn are determined by latitude and by the position of the earth in its revolution around the sun. The
amount of sunshine may likely vary depending on the location of the place. Places located towards
the equator receive considerable amount of sunshine because the sun is overhead twice on the
equator and twice around the equator at 23o
North and South during revolution of the earth and the
sunis inclined at an angle of 66½o
North and South of the equator.Places located within this angle

19
receive less and beyond the angle receives lesser compared to 66½o
North and South. Sunshine is
recorded using an instrument called sunshine recorder.
Clouds
When air cools, some of its water vapour may condense into tiny water droplets. The temperature at
which this occur is called the dew point temperature. Some condensation takes place on tress and
grasses directly on the earth surface. The water droplets forming on these surfaces are called dew,
often formed in some parts of Nigeria at nightin the dry season and it play a significant role in
keeping plants alive.
Clouds are formed by water droplets and ice particles. Mist and fog are also considered as cloud
types because they are formed close to the earthsurface. Meteorologists suggest that, weather can be
determine according to the shape, height and movement of the clouds. Clouds are classified into
three (3);
i) High clouds: whose height is between 6,000 to 12,000m above the earth surface. Examples
include: Cirrus, cirrocumulus.
ii) Middle cloud: Middle cloud has two distinct parts namely: altocumulus and altostratus.
iii) Low cloud: Low clouds consist of three (3) layers namely: stratocumulus, nimbostratus and stratus
clouds. These often bringdull weather to adjacent lands.
Cumulus and cumulonimbus are regarded as clouds with great vertical extent. Cumulus clouds are
round topped and flat-based forming a whitish – grey globular mass, consisting of individual
cloud units. Onthe other hand cumulonimbus cloud is a special type of cloud whose round tops are
spread out in form of anvil. This type of cloud indicates convectional rainfall with a feature of
thunder and lightning.
4.0 CONCLUSION
Weather describes the atmospheric condition of a place over a short period of time while climate
explains the average weather condition of a place over a long period of time say 30 – 50 years. Both
weather and climate are controlled by certain elements which include temperature, rainfall, humidity,

20
cloud, atmospheric pressure, etc. These elements are measured using different instruments which are
peculiar to the elements.
5.0 SUMMARY
In this unit, climate, weather and differences between climate and weather have been discussed. The
study of climate is called climatology and climatologists are professionals who study the climate
while the study of weather is called meteorology and those who study meteorology are called
meteorologists.
In this unit, the elements of climate and weather alongside the instruments used for measuring each
of the elements are considered; thermometer for measuring temperature, raingauge for rainfall, wind
vane for measuring wind direction and anemometer for measuring wind speed. Barometer for
measuring pressure and sunshine recorder for sunshine.
Unit 3. FACTORS CONTROLLING CLIMATE ANDWEATHER
1.0 INTRODUCTION
The climatic elements are controlled on a daily basis by the passage of the sun the nature of the
weather systems and by local atmospheric factors such as local winds and air movements. In the
longer term the climate is determined by the relationship of an area to the sun and by its position
relative to major atmospheric features such as the permanent centres of high or low pressure, or the
main components of the circulation. This unit will discuss the factors that control climate thereby
creating variation in weather.
2.0 OBJECTIVES
Factors Controlling Climate and Weather
Climate varies based on location and as a result is influenced by the following factors:

21
Latitude
: The altitude of the sun is always high at the equator, resulting to hot condition within the latitudes
of this region, those within the region where the sun’s elevation is usually lower experience cold
condition. Changes in latitudes cause changes in temperature and this brings about seasonal
temperature changes.
Altitudes and Relief
This is described as the height of a place above the sea level and thus account for the reduction in
temperature as one ascends higher. Areas of mountains and highlands always experiences cold
climates. Altitudes reduces temperature at an average of about 6.5o
C for every 1000m of ascend
while relief determines orographic rainfall.
The Nature of Ocean Currents
Ocean currents control the average weather which in the long termcharacterizes the climate. Ocean
currents change the effects of winds blowing over them, thereby influencing the temperature of the
coastal lands. A cold current causes the wind blowing over it to be cold and as such dry, whereas, a
warm current causes the wind blowing over it to be warmed and moisture laden. The warm air from
the seas often keep the immediate surrounding environment especially the lowlands warm, while the
cold currents tend to have a reduction effect on summertemperature especially the onshore winds.
Prevailing Winds and the Location of the Main PressureCentre
Wind blows from high pressure belts towards low pressure belts. When this happens, the climate of
places or locations along their paths may be affected. The movement of winds brings about changes
in temperature and relative humidity. This therefore, determines the type of precipitation that may
occur in the location. The low pressure belt is usually situated along the equator while high pressure

22
belts exist North and South of the equator.
Distribution of Land and Sea
This has a complex effect, for the land gains and losses heat rapidly thanthe sea. Thus, temperature
range tends to be greater over the continents than over the oceans. The land surface warms up and
cools down more quickly than the sea surface. Therefore, in temperate latitude, the sea warms
coastal regions in winter, while in summer they are cooled by it. The temperature of such coastal
areas is always affected by the influenceof the cooled wind from the seas in summer and that of the
warm wind from the sea in winter. (Briggs & Smithson, 1985).
UNIT 4 IMPORTANCE OF CLIMATE AND WEATHER
1.0 INTRODUCTION
Climate and weather control virtually all the activities of human beings. The importance of climate
and weather as they affects agriculture and aviation among others will be discussed under this unit.
2.0 OBJECTIVES
• mention relevant areas where climate and weather significantly affect human activities and
existence
• explain the importance of climate and weather on agriculture
MAIN CONTENT
Importance of Climate and Weather
Importance on Agriculture
Adequate knowledge of the atmospheric conditions of a place will assist farmers to plan for their
various farming activities both seasonally and annually especially on rainfall regime. With good
information to farmers, effective steps can be taken against hail, frost, heavy rainfall, drought and
diseases.
Importance on Transportation and Communication
Transportation and communication can be enhanced where the atmospheric conditions is well
understood. Air transport system requiresan effective and reliable weather information before it can

23
besuccessfully operated. Sailors at sea require adequate weather information at all times.
Importance on Mode of Dressing and Nature of HousesBuilt
A good knowledge of weather will assist in building the kind of houses that are suitable for our
climate. The type of dresses used in any area isto a greater extent determined by the climatic and
weather conditions obtainable in that particular area. For instance, in Polar regions where
temperature is reduced, inhabitants wear thick and heavy clothing and a more higher clothing is used
in the equatorial region where temperature is more or less high (Oluwafemi, 1998).
THE DYNAMICS OF THE EARTHATMOSPHERE
1.0 INTRODUCTION
Atmospheric dynamics encompasses all physical processes within atmospheres, including global and
regional-scale circulation, convection, tropical cyclones, and inter-annual variability. Information
about dynamics informs both short range weather forecasting and projections for medium to long
term climate. This unit will explain fundamental set of physical principles and apply them in
understanding large scale atmospheric motions, mathematical description of the atmospheric
dynamics, thermodynamics of the atmosphere, forces of theatmosphere planetary waves, mid latitude
cyclones, the planetary boundary layer, and aspects of the general circulation of the atmosphere.
2.0 OBJECTIVES
explain applications of the thermodynamics of the earthatmosphere
define the planetary boundary layer of the atmosphere
describe the forces in the atmosphere planetary waves
discuss the mid-latitude cyclones of the atmosphere
describe the general circulation of the atmosphere

24
MAIN CONTENT
Thermodynamics of the Earth Atmosphere
Atmospheric thermodynamics is the study of heat to worktransformations and the reverse in the
atmospheric system in relation to weather or climate. Following the fundamental laws of classical
thermodynamics, atmospheric thermodynamics studies phenomena such as properties of moist air,
formation of clouds, atmospheric convection, boundary layer meteorology and vertical stabilities in
the atmosphere. Atmospheric thermodynamics forms the basis for cloud microphysics and
convection parameterizations in numerical weather models, and is in use in many climate
considerations, including convection – equilibrium climate models. (Holton, 2004).
The atmosphere is a typical example of a non-equilibrium system. Atmospheric thermodynamics
focuses on water and its transformations. The major role of atmospheric thermodynamics is
expressed in terms of adiabatic and diabatic forces acting on air parcels included in primitive
equations of air motion either as grid resolved or sub-grid parameterizations.
Applications of Thermodynamics
1) Hadley Circulation
In the application of thermodynamics, the Hadley circulation can be considered as a heat engine,
identified with rising of warm and moist air in the equatorial region with the descent of cooler air in
the subtropics corresponding to a thermally driven direct circulation with consequent net production
of kinetic energy. The thermodynamic efficiency of the Hadley system, considered as a heat engine,
has been relatively constant over the years, averaging 2.6%. The power generated by Hadley
circulation between (1979-2010) according to Holton (2004) has risen atan average rate of about
0.54TW per year. This reflects an increase in energy input to the system consistent with the observed
trend in the tropical sea surface temperatures.
2) Tropical Cyclone Cycle

25
The thermodynamics structure of the hurricane can be modeled as a heatengine running between sea
temperature of about 300k and tropopause which has temperature of about 200k. Parcels of air
travelling close to the surface take up moisture and warm, ascending air expands and cools releasing
moisture (rain) during the condensation. The release of latent heat energy during the condensation
provides mechanical energy for the hurricane. A decreasing temperature in the upper troposphere
close tothe surface will increase the maximum winds observed in hurricanes.
When applied to hurricane dynamics, it defines a carnot heat engine cycle and predicts maximum
hurricane intensity.
3) Water Vapour and Global Climate Change Clausius–Clapeyron relation shows how
the water-holding capacity of the atmosphere increases by about 8% per Celsius increase in
temperature. This water holding capacity can be approximated using August-Roche-Magnus
formula.
E.g. T = 6.1094exp (17.625T)
T+243.04
Where e.g. T is the equilibrium or saturation vapour pressure in pha, andT is temperature in degree
Celsius. This shows that, when atmospheric temperature increases due to greenhouse gases, the
absolute humidity should also increase exponentially, assuring a constant relative humidity.
However, this pure thermodynamics argument is subject of consideration because convective
processes might cause extensive drying due to increased areas of subsidence and efficiency of
precipitation could be influenced by the intensity of convection andbecause cloud formation is
related to relative humidity.
Forces in the Atmosphere Planetary Waves
Planetary waves are often called Rossby waves. They are natural phenomena in the atmosphere and
oceans of planets that largely owe their properties to rotation. In other words, it is a periodic
disturbance in the fields of atmospheric variables such as surface pressure or geo-potential
height, temperature or wind velocity which may either propagate (travelling wave) or not (standing
wave). Atmospheric waves range in spatial and temporal scale from large scale planetary waves

26
(Rossby waves) to minute sound waves. Atmospheric waves with periods which are harmonics of
one solar day are known as atmospheric tides.
The mechanism for the coring of the waves can vary significantly.Generally waves are either exerted
by heating or dynamic effects. For instance the obstruction of the flow by mountain barrier like
Rocky Mountains in the USA or the Alps in Europe. Heating effects can be ona small-scale like
the generation of gravity waves by convection orlarge-scale (formation of Rossby) waves by the
temperature contrast between continents and oceans in the Northern hemisphere winter. Atmospheric
wave transport momentum which is fed back into the background flows as the wave dissipates. This
wave forcing of the flow is particularly important in the stratosphere where this momentum
deposition by planetary scale Rossby waves gives rise to sudden
stratospheric warming and the deposition by gravity waves gives rise to the quasi-biennial
oscillation. In the mathematical description of the atmospheric waves, spherical harmonics are used
when considering a section of a wave along a latitude circle, this is equivalent to a sinusoidalshape.
Mid-Latitude Cyclone
These are large travelling atmospheric cyclonic storms up to 2000 kilometers in diameter with
centres of low atmospheric pressure. An intense mid-latitude cyclone may have a surface pressure as
low as 970 millibars, compared to an average sea-level pressure of 1013 millibars. Mid-latitude
cyclones are the result of the dynamic interaction of warm tropical and cold polar air masses at the
polar front. This interaction causes the warm air to be cyclonically lifted vertically into the
atmosphere where it combines with colder upper atmosphere air. This process helps to transport
excess energy from the lower latitudes to the higher latitudes. The mid-latitude cyclone is rarely
motionless and commonly travels about 1200 kilometers in one day. Its direction of movement is
generally eastward. Precise weather movement of weather system is controlled by the orientation
of the polar jet stream in theupper troposphere. Mid-latitude cyclones can produce a wide variety
of precipitation types such as rain, freezing rain, hail, sleet, snow pellets, and snow.
The Planetary Boundary Layer (PBL)
The lowest layer of the atmosphere is called the troposphere. The troposphere can be divided into
two parts: a planetary boundary layer (PBL) extending upwards from the surface to a height that
ranges anywhere from 100 to 2000m and above it, (the free atmosphere). The PBL is directly
influenced by the presence of the earth surface, responding to such forcing as frictional drag, solar

27
heating and evaporation. Each of these generates turbulence of various sized eddies, which can be as
deep as the boundary layer itself lying on top of each other. PBL model is used for weather
forecasting.
General Circulation of the Atmosphere
Climate and general circulation of the atmosphere are related to energy balance, transportation
processes and the three cell model. Energy balance of the incoming solar radiation and the outgoing
terrestrial radiation emitted by the earth is nearly balance over the year. When average over a latitude
band, incoming radiation is a surplus in the tropics and deficit of radiation is found in the polar
region due the outgoing terrestrial radiation being larger than the absorbed solar
radiation. To compensate for the surplus and deficit of radiation indifferent regions of the globe,
atmospheric and oceanic transport processes distribute the energy equally around the earth. This
transportis accomplished by atmospheric winds and ocean currents.
SELF-ASSESSMENT EXERCISE
1. Explain the 3 applications of thermodynamics
2. Describe the general circulation of the atmosphere
4.0 CONCLUSION
The dynamics of the earth atmosphere has been discussed with referenceto thermodynamic, forces of
planetary waves, mid-latitude cyclones planetary boundary layers, and general circulation of the
atmosphere.
5.0 SUMMARY
It is agreed that, corriolis force plays significant role in the activities of the atmospheric processes

28
during rotation of the earth, the severity of effects depends on the magnitude of effects exerted at a
given location. The earth atmosphere is not stationed but changes occur at different times and
different region thereby bringing variation in weather and climatic conditions.

29
MEANING, PRINCIPLES AND LAWS OFRADIATION
The sun provides 99.97% of the energy required for all the physical processes that take place on the
earth and the atmosphere. As a result of absorbed insolation, different types of radiant heat or
radiation flow throughout the earth-atmosphere system, and inputs and outputs of radiation are
balanced at the planetary system. This unit focuses attention on the meaning of radiation,
principles of radiation and thelaws of radiation.
MAIN CONTENT
Meaning of Radiation
Radiation may be regarded as a transmission of energy in the form of electromagnetic waves. The
wave length of radiation is the distance between two successive wave crests. This wave length caries
in differenttypes of radiation and is inversely proportional to the temperature of the body that send it
out. The higher the temperature at which the radiationis emitted, the shorter the wavelength of the
radiation. The sun has a surface temperature of about 6900o
C, whereas the average surface
temperature of the earth is approximately 15o
C. Thus, radiation coming from the sun is short-wave
radiation and that emitted from the earth is long wave radiation. There is a wide spectrum ranges
from very short waves, such as cosmic rays and gamma rays to very long waves, such asradio-and
electric – power waves. (Briggs and Smithson, 1985; Blij, Muller, Williams, Conrad & Long, 2005).
Principles of Radiation
Radiant energy consist of electromagnetic waves of varying lengths. Any object whose
temperature is above absolute zero (0K or -273.15o
C) emits radiant energy. The intensity and the
character of this radiation depends on the temperature of the emitting object. As the temperature
rises, the radiant energy increases in intensity but its wavelength decreases as the wavelength
expands. The amount of radiation reaching any object is inversely proportional to the square of

30
the distance from the sources. This distance decay factor account for the difference in solar
inputs to the various planets in the solar system.
To a certain extent radiation is able to penetrate matter as exemplified inthe x-rays which can pass
through the human body. However, most radiant energy is either absorbed or reflected by objects in
its path. An absorption occurs when the electromagnetic waves penetrate but do not pass through the
object. The ability of an object to absorb or reflect radiant energy depends on a number of factors,
including the detailed physical structure of the materials, its colour and surface roughness, the angle
of the incident radiation and the wavelengths of the radiant energy.
An object which is able to absorb all the incoming radiation is referredto as a black body, although
this has conceptual value. A perfect black body does not exist in reality. All objects absorb a
proportion of the incoming energy and reflect the remainder. Variation also occurs according to the
wave length of the energy.

31
When radiant energy is reflected by an object, very little change in the nature of the radiation occurs,
although the effect may be to scatter the radiation. Scattering changes the direction of the
incoming radiationwithout directly affecting its wavelength.
Laws of Radiation
The following are the radiation laws;
1. All substances emit radiation as long as their temperature is above absolute zero (0K or -237.15o
C)
2. Some substances emits and absorb radiation at certain wavelengths only. This is mainly true of gases
3. If the substance is an ideal emitter (black body) the amount of radiation given off is proportional to
the fourth power of itsabsolute temperature. This is known as the Stefan-Boltzmann lawand can be
represented as E=GT4 where G is a constant (the Stefan-Boltzmann constant) which has a value 5.67
x 10-8
WM- 2
K-4
and T is the absolute temperature.
4. As substances get hotter, the wavelengths at which radiation is emitted will become shorter. This is
called Wien’s displacement law which can be represented as Xm = a/T where Xm is the wavelength,
T is the absolute temperature of the body and ‘a’ is a constant with a value of 2898 if Xm is
expressed in micrometers.
5. The amount of radiation passing through a particular unit area is inversely proportional to the square
of the distance of that area from the source (1/d2
).
released by the sun is called a perfect black body, but unfortunately nobody can absorb all.
Instead, bodies absorb some and reflect some.
ENERGY IN THE ATMOSPHERE
INTRODUCTION
The atmosphere is described as a dynamic, constantly churning component of a gigantic heat engine.
The engine is being fuelled by incoming solar radiation (insolation). This unit will expose you to the
concept of atmosphere and its energy systems.

32
2.0 OBJECTIVES
• explain the concept of atmosphere
• discuss the regions of atmosphere
• illustrate the vertical region of the atmosphere
• illustrate the variation of atmospheric temperature with height
• provide explanation on the energy system
• illustrate the schematic presentation of the solar energy cascade
MAIN CONTENT
The Atmosphere
The atmosphere may be broadly divided into two vertical regions. The lower region called the
hemisphere, extends from the surface to 80 – 100km above the earth and has a more or less,
chemical composition. Beyond this level, the chemical composition of the atmosphere changes in the
upper region known as heterosphere. The hemisphere is the more important of the two atmospheric
regions for human beings because we live in it. It contains three major groups of components; they
are constant gases, variable gases and impurities (Briggs and Smithson,1985; Blij, Muller, Williams,
Conrad and Long, 2005).
Constant Gases: Two major constant gases make up 99% of the air by volume, and both are critical
to sustaining human and other forms of terrestrial life. They are nitrogen (n) which constitutes
78% of the airand oxygen (O2) which accounts for another 21%. Survival depends on oxygen and
nitrogen. The oxygen and nitrogen is relatively inactive but bacteria convert it into other nitrogen (n)
compounds essential for plant growth.

33
Variable Gases: They collectively constitute only a tiny proportion of the air. They contain
certain atmospheric grades in varying quantitiesessential to human well-being. Examples include;
carbon-dioxide (Co2)0.04% of dry air and is a significant constituent of the atmosphere interms
of its climatic influence and vital function in photosynthesis.Water vapour is an invisible gaseous
form of water (H2O), moresufficient in capturing radiant energy because of its storage capacity.
Ozone (O3) is a rare type of oxygen molecule, composed of threeoxygen atoms instead of two,
laying between 15km and 50km above the earth. It has the ability to absorb radiant energy, in
particular the ultraviolet radiation associated with incoming solar energy. Other variable gases
present in the atmosphere include; hydrogen, helium,sulphur dioxide, oxides of nitrogen, ammonia,
methane and carbonmonoxide. Some of these are air pollutants. They can produce harmfuleffects
even when concentrations are one part per million (ppm) or less. Impurities: The atmosphere
contains great number of impurities in formof aerosols (tiny floating particles suspended in the
atmosphere). Impurities play an active role in the atmosphere. Many of them help in the
development of clouds and rain drops. Examples of aerosols include; dust, smoke, salt crystals,
bacteria and plants spores.
100km
80km
Fig. 1: Vertical Region of the Atmosphere
The atmosphere is an envelope of transparent, odourless gases held tothe earth by gravitational
attraction. The furthest limit of the atmosphere is said by international convention to be
10000km.Most of the atmosphere, and therefore our climate and weather is concentrated within
16km of the earth’s surface at the equator and 8km at the poles. Fifty percent of atmospheric mass is
within 5.6km of sea level and 99 percent is within 40km. Atmospheric pressure decreases rapidly
with height and temperature. Changes in temperature means that the
Heterosphere
-----------------------
-----------------------
Homosphere

34
atmosphere can be conveniently sub-divided into four distinct layers: bottom layer (troposphere),
upper boundary (tropopause), stratosphere and stratospause.
The bottom layer of the atmosphere, where temperature usually decreases with an increase in
altitude, is called troposphere. The rate ofa decline in temperature is known as Lapse rate, and in the
troposphere, the average lapse rate is 6.5o
C/1000m. The upper boundary of the troposphere, which
temperature stop decreasing with height, is called thetropopause.
Beyond this continuity, is a layer called stratosphere, temperatures either remain constant or start
increasing with altitude. Layers in which the temperature increases with altitude exhibit positive
lapse rates. Theseare called temperature inversions because they inverse or reverse what isbelieved
to be the normal state of temperature change with elevation i.e. a decrease with height.
As the top of the stratosphere is approached, beyond 52km above the earth, temperatures remains
constant with increasing altitude. This boundary zone is called the stratopause, and is topped by a
layer known as the mesosphere. In the mesosphere, temperatures again fall withheight, as they did in
the troposphere. Eventually the decline in temperature stops at a boundary called menopause. This
occurs at about 80km above the earth’s surface. Not far beyond the mesopause, temperature once
more increase with height in a layer called the thermosphere.
120
100
80
60
40
20
0
Height
Thermosphere
Mesosphere

35
Stratosphere
Ozonosphere
Trospophere
Stratospause
Tropopause

36
-100 -50 0 50 100
Temperature
Fig. 2: Variation of Atmospheric Temperature with Height
The Energy System
The sun is the earth’s prime source of energy. The earth receives energy as incoming short wave
solar radiation (also referred to as insolation). It is this energy that controls the planet’s climate and
weather, which when converted by photosynthesis in green plants, supports all forms of life. The
amount of incoming radiation received by the earth is determinedby four astronomical factors;
i. The solar constant
ii. The distance from the sun
iii. The altitude of the sun in the sky
iv. The length of night and day
It is a theoretical assumption that there is no atmosphere around the earth. In reality, much
insolation is absorbed, reflected and scattered asit passes through the atmosphere. Absorption of
incoming radiation is mainly by ozone (O3), water vapour, carbon dioxide and particles ofdust,
ice and clouds and, to a lesser extent, the earth’s surface reflects a considerable amount of radiation
back to space. The ratio between incoming radiation and the amount reflected expressed as a
percentageis known as albedo. The albedo varies with cloud type from 30-40percent in thin
clouds, to 50-70 percent in thicker stratus and 90 percent in cumulo-nimbus (when only 10 percent
reaches the atmosphere below cloud level). Albedos also vary over different land surfaces, from less
than 10 percent over oceans and dark soil, to 15 percent over coniferous forest and urban areas, 25
percent over grasslands and deciduous forest, 40 percent over light coloured deserts and 85 percent
over reflecting fresh snow. Where deforestation and overgrazing occur, the albedo increases. This
reduces the possibility of cloud formation and precipitation and increases the risk of desertification.
Scattering occurs when incoming radiation is diverted by particles of dust, as from volcanoes and
deserts, or by molecules of gas. It takes place in alldirections and some of the radiation will
reach the earth’s surface as diffuse radiation. As a result of absorption, reflection and scattering,only
about 24% of incoming radiation reaches the earth’s surface directly, with a further 21% arriving at
ground level as diffused radiation. Incoming radiation is converted into heat energy when it reaches

37
the earth’s surface. As the ground warms, it radiates energy backinto the atmosphere where 94% is
absorbed (only 6% is lost to space), mainly by crater vapour and carbon dioxide, the green house
(effect which traps so much of the outgoing radiation). This outgoing (terrestrial) radiation is known
as long-wave or infra-red radiation.
Schematic presentation of the solar energy cascade
Incoming
radiation (100%) Clouds absorb (3%)
and reflect (23%)
(1%) absorb in
stratosphere
(21%) scattered and
reaches the earth as
diffuse radiation
(4%) is reflected
back into space
45% reaches the
earth’s surface:
direct 24% +
diffuse 21%
radiation

38
24% absorb by theatmosphere
24% directly reachesby the atmosphere
Fig. 3: Earth’s surface
Radiant Energy: is the most relevant to our discussion, for it is in this form that the sun’s energy is
transmitted to the earth. The heat from the sun exerts or disturbs electric and magnetic fields, setting
up a wave-like activity in space. The length of these waves – that is, their distance apart varies
considerably, so that solar radiation comprises a wide range of electromagnetic wave length, only a
very small proportion of these are visible to the human eye reaching the earth surface as light.
However, it takes about 81/3 minutes to transit energy from the sun the earth (15- 107km). On
passing through the atmosphere which surrounds the earth, some of this radiant energy is reflected or
absorbed. Because of this interception not all the radiant energy finds its way to the earth’s surface.
That which does, and that which is absorbed by the atmosphere is converted from radiant to other
forms of energy. (Briggs and Smithson, 1985)
Thermal Energy: This is obtained from the conversion of radiant energy. It warms the earth’s
surface and the atmosphere by exerting the molecules of which they are composed. In simple terms,
the radiant energy is transmitted into the molecules making up the earth and atmosphere.
Thermal energy which involves disturbance of magnetic and electric fields can therefore be
considered as energy involved in the motion of extremely small components of matter, sometimes
referred to as kinetic energy of molecules.
Kinetic Energy: This is the energy of motion. Any moving objectpossesses kinetic energy, and it is
through the utilization of this energy that a stone thrown into a lake can disturbed the water to the
extent of
producing waves. It is also through the exploitation of kinetic energythat turbines and engines
are able to produce heat, light and so on.
Potential Energy: This is related to gravity because of the apparent pullthat the earth exerts upon

39
objects within its gravitational field, materialis drawn toward the earth’s centre. Thus, objects lying
at greater distances from the earth centre.

40
Clou
Clou
Clou
Clou
Clou Cloud
HEATING OF THE ATMOSPHERE
INTRODUCTION
In unit one, you studied the meaning, principles and laws of radiation, whereby, you learn that
radiation may be regarded as a transmission of energy in the form of electromagnetic waves. You
also discovered that any object whose temperature is above absolute zero (OK-273.15E)emits radiant
energy. And as the temperature rises, the radiant energy increase in intensity. The amount of
radiation passing through a particular unit area is inversely proportional to the square of the distance
of that area from the source – (1/d2
).
While in unit 2, you learned about the atmosphere, what happens within the regions of the earth’s
atmosphere and its energy system. You saw that, not all the radiant energy finds its way to the
earth’s surface because, on passing through the atmosphere, some of this energy is reflected or
absorbed.
In this unit, you will learn how the atmosphere is being heated.
MAIN CONTENT
Processes of Heating
The earth does more than absorb or reflect shortwave insulation; itconstantly gives off long wave
radiation on its own. When the earth’s land masses and oceans absorb shortwave radiation. It
triggered rise in temperature, and the heated surface now emits long wave radiation. One or two
things can happen to this radiation leaving the planetary surface; either it is absorbed by the
atmosphere or it escapes into space.

41
Surface
Fig. 4: Long Wave Radiation Emitted By the Earth
The major atmospheric constituent that absorbs the earth’s long wave radiation are carbon dioxide,
water vapour, and ozone. Each of these variable gases absorbs radiation at certain wavelengths but
allows other wavelengths to escape through an atmospheric “window.” Up to 9 percent of all
terrestrial radiation is thereby lost to space, except when the window is shut by clouds. Clouds
absorb or reflect back to earth almost all the outgoing long wave radiation. Therefore, a cloudy
winter night is likely to be warmer than a clear one.
The atmosphere is heated by the long wave radiation it absorbs. Most of this radiation is absorbed at
the lower, dense levels of the atmosphere, a fact that helps account for air’s higher temperatures near
the earth’s surface. Thus, our atmosphere is actually heated from below, not directly by the sun
above. The atmosphere itself, being warm, can also emit long wave radiation. Some goes off into
space, but some known as counter-radiation, is reradiated back to the earth. Without this counter
radiation from the atmosphere, the earth’s mean surface temperature would be about -20o
C, 35o
C
colder than its current average of approximately 15o
C. The atmosphere, therefore, acts as a blanket.
The blanket effect of the atmosphere is similar to the action of radiation and heat in a garden
greenhouse. Shortwave radiation from the sun is absorbed and transmitted through the greenhouse
glass windows, strikes the interior surface, and is converted to heat energy. The long wave radiation
generated by the surface heats the inside of the greenhouse.But the same glass that let the short
wave radiation now acts as a trap to prevent that heat from being transmitted to the outside
environment, thereby raising the temperature of the air inside the greenhouse.
A similar process takes place on the earth, with the atmosphere replacing the glass. Not

42
surprisingly, this is called the basic natural process of atmospheric heating greenhouse effect.
Human being may be influencing the atmosphere’s delicate natural processes through series of
activities which may trigger sequence of events that could heighten a global warming trend with
possibly dire consequences for near-future environmental change. (Blij, Muller, Williams, Conrad,
Long, 2005).
Short-Wave Energy in the Atmosphere
Sunlight first enters the atmosphere and passes through the mesosphere with little change. In the
stratosphere, the density of atmospheric gases increases. There is more oxygen available which
reacts with the shortest or ultra-violet wavelengths and effectively removes them, warming the
atmosphere in the process. It is in the troposphere that most effects take place. In the upper
troposphere, the atmosphere is relatively dense with apressure of about 20% of that at the surface.
The size of the gas molecules of the air is such that they interact with the insolation, causingsome of
it to be scattered in many directions. This process depends on wavelength. The shorter waves are
scattered more than the longer wavesso we have these scattered waves as blue sky. If the reverse
were true, the sky would be permanently red, and if there were no atmosphere, as on the moon, the
sky would be black. Dust and haze in the atmosphere produce further scattering, but not all of this is
lost.
Some of the scattered radiation is returned to space, but much is directeddownwards the surface as
down-scatter or diffuse radiation. This is also the type of radiation which is experience during cloudy
conditions with no direct sunlight when the solar beam is ‘diffused’ by the water droplets or ice
particles. Without diffuse radiation, everything we see would either be very bright, when in direct
sunlight, or almost black when in shadow.
Another type of short wave energy loss is absorption. The gases in the atmosphere absorb some
wavelengths as cloud equally do. In this manner, the atmosphere is warmed though the amounts
involved are small. The most important loss of short-wave radiation in its path
through the atmosphere is by reflection. The water droplets or ice crystals in clouds are very
effective in reflecting insolation. The degree of reflection is usually called the albedo. Albedo is
normally expressed as a ratio of the amount of reflected radiation divided by the incoming radiation,
if multiply by 100, this can be expressed as a percentage.
The sunlight reaching the earth’s surface which is not reflected, the radiation is returned to space in
the short wave form and becomes part ofthe outflow of energy from the earth.
Long Wave Energy in the Atmosphere

43
All substances emit long wave radiation in proportion to their absolute temperatures. The earth’s
surface receives most short wave radiation andtherefore normally has the highest temperatures. It
follows from this form that, most long wave emission will be from the ground surface. The
atmosphere is much more absorbent to long wave radiation than to shortwave radiation. Carbon
dioxide and water vapour are much more effective absorbers of much of the longer part of the
spectrum.
Clouds are also more effective at absorbing long wave radiation hence, their temperature will be
higher than otherwise. This cloud effect is most noticeable at night. With clear skies, radiation is
emitted by the surface but little is received from the atmosphere and therefore, the temperature falls
rapidly. If the sky is cloudy, the clouds will absorbs much of the radiation from the surface and,
because they are also emitters, more of the radiation will be returned to the grounds as counter
radiation than if the sky had been clear (Briggs, Smithson, 1985).
Some of the radiation given off by the surface is lost to space butmajority gets caught up in
the two-way exchange between the surface and the atmosphere.
Heat Balance
Climate is often considered to be something derived from the atmosphere, and it is true that the
climate of a place is essentially the result of the redistribution of heat energy across the face of the
earth. However, the events of the atmosphere are greatly affected by the processes that operates on
the earth’s surface itself. Flows of heat energyto and from the surface are as much as part of the
climate of an area as the winter snow or summer thunderstorm, in fact, because these heat energy
flows operate continuously.
The heat energy balance of the earth’s surface is composed in its simplest form of four different
kinds of flows. One of these is thecomposite flows of radiant heat that makes up net radiation the
second is
latent heat which causes evaporating liquids to change to gases. All the air molecules contain heat
energy, the heat that we feel on our skin, and this sensed heat is termed sensible heat flow. Usually
during the day, theground warms the air above it. Warm air rises, and parcels of air move upward in
a vertical heat-transfer process known as convection, thereby causing a sensible heat flow.
Whereas, sensible heat flow depends on convection, the heat that flows into and out of the ground
depends on conduction, the transport of heat energy from one molecule to the next. The heat that is
conducted into and out of the earth’s surface is collectively called ground heat flow or soil heat flow.

44
This is the smallest of the four heat balance components.
Generally, the heat that passes into the ground during the day is approximately equal to that flowing
out at night. Thus, over a 24 hour period, the balance of ground heat flow often is so small that it can
be disregarded. Except for the usually small amount of energy used by plants in photosynthesis, the
total heat balance of any part of the earth is made up of the flows of radiant heat, latent heat, sensible
heat and ground heat.

45
ATMOSPHERIC MOISTURE
FORMS OF WATER AND HEAT TRANSFERS
INTRODUCTION
The physical world is characterized by energy flow which continually pass from one place to another
on land, within the land and in the atmosphere. This unit looks at the ability of water to exist in three
physical states and the process of transfer and energy required to transfer water from one state to
another.
2.0 OBJECTIVES
At the end of the unit, you should be able to;
• discuss the various forms of water
• explain the different processes of heat transfer
• identify the energy required to transfer water from one state toanother
Forms of Water
The Solid Form
The solid form of water, ice is made up of molecules that are linked together in a uniform manner.
When there is enough heat energy, the bonds that link molecules together are broken making ice to
change its state to become the liquid form which is known as water. Molecules in the liquid form are
not evenly spaced though they exist together, movingaround freely. (Goudie, 1986).
The Liquid Form
Water is a liquid compound which is converted by heat into vapour (gas)and by cold into solid (ice).
The presence of water serves three essentialspurposes:
a. It maintains life on earth: Flora, in the form of natural vegetation (biomes) and crops, and fauna, i.e.
all living creatures, including humans.

46
b. Water in the atmosphere, mainly as a gas, absorb reflects and scatters insolation to keep our plant at
a habitable temperature
c. Atmospheric moisture is of vital significance as a means of transferring surplus energy from tropical
areas either horizontally to polar latitudes or vertically into the atmosphere to balance the heat
budget.
Despite this need for water, its existence in a form readily available to plants, animals and humans is
limited. It has been estimated that 97.2% of the world’s water is in the oceans and seas. In this form,
it is only useful to plants tolerant to saline conditions (halophytes) and to the populations of a few
wealthy countries that can afford desalinization of plants.
Approximately 2.1% of water in the hydrosphere is held in storage as polar ice and snow. Only 0.7%
fresh water found either in lakes and rivers (0.1%) as soil moisture and ground water (0.6%) or in the
atmosphere. At any given time, the atmosphere only holds, on average, sufficient moisture to give
every place on the earth 2.5cm (about 10 dayssupply) of rain. There must therefore be a constant
recycling of water between oceans, atmosphere and cloud. This recycling is achieved through the
hydrological cycle.
Measuring Water Vapour
Humidity is a measure of the water vapour content in the atmosphere. Absolute humidity is the mass
of water vapour in a given volume of air measured in grams per cubic metre (g/m3
). Specific
humidity is similar but expressed in grams of water per kilogram of air (g/kg). Humidity depends
upon the temperature of the air. At any given temperature, thereis a limit to the amount of moisture
that the air can hold. When this limitis reached, the air is said to be saturated. Cold air can hold only
relatively small quantities of vapour before becoming saturated but this amount increases rapidly as
temperatures rise.
This means that the amount of precipitation obtained from warm air is generally greater than that
from cold air. (Briggs, and Smithson, 1985).
Relative humidity (RH) is the amount of water vapour in the air at a given temperature expressed as
a percentage of the maximum amount ofvapour that the air could hold at that temperature. If the RH
IS 100% theair is said to be ‘moist’ and the weather is humid or clammy. When the RH drops to
50%, the air is ‘dry”. Figures as low as 10% have been recorded over hot deserts. If unsaturated air is
cooled and atmospheric pressure remains constant, a critical temperature will be reached when the
air becomes saturated (i.e. RH – 100%). This is known as dew point.Any further cooling will result
in the condensation of excess vapour, either into water droplets where condensation nuclei are

47
present, or into ice crystal if the air temperature is below 0o
C.
This is shown in the following work examples.
1. The early morning air temperature was 10o
C. Although the air could have held 100 units of water at
that temperature, at the timeof reading it held only 90. This means that the RH was 90%.
2. During the day, the air temperature rose to 12o
C. A s the air warmed it became capable of holding
more water vapour, up to 120 units. Owing to evaporation, the reading reached a maximumof 108
units which meant that the RH remained at 90% i.e.108/120) x 100.
3. In early evening, the temperature fell to 10o
C at which point, as stated above, it could hold only 100
units. However, the air at thattime contained 108 units, so, as the temperature fell, dew points was
reached and the 8 excess units of water were lost through condensation.
Hydrological Cycle
The hydrological cycle model consists of a number of stages showing the relative amounts of
water involved in each.
1. The largest amounts of water transferred in any component of thetotal cycle are those involved in the
direct evaporation from the sea to the atmosphere and in precipitation back to the sea. Evaporation is
the process by which water changes from liquid togaseous (vapour) form. Precipitation includes any
liquid water or ice that falls to the surface through the atmosphere.
2. The passage of water to the atmosphere through leaf pores is called transpiration and the term
evapotranspiration encompasses the joint processes by which water evaporates from land surface and
transpires from plants. Evapotranspiration combines the precipitation of water on land and plant
surface to play a quantitatively smaller, but possibly more important part in the hydrological cycle.
3. If surplus precipitation at the land surface does not evaporate, it isremoved via the surface network
of streams and rivers, a phenomenon called Runoff. The run off value includes some water that
infiltrates (penetrates) the soil and flows beneath the surface, eventually finding its way to rivers and
the ocean.
CAUSES OF ATMOSPHERIC CIRCULATION
1.0 INTRODUCTION

48
This unit will explain the causes of atmospheric circulation. The basic factors that explain the
circulation of air in the atmosphere; latitudes andthe earth rotation and what makes the earth receives
an unequal amount of heat energy at different latitudes and the earth rotation will be explained.
Latitudes and Earth Rotation as Cause af AtmosphericCirculation
The sun strike the earth’s surface at higher angles, and therefore at great intensity in the lower
latitudes than in the higher latitudes. The equator receives about two and one-half times as much
annual solar radiation as the poles do.
If this latitudinal imbalance of energy were not somehow balanced, the low-latitude regions would
be continually heating up and the Polar Regions cooling down. Energy in the form of heat is
transferred by atmospheric circulation (and to a much lesser extent oceanic circulation). Briggs,
Smithson, 1985).
The simple rotation of the earth complicates the operation of the general atmospheric circulation.
The most important effect is expressed as an apparent deflective force. This deflective force
affecting movement on a rotating body is called the coriolis force. If the earth did not rotate and was
composed of entirely land or water, there would be one large convection cell in each hemisphere.
Surface winds would be parallel to pressure gradients and would blow directly from high to low
pressure areas. In reality, the earth does rotate and the distribution of land and sea is uneven,
consequently more than one cell is created as rising air warm at the equator loses heat to space and
there is less cloud cover to retain itas it travels further from its source of heat.
Moving air tends to be deflected to the right in the northern hemisphere and to the left in the southern
hemisphere by coriolis force.

49
Fig. 1: Air Movement on a Rotation Free Earth
The earth’s rotation through 360o
every 24 hours means that a wind blowing in a northerly direction
in the northern hemisphere appears to have been diverted to the right on a curved trajectory by 15o
of
longitudefor every hour. This helps to explain why the prevailing winds blowing from the tropical
high pressure zone approach Britain from the south- west rather than south. In theory, if the Coriolis
force acts alone, the resultant wind would blow in a circle.
Winds in the upper troposphere, unaffected by friction with the earth’s surface, shows that there is a
balance between the forces exerted by the pressure gradient and the coriolis deflection. The result is
the geostrophic wind which blows parallel to isobars. The existence of the geostrophic wind was
recognized in 1857 by a Dutchman, Buys Ballot, whose law states that ‘if you stand in the northern
hemisphere with your back to the wind, low pressure is always to your left and high pressureto
your right.’
Friction caused by the earth’s surface upsets the balance between the pressure gradient and the
coriolis force by reducing the effect of the latter. As the pressure gradient becomes relatively more
important when friction is reduced with altitude, the wind blows across isobars towards the low
pressure. Deviation from the geostrophic wind is lesspronounced over water because its surface is
smoother than that of land as indicated in the diagram

50
Fig 2: Latitudinal Variation in the Coriolis Force
Fig 3: Formation of Geostrophic Wind in the Northern Hemisphere
These are derived by combining these characteristics of latitude and humidity. When air masses
move from their source region they aremodified by the surface over which they pass and this
alters their temperature, humidity and stability. For instance tropical air moving northwards is cooled
and becomes more stable while polar air moving south becomes warmer and increasingly unstable.
Each air mass therefore brings its own characteristic weather conditions to the location found. Each
air mass is unique and dependent on climatic conditions in the source region at the time of its
development; the path which itsubsequently follows; the season in which it occurs; and since it
has a three-dimensional form, the vertical characteristics of the atmosphere at the time.
The tropical maritime (TM) air and tropical continental (TC) air masses are dominant in Nigeria.
They determine the occurrence of wet and dry seasons. TM dominates the wet season due to it
sources of origin and direction of movement and is sometimes called the south-westerly trade winds

51
while the TC dominates the dry season and is sometimes calledthe north-easterly trade winds.
UNIT 4 PLANETARY SCALE
INTRODUCTION
Despite many modern advances using radiosonde readings, satellite imagery and computer
modeling, the tricellular model still forms the basis of our understanding of the general circulation of
the atmosphere. This unit explains the tricecullar model of understanding the general pattern of
atmospheric circulation.
This unit also explains other scales used to provide understanding of the atmospheric circulation
pattern. The concept of the synoptic systems is also the focus of this unit.
Local wind systems are often more significant in day-to-day weather because they respond to much
more subtle variations in atmospheric pressure than are depicted. Moreover, because smaller
distances are involved, the effect of the coriolis force can usually be disregarded. A number of
common local winds serve to illustrate how topography and surface type can influence the pressure
gradient and its resultant wind flow of the three meso-scale circulations described here, land and sea
breezes and mountain and valley winds are caused by local temperature differences while fohn
results from pressure differences on either side ofa mountain range.
Tricellular Model and Atmospheric Circulation
The meetings of the trade winds in the equatorial region form the intertropical convergence zone
(ITCZ). The trade winds which pick up latent heat as they cut across warm, tropical oceans, are
forced to rise by violent convection currents.
The unstable, warm, moist air is rapidly cooled adiabatically to produce the towering cumulonimbus
clouds, frequent afternoon thunderstorms and low pressure characteristics of the equatorial climate.
It is these strong upward currents that form the ‘powerhouse’ of the general global circulation and
which turns latent heat first into sensible heat and later into potential energy. At ground level, the
ITCZ experiences only very gentle, variable wind known as the doldrums. Briggs & Smithson,1985).

52
As rising air cools to the temperature of the surrounding environmental air, uplift ceases and it
begins to move away from the equator. Further cooling, increasing density and diversion by the
coriolis force cause the air to slow down and to subside, forming the descending limb of the Hadley
cell. In looking at the northern hemisphere, the southern is its mirror image; it can be seen that air
subsides about 30o
N of the equator to create the sub-tropical high pressure belt with its clear dry sky
and stable conditions on reaching the earth’s surface, the cell is completed as one of the air is
returned to the equator as the north-east trade winds.

53
Macro Scale
The concept of air masses is important because air masses help tocategorized world climate types. In
regions where one air mass is dominant all year, there is little seasonal variation in weather, for
example at the tropics and at the poles. Areas such as the British Isles, where air masses constantly
interchange, experience much greater seasonal and diurnal (daily) variation in their weather. (Blij
and Muller, 2005).
If air remains stationary in an area for several days, it tends to assume the temperature and
humidity properties of that area. Stationary air is mainly found in the high pressure belts of the
subtropics and in high latitudes. The areas in which homogenous air masses develop are called
source regions. Air masses can be classified according to the latitudes in
which they develop which determines their temperature – Artic (A), polar (P) or tropical (T) and the
surface over which they develop, which affects their moisture contents – maritime (M) or continental
(C).
The five major air masses which affect a location at various times of the year are as follows:
1. Artic Maritime Air Mass (AM)
2. Polar Maritime Air Mass (PM)
3. Polar Continental Air Mass (PC)
4. Tropical Maritime Air Mass (TM)
5. Tropical Continental Air Mass (TC)
Meso Scale
Land and sea breeze systems: Land surfaces and water bodies displays sharply contrasting thermal
responses to energy input. Land surfaces heat and cool rapidly, whereas water bodies exhibit a more
moderate temperature regime. During day, a land surface heats up quickly and the air layer in
contact with it rises in response to theincreased air temperature. This rising air produces a low
pressure cell over the coastal land or island. Since the air over the adjacent water is cooler, it

54
subsides to produce a surface high pressure cell. A pressure gradient is thereby produced, and air in
contact with the surface now moves from high pressure to low pressure. Thus, during the day, shore-
zone areas generally experience air moving from water to land. This is called sea breeze.
Fig 6: Sea Breeze
At night, when the temperature above the land surface has dropped significantly, the circulation
reverses because the warmer air (and lower pressure) is now over the water. This result in air moving
from land to water. This is called land breeze.
Fig 7: Land Breeze
When the system generates, sea and land breezes, it produces a circulation cell composed of the
surface breeze, rising and subsiding air associated with the lower-and higher pressure areas
respectively, airflow aloft in the direction opposite to that of the surface. Although it modifies the
wind and temperature conditions at the coast, the effect of this circulation diminishes rapidly as one

55
move inland. Note also that we use the word breeze. This accurately depicts a rather gentle
circulationin response to a fairly weak pressure gradient. The sea/land breezephenomena can easily
be overpowered if stronger pressure systems are nearby. (Williams, Conrad & Long, 2005).
Mountain/Valley Breeze Systems
Mountain slopes are subject to the reversal of day and night local circulation systems. This wind
circulation is also thermal, meaning thatit is driven by temperature differences between adjacent
topographic features. During the day, mountains terrain facing the sun tends to heat up more rapidly
than the surrounding slopes. This causes low pressure to develop, spawning an up sloping valley
breeze. At night, greater radiative loss from the mountain slopes cools them more sharply, high
pressure develops, and a down sloping mountain breeze results. The wind that blows up the valley is
also known as an anabatic wind while the down valley wind is called the Katabatic wind, which are
usually gentle but much stronger if they blow over glaciers or permanently snowcovered slopes.
Fohn
The fohn is a strong, warm and dry wind which blows periodically to thelee of a mountain range. It
occurs in the Alps when a depression passes to the north of the mountains and draws in warm, moist
air from the Mediterranean. As the air rises it cools at the DALR of 1o
C per 100m. If condensation
occurs at 1000m, there will be a release of latent heat and the rising air will cool more slowly at the
saturated adiabatic lapse rate (SALR) of 0.5o
C per 100m. This means that when the air reaches
3000m it will have a temperature of 0o
C instead of the -10o
C had latent heat not been released.
Having crossed the Alps, the descending air is compressed and warmed at the dry adiabatic lapse
rate (DALR) so thatif the land drops sufficiently, the air will reach sea level at 30o
C. This is 10o
C
warmer than when it left the Mediterranean. Temperatures mayrise by 20o
C within an hour and
relative humidity can fall to 10 percent. (Williams, Conrad, Long, 2005).
This wind, also known as Chinook on the American prairies, has considerable effects on human
activities. In spring, when it is mostlikely to blow, it melts snow and enable wheat to be grown.
Conversely, it warmth can cause avalanches, forest fires and premature budding of trees.

56
4.0 CONCLUSION
The overall pattern as explained by the tricellular model is affected by the apparent movement of the
overhead sun to the north and south of theequator (0o
C). This movement causes the seasonal shift of
the heatequator, the ITCZ, the equatorial low pressure zone and global wind systems and rainfall
belts. Any variation in the characteristics of the ITCZ i.e. its location or width can have drastic effect
on the surroundingclimates, as seen in the sahel droughts of the early 1970s and most of the1980s.
Categories of world climatic types are determined by air masses, seasonal variations and daily
changes in weather are equally determined by air masses. Five major air masses affect the weather of
a location. They include: artic maritime air mass (AM), polar maritime (PM), polar continental (PC),
tropical maritime (TM) and tropical continental (TC). All these air masses have effects on the
locations where they havedominance.
Local winds connote the winds that are peculiar to a relatively small area and are of local
importance. They are seasonal and often confinedto the lowest part of the atmosphere which result
to differential heating and cooling of land and sea.

57
Unit 5. THE DYNAMICS OF PRESSURE AND WINDSYSTEMS
INTRODUCTION
The movement of air on the earth’s surface is controlled by the imbalance or difference in air
pressure of different places. Air pressure on the other hand is determined by the height of the column
of air over a given place and its temperature. Thus, air pressure is a function of elevation and
temperature. The general principle is that air moves from areas of high pressure to areas of low
pressure. The movement of air in the atmosphere system may be vertical and horizontal. The two
movements are called descending and ascending dynamic of air system. Near the surface of the
earth, below an elevation of about 1000m,frictional forces come into play and disrupt the balance
represented by the geostrophic wind. Friction both reduces the speed and alters the direction of a
geostrophic wind. The frictional force acts in such that pressure gradient is forced over the coriolis
force so that the wind at the surface blows across the isobars instead of parallel to them. This
produces a flow of air out of high pressure areas and into low pressure areas, but at an angle of the
isobars rather than straight across them. Thisunit will explain some wind systems caused by friction
with particular focus on tropical cyclones and anticyclones.
Concept of Wind and Pressure Systems
Winds results from differences in air pressure which in turn may be caused by differences in
temperature and the force exerted by gravity as pressure decreases rapidly with height. An increase
in temperature causes air to be heated, expanded, becomes less dense and rises creating an area of
low pressure below. Conversely, a drop in temperature produces an area of high pressure.
Differences in pressure are shown on maps by isobars, which are lines joining places of equal
pressure. Pressure is measured in millibars (mb) and it is usual for isobars to be drawn at 4mb
intervals. Average pressure at sea level is usually 1013mb.However, the isobars pattern is usually
more important in terms ofexplaining the weather than the actual figures. The closer together the
isobars, the greater the differences in pressure, the pressure gradient and the stronger the wind. On
the other hand, the further apart the isobars,the lower the difference in pressure gradient and the
weaker the wind. Wind is nature’s way of balancing out differences in pressure as well as

58
temperature and humidity. (Blij, Muller, Williams, Conrad &Long, 2005).
Patterns of Movement of Wind System
Tropical Cyclones
Tropical cyclones are systems of intense low pressure known locally as hurricanes, typhoons and
cyclones. They are characterized by winds of extreme velocity and are accompanied by torrential
rainfall which may cause widespread damage and loss of life. Tropical cyclones are associated with
rising air at their centres, other sources of development may include;

59
i) They tend to develop over warm tropical oceans where sea temperatures exceed 26o
C and where
there is a considerable depth of warm water.
ii) In autumn, when sea temperatures are at their highest
iii) In trade winds belt; where the surface winds warm as they blow towards the equator
iv) Between latitude 50o
C and 20o
C north and south of the equator (nearer to the equator, the coriolis
force is insufficient to enable the feature to ‘spin’). Once formed, they move westwards, often on
erratic, unpredictable courses, swinging poleward on reaching land, where their energy is rapidly
dissipated. They are another mechanism by which surplus energy is transferred away from the
tropics.
Hurricanes are tropical cyclones of the Atlantic. They form after the ITCZ has moved to its most
northernly extent enabling air to convergeat low levels, with a diameter of up to 650km. Hurricane
rapidly declines once the source of heat is removed, i.e. when it moves over colder water or a land
surface; this increases friction and so cannot supply sufficient moisture. The average life span of a
tropical cyclone is 7 – 14 days. Tropical cyclones are a major natural hazard which often causes
considerable loss of life and damage to property and crops. Thereare four main causes of damage. (i)
High winds which often exceed 160km/hr and in extreme cases 300km/hr. (ii) ocean storm (tidal
surges),resulting from the high winds and low pressure, may inundate coastal areas many of which
are densely polluted. (iii) floodingwhich can be caused either by a storm of tidal surges or by the
torrential rainfall (iv) landslides can result from heavy rainfall where buildings have been erected on
steep unstable slopes.
This is an air circulation pattern associated with a tropical cyclic low pressure cell.
Anticyclones
An anticyclones is a large mass of subsiding air which produces an area of high pressure on the
earth’s surface. The source of the air is the upperatmosphere, where amounts of water vapour are
limited. On its decent, the air warms at the dry adiabatic lapse rate (DALR). So dry conditions result
pressure gradients that are gentle, resulting in weak winds orcalms. The winds blow outwards

60
in anticlockwise in the northern hemisphere. Anticlines may be 3000km in diameter, much larger
than depressions and, once established, they can give several days or, under extreme conditions,
several weeks of settled weather.
Blocking anticyclones often occur when cells of high pressure detach themselves from the major
high pressure areas of the subtropics orpoles. Once created, they last for several days and ‘block’
eastwards- moving depressions to create anomalous conditions such as extremes of temperature,
rainfall and sunshine.
This is an air circulation pattern associated with an anticyclonic high- pressure cell.
Frictional Surface Wind Systems
Below an elevation of about 1000m, near the earth surface, frictional forces play significant role and
disrupt the balance represented by the geostropic wind. Friction reduces the speed and alters the
direction of a geostrophic wind, causing the pressure-gradient force to overpower the coriolis force
so that the wind at the surface blows across the isobars instead of parallel to them. This produces a
flow of air high pressure areas into low-pressure area at an angle of the isobars and not straight
across them.
Surface pressure systems are often circular when viewed from above,the winds converge toward
a cyclone (low-pressure cell), this converging air has to feed and move to somewhere else so
it risesvertically in the centre of the low-pressure cell. The reverse is the casein the centre of an
anticyclone (a high- pressure cell); the air diverges and moves outward. Thereby drawing air down in
the centre of the high pressure cell. Cyclones are associated with rising air at their centres while
anticyclones are associated with subsiding air at their centres. Thismovement of air produces varied
weather conditions associated with each type of pressure system.
Planetary Winds
These are wind systems that result from planetary pressure distribution. These are the trade winds,

61
the mid-latitude westerlies and the polar easterlies. Trade winds are winds that blow from sub-
tropical belts of high pressure to the equatorial belt of low pressure. Their meeting point at the
equatorial low pressure belt is called inter-tropical convergence zone (ITCZ), and the area occupied
by the zone, doldrums. Mid-latitude westerlies are winds that blow from subtropical belt of high
pressure to the sub-polar belt of low pressure in both hemispheres. Those blowingin the northern
hemisphere are called south-westerlies and those blowingin the southern hemisphere are called the
north-westerlies. They are generally inconsistent in direction and speed due to the influence of
local pressure gradients which in turn are produced from the varied effects of land and sea. Polar
easterlies are polar wind systems that tend
to move towards the sub-polar low pressure belts of the northern and southern hemisphere. These
result in the formation of a wind belt of complex condition and characteristics. The complexity is
more definedin the northern hemisphere where the distribution of land and sea is widely varied.
(Robert, Robert, Daniel and James, 1999)
Local Winds
These winds are peculiar to a relatively small area and are of local importance though seasonal in
nature and usually confined to the lowest part of the atmosphere. They occur due to variation in
temperature of land and sea and the effects includes land and sea breezes. They arelocal winds
on daily basis. They are monsoon winds and the variations exist in areas adjacent to large water
bodies, rivers or sea (Oluwafemi, 1998).
A sea breeze is a very cool moisture-laden wind that blows from the sea during the day towards the
low pressure on the land due to the heating effect of the sun during the day. The land gets heated
more rapidly than the sea during the day. The heated air on the land expands and becomes lighter
and rises thereby creating a region of low pressure. The sea atthat point remains comparatively
cooler with a higher pressure making way carefully from the sea to replace the warm air rising on the
land. At night the land cool more rapidly than the sea so that cold and heavy airis developed thus
resulting to a high pressure condition over the land and a low pressure is created on the sea. As the
land cools off at night, the sea retains much of its day time heat. An outward blowing land breeze is
set up to replace the warm rising air on the sea.

62
2
CAUSES OF ATMOSPHERIC CIRCULATION
1.0 INTRODUCTION
Atmospheric circulation is the large – scale movement of air, and the means by which thermal
energy is distributed on the surface of the earth.The scale of circulation varies from year to year but
the basic structure remains fairly constant. Individual weather systems, mid-latitudedepressions or
tropical convective cells occur randomly. Causes forthese and other issues related to atmospheric
circulation will be discussed and explained in this unit.
Causes of Atmospheric Circulation
There are two factors that explain the circulation of air in the atmosphere. These include;
i) the amount of heat energy the earth receives at different latitudes
ii) the rotation of the earth on its axis
There is a marked surplus of net radiation between the equators, latitudes 35o
N. Pole wards where
outgoing radiation exceeds incoming radiation. This is because the sun’s rays strike the earth’s
surface at higher angles and therefore at greater intensity and magnitude in the
lower latitudes than the other latitudes. As a result about two and one- half times (21/
) as much as
annual solar radiation received at the poles and the one received at the equator (Blij, Muller,
Williams, Conrad & Long, 2005).
If this imbalance in energy continues without any balance, the lowlatitudes regions would experience
great heating up and the Polar regions cooling down faster than expected. The weather at these
latitudes is characterized by frequent north-south movement of air masses since considerable amount
of energy in form of heat is transferred by atmospheric circulation polewards due to intense heat
received at the equator which causes warm air to rise.
Heat transfer could occur by a simple cellular movement when the earth is stationary and there are
no thermal variations between landmasses and ocean. Warm air at low latitudes however travels

63
toward the poles at a high altitude and descends as it cools and then returns to the low latitudes as a
surface wind.
When the earth rotates on its axis, a significant effect is expressed as an apparent deflective force.
The deflective force affecting movement on a rotating body is called the coriolis force. Anything that
moves over the surface of the spinning planet; from stream currents to missiles to air particles is
subjected to the coriolis force. When other forces are absent, moving objects are deflected to their
right in the northern hemisphere and to their left in the southern hemisphere. Therefore, if the wind is
blowing from the North pole, it would be deflected to the right and becomes an easterly wind which
blows towards the west. However, if it is blowing from the sub-tropical region, it would be deflected
to the left and becomes westerly wind which blows towards the east (Wikipedia, 2015).
CHANGES IN PRESSURE AND WINDSYSTEMS
1.0 INTRODUCTION
The atmosphere is made up of gases and atmospheric pressure pushes against everything on the
planet earth. The force is Omni-directional andis more like a gloved hand than a blanket. The bodies
of all living thingsare balanced to the same pressure because of evolution. This unit will discuss the
changes that occur in pressure of the wind systems.
Vertical Changes in Pressure
The earth’s gravity holds the atmosphere in place otherwise it would escape into space. Gravity is
the invisible glue which keeps the universe together. The force of the earth’s gravity is stronger on
low altitude molecules than high altitude molecules. The effects of gravity on the lower altitudes is
that the gas molecules are more concentrated due to gravitational force that is, the molecules in the
lower atmosphere are denser therefore, more collisions occur and pressures tend to be higher.
(Robert, Robert, Daniel & James, 1999).
Horizontal Changes in Pressure
Density and temperature can affect the pressure in the atmosphere and changes can occur. Horizontal
changes in pressure can be classified into two categories: thermal – caused by temperature and
dynamic-caused bymotion.

64
Pressure and Thermal Changes
This describes how different surfaces heat and cool. One of the basic rules of gasses is that pressure
and density of a given gas vary inversely with temperature. During the day, temperature increases
thereby given room for the air to expand in volume while density decreases. Theregion around the
equator is a region of low pressure. Air density increases towards the poles and decreases in volume.
This condition makes the air subside and the pressure high. Though this might becontrary to the
common principle where warm temperatures are related with low pressure and cool weather with
high pressure. Pressures like temperatures are relative to each other.
Dynamic Changes
It is logical to assume that there would be a progressive increase in pressure from the equator to the
poles, but in the real sense pressure is different and it changes apparently. There are areas of high
pressure in the sub-tropics and areas of low pressure in the sub-polar regions. The zones of high or
low pressure are more complicated than just thermal activity. The reason for this apparent
inconsistency is the dynamic (motions) action of the earth.
The dynamic factors are the rotation of the planet and the great patternof circulation of the ocean
and atmosphere. Warm air rises from the equator and moves poleward direction but, the earth’s
rotation deflects
the air to the east. When it reaches the subtropics, the air has changed direction and is now flowing
west to east. The deflection causes the air to stack up over the subtropics which increases air
pressure at those locations.
Secondly, there are high pressure areas over the poles and subtropical zones. Dynastically reduced
zones of low pressure are formed between them in the sub-polar regions. One of the consequences is
air flows or downhill from highs to lows where it will rise. This leads to another concept called
pressure gradient.
Changes during the Seasons
Atmospheric pressure belts change positions because of the seasons. They meander northward in
July and southward in January in the northern hemisphere. This migration is between latitude 23½o
N

65
& S (Tropics of Cancer and Capricorn) because of changes in temperature. The pressure systems do
not change very much in low latitudes because of the small temperature changes. However, in high
latitudes where there is a variation between hours of sunlight and the angle of the sun, there will be
more changes in pressure and temperature. The temperature extremes are more substantial in the
northern hemisphere where the landtakes up 40% of the total surface while the southern hemisphere
takes only 20% land.
Changes during Winter
In the North Atlantic low-pressure cell called the Icelandic low another cell of low pressure
develops in the north pacific called the Aleutianlow. Since the air of the two low pressure cells
have relatively lower pressures compared to the two subtropical or polar high systems, the air moves
towards these cells from the north and south. These low pressure cells are associated with cloudy,
unstable weather and forms the originof winter storms. In winter in the mid latitude high-pressure
cells are associated with clear blue skies, calm, starry nights, and cold stable weather. In the winder,
cloudy conditions are tied to the oceanic lows, while clear weather is tied to the continental highs.
Changes during Summer
During summer, the anticyclone is very weak over the North pole due tothe heating of the ocean and
the continents as the length of day increases. The Aleutian and Icelandic lows almost disappear.
Over the landmasses of Eurasia and North America, low pressure cells develop,in Asia a low-
pressure system is formed but broken into two separate cells by the Himalayas. The low-pressure
system above northwest India
is strong enough to combine with the equatorial trough which has shifted from its winter location.
The subtropical highs in the northern hemisphere are more developed over oceans than continents.
They also journey northward and are extremely important in the climate of the continents. In the
pacific, the sub-tropic is designated the pacific high and has a very important part in moderating the
temperatures of the west coast of northern America. In the Atlantic, a similar formation serving the
same function is called the Bermuda high by Americans and Azores high to Europeans.

66
CONDENSATION AND PRECIPITATION
PROCESSES
PROCESSES OF CONDENSATION ANDPRECIPITATION
INTRODUCTION
Condensation is a process by which water vapour in the atmosphere is changed into liquid or, if the
temperature is below 0o
C, a solid. It usuallyresults from air being cooled until it is saturated. Cooling
may be achieved by: Long wave radiation, advection, orographic and frontal uplift and convective or
adiabatic cooling.
Condensation produces minute water droplets less than 0.05mm in diameter, or if the dew point
temperature is below freezing, ice crystals. The droplets are so tiny and weight so little that they
are kept buoyantby rising air currents which created them. Although condensation forms clouds,
clouds do not necessarily produce precipitation. As rising air currents are often strong, there has to
be a process within the clouds which enables the small water droplets and or ice crystals to become
sufficiently large enough to overcome the uplifting mechanism and fall to the ground.
Mechanisms of Condensation
Condensation takes place when the following mechanisms fully mature:
a) Radiation and contact cooling: This typically occurs on calm, clear evenings. The ground loses heat
rapidly through terrestrial radiation and the air in contact with it is then cooled by conduction. If the
air is moist, some vapour will condense to formradiation fog, dew or if the temperature is below
freezing point, hoar frost will occur.

67
b) Advection closing: This results from warm, moist air movingover a cooler land and sea surface.
This is formed when warm air from the land drifts over cold offshore ocean currents. Both radiation
and advection involve horizontal rather than vertical movements of air. When this happens, the
amount of condensation is reduced or limited.
c) Orographic and frontal uplift: Warm moist air is forced to rise either as it crosses a mountain barrier
(orographic ascend) or when it meets a cooler, denser mass of air at a front and results in the
formation of water droplets.
d) Convective or adiabatic cooling: This is when air is warmedduring the day time and rises in pockets
as thermals. As the air expands, it uses energy and so loses heat and the temperature drops. Because
air is cooled by the reduction of pressure with height rather than by a loss of heat to the
surrounding air, it issaid to be adiabatically cooled.
Both orographic and adiabatic cooling involve vertical movements ofair, they are more effective
mechanisms of condensation. Condensation does not occur readily in clear air. Indeed, if air is
absolutely pure, it canbe cooled below its dew point to become super-saturated with an RH in excess
of 100%. Laboratory tests have shown that, clean saturated air can be cooled to -40o
C before
condensation or in this case, sublimation. Sublimation is when water vapour condenses directly
into ice crystals
without passing through the liquid state. However, air is rarely pure and usually contains large
numbers of condensation nuclei. These microscopic particles referred to as hydroscopic nuclei
because they attract water, include volcanic dust (heavy rain always accompaniedwith volcanic
eruptions), dust from windblown soil, smoke and sulphuric acid originating from urban and
industrial areas and salt from sea spray.
Hygroscopic nuclei are most numerous over cities, where these may be up to 1 million per cm3
and least common over oceans (only 10 per cm3
). where large concentrations are found,
condensation can occur withan RH as low as 75%. Clouds are visible masses of suspended, minute
water droplets and/or ice crystals. Two conditions are necessary for the formation of clouds.
i) The air must be saturated, either by cooling below the dew point (causing water vapour to condense)
or by evaporating enough water to fill the air to its maximum water-holding capacity.
ii) There must exist a substantial quantity of small airborne particles called condensation nuclei around
which liquid droplets can formwhen water vapour condenses. Condensation nuclei are almost always

68
present in the atmosphere in form of dust and salt particles.
Processes of Precipitation
There are two theories which explain the processes by which droplets grow large enough to fall out
of a cloud as precipitation. (Blij, Muller, William, Conrad & Long, 2005).
The Ice-crystal Process
The ice-crystal process was first identified in the 1930s by meteorologists Tor Bergeron and Von
Findeisen. This process requires both liquid droplets and ice particles in the cloud. Ice particles are
normally present if the temperature is below 0o
C and if there are small particles called freezing
nuclei. Freezing nuclei perform the same function for ice particles as condensation nuclei perform
for water droplets.
When the cloud contains both ice particles and water droplets, the water droplets tend to evaporate
and then the resultant water vapoursublimates (changes from a vapour to a solid) directly onto
the icecrystals. The ice crystals attract more of the water vapour because the vapour pressure over
the ice crystal grows at the expense of the liquid droplets. The ice crystals become larger and often
joint together to form a snow flake. When the snow flake is heavy enough, it usually
encounters higher temperatures and melts, eventually reaching the surface as a liquid rain drop. Most
rainfall and snow fall in the mid- latitudes are formed by the ice-crystals process, but in the tropics
the temperature of many clouds do not necessarily drop below freezing point. Therefore, a
second process is initiated to make raindrops large enough to fall from cloud.
The Coalescence Process
The coalescence process (sometimes called the collisions – coalescence process) requires some
liquid droplets to be larger than others, which happens when there are giant condensation nuclei.
As they fall, thelarger droplets collide and join with smaller ones. But narrowly mixed smaller
droplets may still be caught up in the wake of the large ones and drawn to them. In this case, the
larger droplets grow at the expense ofthe smaller ones and soon become heavy enough to fall to
earth.
Types of Precipitation
Precipitation includes rain, snow, sleet, hail, dew, hoar frost, fog and rime.Among these onlyrain and
snow provide significant totals in the hydrological cycle. Precipitation reaches the earth’s surface in

69
several forms.Large liquid water droplets form rain. If the ice crystals in the ice-crystal process do
not have time to melt before reaching the earth’s surface, the result is snow. Sleet refers to pellets of
ice produced by the freezing rain before it hits the surface. If the rain freezes after reaching the
ground, it is called freezing rain (or glaze). Soft hails pellets (sometimes called snow pellets) can
form in a cloud that has more ice crystals than water droplets and eventually fall to the surface. True
hailstones result when falling ice crystals are blown upward from the lower, warmer part of a cloud,
where they gain a water surface to the higher, freezing part where the outer water turns to ice. This
process, which often occurs in the vertical air circulation of the thunder forms, may be repeated over
and over to form larger hailstones.
a) Rain: Main types of rainfall, distinguished by the mechanisms which cause the initial uplift
of air. Rarely does each mechanism operatein isolation. The types are discussed as follows.
1. Convergent and Cyclonic (frontal) Rainfall
This form of rainfall results from the meeting of two air streams in areas of low pressure. Within the
tropics, the trade winds, blowing towards the equator meets at theinter-tropical convergence zone
(ITCZ). The air is forcedto rise and in conjunction with convection currents, produces the heavy
afternoon thunderstorms associated with the equatorial climate. While in temperate latitudes,
depressions form at the boundary of two air masses. At theassociated fronts, warm, moist, less dense
air is forced to rise over colder, denser air, giving periods of prolonged and sometimes intense
rainfall. This is often augmented byorographic precipitation.
2. Orographic or Relief Rainfall
This type of rainfall results when near-saturated, warm maritime air is forced to rise when
confronted by a coastal mountain barrier. Mountains reduce the water holding capacity of rising air
by enforced cooling and can increase the amounts of cyclonic rainfall by retarding the speed
depression movement. Mountains also tend to cause air streams to converge and form through
valleys. Rainfall total increases where mountains are parallel to the coast. As air descends on the
leeward side of a mountain range, it becomes compressed and warmed and condensation ceases,
creating a rain shadow effect where little rain falls.
3. Convectional Rainfall
This occurs when the ground surface is locally overheated and the adjacent air, heated by
conduction, expands and rises. During its ascent, the air mass remains warmer than the surrounding

70
environmental air and it is likely to become unstable with towering cumulonimbus cloud forming.
These unstable conditions, possibly augmentedby frontal or orographic uplift force the air to rise in
a ‘chimney’. The up draught is maintained by energy released as latent heat at both condensation and
freezing levels. The cloud summit is characterized by ice crystals inan anvil shape and the top of the
cloud being flattened by upper-air movements. When the crystals and frozen water droplets, i.e. hail,
become large enough, they fall in adowndraught. The air through which they fall remains coolas heat
is absorbed by evaporation. The downdraught reduces the warm air supply to the ‘chimney’ and
therefore limits the lifespan of the storm. Such storms are usually accompanied by thunder and
lightning. How storms develop immense amount of electric charge is not fully understood. One
theory suggests that as raindrops arecarried upwards into colder regions, they freeze on the outside.
This ice-shell compress the water inside it until the shell bursts and the water freezes into
positively charged ice-crystals while the heavier shell fragments which are negatively charged,
towards the clouds and the
cloud base including a positive change on the earth’s surface.
Lightning is the visible discharge of electricity between the clouds or between clouds and the
ground. Thunder is the sound of the pressure wave created by the heating ofair along a lightning
flash. Convection is one process by which surplus heat and energy from the earth’s surface are
transferred vertically to the atmosphere in order tomaintain the heat balance.
b) Snow: Snow forms under similar conditions to rain except that at dew point temperatures
are under 0o
C, then the vapour condenses directly into a solid (sublimation). Ice crystals will form if
hygroscopic or freezing nuclei are present and these may aggregate to fire snow flakes. As warm air
holds more moisture than cold, snowfalls are heaviest when the air temperature is just below
freezing. As temperaturedrops it becomes too cold for snow.
c) Hailstone: Hail is made up of frozen raindrops which exceed 5mm in diameter. It is
usually formed in cumulo-nimbus clouds, resulting from the uplift of air by convection currents or
at a cold front.It is more common in areas with warm summers where there is sufficient heat to
trigger the uplift of air and less common in colder climates.
d) Dewfall, Hoar Frosts, Fog And Rime: Dew, hoar frost andradiation fog are all formed
under calm, clear, anticline conditions when there is rapid terrestrial radiation at night. Dew point
is reached as theair transpired from plants, condenses. If dew point is above freezingpoint dew will
form. If it is below freezing, hoar frost develops. Frost may also be frozen dew. Dew and hoar frosts
usually occur within 1m of ground level.

71
If the lower air is relatively warm, moist and contains hygroscopicnuclei, and if the ground cools
rapidly, radiation fog may form.Wherevisibility is more than 1km it is mist, if less than 1km, it is
called fog. In order for radiation fog to develop, a gentle wind is needed to stir the cold air adjacent
to the ground so that cooling affects a greaterthickness of air.
Advection fog is formed when warm air passes over or meets with cold air to give rapid cooling.
Sufficient droplets fall to the ground as fog- drop to enable some vegetation grow.
Rime occurs when super cooled droplets of water, often in the form of fogcome into contact with and
freeze upon solid objects such as masks, poles and trees.
e) Sleet and Glazed Frost: Sleet is a mixture of ice and snowformed
when the upper air temperature is below freezing point allowing snowflakes to form and the lower
air temperature is around 2o
C to 4o
C, which allow their partial melting. Glazed frost is the reverse of
sleet and occurs when water droplets form in the upper air but turn to ice on contact with a freezing
surface. When glazed frost forms on roads, it is known as ‘black ice.’
f) Acid Rain: This is an umbrella term for the presence in rainfallof a series
of pollutants which are produced mainly by the burning fossilfuels. Coal-fired power stations, heavy
industry and vehicle exhausts emit sulphur dioxide and nitrogen oxides. These are carried by
prevailing winds across seas and national frontiers to be deposited either directly onto the earth’s
surface as dry deposition or to be converted intoacids (sulphuric and nitric acid).
Clean rainwater has a PH value of between 4 and 5; the lowest ever recorded was 2.4. The effects of
acid rain include the following:
1. Increase in levels of water acidity, causing death of fish andplants life in rivers and lakes
2. Pollution of fresh water supply for human consumption and animal survival
3. Destruction of forests and important soil nutrient (calcium and potassium) are washed away to be
replaced by manganese and aluminum which are harmful to root growth.
4. Acid rain has been linked with a decline in human health as seen by the increasing incidences of
Alzheimer’s diseases (which may result from higher concentrations of aluminium), bronchitis and
lungs cancer

72
5. As soil becomes more acidic, crops yields are likely to fall
6. Chemical weathering may likely occur and this will erode buildings (Briggs and Smithson, 1985).
MEASUREMENT OF PRECIPITATION
INTRODUCTION
In Unit 1, you read about the mechanism of condensation, the twoconditions for the formation of
clouds and the types of precipitation.You also read about the two theories that explain the process
by which droplets grow large enough to fall as precipitation.
In this unit, you will be exposed to the ways to which precipitation is measured using the rainguage
as the instrument for measurement.
2.0 OBJECTIVES
• identify the instrument and explain how it is used in themeasurement of rain
• explain the methods use in measuring snow
• discuss the process involved in the measurement of hail
• discuss how fog-drip and dew fall will be measured
MAIN CONTENT
Measurement of Rain
In normal operation, the amount of rainfall is collected in a gauge and measured once a day. An

73
appropriately calibrated stick is used to measure the depth of water which has accumulated in
the gauge toobtain the quantity of rainfall. The gauge is to be sited away frombuildings and
trees and the amount collected does not only depend on the gauge positioned away from obstacles
but also on the diameter and height of the gauge above the ground.
In some instances, the rain water is collected and poured into a glass measuring cylinder, where the
rainfall equivalent can be read directly. Astandard rain gauge will only record the total rain which
has fallen between readings. In many cases, it is important to know when the rain fell and at what
intensity.
Measurement of Snow
Snow can be measured using different method. The water equivalent of snowfall can be obtained by
melting the snow which has accumulated in the gauge. This method is not accurate especially during
heavy snowfall when allowed gauge percent may be totally covered. A fall gauge maybe used to
prevents this happenings but the gauge may tend to underestimate the amount of snow reaching the
ground. Experiments have also been made to measure snow depth photogrammetrically, eitherwith
aerial or satellite photography. Where the snow fall is substantial, the depths can be obtained fairly
accurately, but without ground observations the water-equivalent of the snow is unknown.
Whichever approach is used, measurements of the water-equivalent of snowfall always presents
problems and probably inaccuracies.Apartfroma few areas of intensive observations, precise inputs
of water to the ground surface by snow cannot be known.
Measurement of Hail
Hail stones posses considerable kinetic energy and many will bounceout of a convention gauge,
causing underestimation of the total fall. The size distribution of hail stones can be obtained from a
hail pad which measures the degree of impaction made by the stones. If pads are left outfor known
times, the amount of ice and water-equivalent can be found. Fortunately, hail is normally
insignificant as a precipitation input to the hydrological cycle. So it is normally recorded separately
in terms of the number of days with hail.
Measurement of Fog-Drip and Dewfall
The water content of fog-drip and dew fall is small, therefore special measurement techniques have
to be used. Fog drip falls to the surface after contact with the leaves or trees so trough-shaped rain
gauges have been designed to increase the sampling area and make measurements more accurate. In

74
principle, they work like an ordinary gauge. (Briggs and Smithson, 1985). The most commonly used
instrument for dew fall
is an accurate weighing device. The dew drops collect on hygroscopic plates which are attached to a
balancing system to weight the amount of water collected. All methods suffer from the basic
uncertainty of how accurately the gauges collect dew compared to natural surfaces. Fortunately,
water quantities are minute so that even large errors are insignificant in relation to the total
precipitation input. (Blij, Muller, Williams, Conrad & Long, 2005).

75
VARIATIONS IN PRECIPITATION
INTRODUCTION
The circulation of the atmosphere is driven by the contrast in surface heating between the equator
and the poles. That contrast results from thedifference between incoming shortwave solar heating
and outgoing loss from the surface through various modes of energy transport including traditional
heat loss as well as heat loss through convection and latent heat release through evaporation. (Muller
& Williams, 2005).
It therefore stands to reason that climate change which in principleinvolves changing the balance
between incoming and outgoing eradicative loss via changes in the greenhouse effect is likely to
alter the circulation of the atmosphere itself, and thus, large-scale precipitation patterns. The
observed changes in precipitation patterns are far very variable and difficult to interpret than
temperature changes however.
Regional effects related to topography (e.g. mountain ranges that force air upward leading to wet
windward and dry leeward condition), ocean, atmosphere heating contrasts that drive regional
circulation patterns such as monsoons, etc, lead to very heterogenous patterns of changes in rainfall,
in comparison with the pattern of surface temperature changes. This unit discusses the variation in
precipitation and the causes of the variation.
Short Term Variability
The variations in rainfall overtime are of vital significance to hydrologists. Decisions about bridge
size, storm sewage construction, culvert dimensions and even flood protection measures must be
taken onthe basis of the expected inputs of precipitation e.g. knowing whatperiod of time or 25mm
of rainfall in a day may not be significant, but ifthe amount of rainfall in an hour, or even less, then
there should be drastic consequences. Surface run off may occur, soil erosion might be initiated,
streams might start to swell and flooding might result. Clearly the precipitation intensity is extremely
important.
The amount of rainfall per unit varies considerably. Heavy rainfall are normally seen to alternate
with relatively quite periods. All types of rainfall show these variations; there is no such thing as
steady rain. In convectional storms, the variation are often associated with the passage of the main
convection zones across the land where the up draughts are strong, the raindrops are held in the

76
cloud and prevented from falling,but as the up draughts weakens, the drops fall more easily to the
ground,giving periods of higher intensity. Considerable variation too is seen in cyclonic rain often
associated with temperature zone of instability in the cyclone.
Only when the source of precipitation is held stationary that is when we can get anything like the
steady rainfall. The most common situation is when moist air is forced to rise over a mountain
barrier. If the moist airis blowing over a sea at a constant speed, the air will be fairly uniform and
the conversion of vapour to water droplets will proceed at a constantrate. In such cases rainfall is
often prolonged and steady.
The short term variability of rainfall differs greatly from one area to another. It tends to be greatest in
the tropics. For instance Djakarta recorded an annual rainfall of 1800mm in only 360 hours on
average. Bycontrast, the average rainfall in London is only 600mm, yet this takes 500 hours to fall.
Variability in precipitation is often most important in the more arid areas of the world, for quite
small storms may be a rare
event. Channels that have been dry for months or even years may fill with water, and the baked clay
used to make houses may crumble and bewashed away. Within a matter of hours the rainfall may
have caused flood, the water almost vanished and within weeks the vegetation will have disappeared
again.
Seasonal Variability
There is a predictable and consistent cycle of rainfall during the courseof the year related to the
latitudinal migration of the wind and pressure systems. Precipitation associated with areas of
convergence and uplift tend to shift Polewards in summer and equator wards in winter, making areas
within the same pressure system throughout the year and so seasonal variations are subdued. This is
also true in the equatorial troughzone where rainfall can occur at any time throughout the year and in
deserts, where rainfall is almost negligible. The brief rare storms which do occur can come at any
time, so monthly rainfall, averaged over the long term shows little variation. Even within the same
pressure system, some seasonal pattern may be evident.
In the mid latitudes where rainfall is associated with the activity of the rain bearing cyclones, the
winter and autumn are relatively wet, for it is at these periods that the westerlies bring the most

77
intense storms. In the tropics and sub-tropics, where convectional rainfall is more important,
precipitation tends to be more abundant during summer months. The magnitude of these seasonal
variations is even more marked in themonsoonal areas of the world where the year can be subdivided
into wet and dry seasons.
Precipitation is more abundant during winter months in areas which experience the mid-latitude
depressions during winter only. TheMediterranean Sea area is the best known example of this type
of precipitation regime. In some areas where, the seasonal patterns may be more complex, there may
be more than one peak in rainfall totals as found in many areas of supposedly Mediterranean climate
and in tropics where seasonal migration of the equatorial trough produces two maxima.
Causes of Variations in Precipitation
Annual rainfall total varies from one part of the world to another, even when altitude allows for.
Some atmospheric factors are responsible for spatial variation in precipitation e.g. convectional
storms give high levels of spatial variation, while cyclonic rainfall is spatially much more uniform.
In the tropics, where a greater proportion of rainfall comesfrom convectional storms, the spatial
variation is particularly marked. It is clear that where the totals are very different, it is
because, mostrainfalls derived from individual cumulonimbus clouds produce intense precipitation
over an area of about 2 to 60km2
. The storms often buildup without any significant movement so
that areas just beyond the limitsof the cloud may receive no rainfall at all.
Sometimes the storms develop over a wider area; perhaps 500km2
, but even so, they do not give rain
everywhere. If rainfall were high for a particular period in one area, it would be below 100km. Over
the short term these differences might be considerable, but in the long term they are expected to
balance out.
Water balance
As far as humans are concerned, the crucial segment of the hydrological cycle occurs at the planetary
surface. Here at the interface between earthand atmosphere, evaporation and transpiration help plants
grow andprecipitation provides the water needed for that evapotranspiration and itis here at the
surface that we may measure the water balance. Thebalance of water at the earth’s surface can
be describe in terms ofsurplus (gain) and deficit (loss) using methods devised by climatologist

78
C. Warrant Thornthwaite.
Water can be gained at the surface by precipitation or more rarely by horizontal transport in rivers,
soil or groundwater. Water maybe lost by evapotranspiration or through runoff along or beneath the
ground. The water balance at a location is calculated by matching the gains from precipitation with
the loss through runoff and evapotranspiration. When actual evapotranspiration is used for the
computation, the balance is always zero because no more water can run off or evaporate than is
gained from precipitation. However, when potential evapotranspiration is taken into account, the
balance may range from a constant surplus of water at the earth’s surface to a continual deficit.
However, the range of water balance conditions may change when the potential evapotranspiration
exceeds the water gained in precipitation, the soil contains less water than it could hold. Because
plants depend on water, the vegetation in such location may appear sparse except where irrigation is
possible.
The above situation is reversed where rainfalls sometimes as much as 45cm in a single month may
exceed the amount of water than can be lostthrough evapotranspiration. The surplus water provides
all that is needed for luxuriant vegetation and still leaves copious quantities to run off the land
surface.
The amount of runoff in any location cannot exceed the amount of precipitation, and usually there is
much less run off than precipitation. This is because some water almost always evaporates and/or
infiltrates the soil.
SEASONAL VARIATIONS IN
TEMPERATURE, DAY LENGTH, RADIATION, RAINFALL AND
EVAPOTRANSPIRATION
SEASONAL VARIATIONS IN CLIMATIC ELEMENTS OF TEMPERATURE, DAY LENGTH,
RADIATION, RAINFALL ANDEVAPOTRANSPIRATION
INTRODUCTION
A season is a division of the year marked by changes in weather as a result of the yearly orbit of the
earth around the sun and the tilt of the earth’s rational axis. The seasonal variations usually pose
some impact on:

79
i. Temperature
ii. Day length
iii. Radiation
iv. Rainfall and
v. Evapotranspiration
The main content of this unit discusses the listed elements in terms of the meaning of each
element, seasonal variation and factors responsible for such variations.
Variation in Temperature
Meaning
Temperature is the degree of hotness or coldness of a place. Temperature varies with changes in
seasons. The different locations on the globe experience temperature variations in length of hours,
intensity and duration.
Causes
The earth’s revolutions round the sun in every 365/6 days (1 year) are mark by changes in
temperature. The tilting of the earth at an angle of 23½o
causes the earth’s orientation to change,
continually as the planet revolves about the sun.
Factors
Temperature can be affected by time of the year, cloud cover, latitude, nature of the surface cover
and altitude. On latitude 23½o
N (tropic of Cancer), temperature is generally higher in the month of
June as it is marked by summer solstice. The sun is overhead at the tropic as solar insolation is
longer. However, temperature is higher during the day under a clear atmosphere than a cloudy
atmosphere, on a bare ground than on snow covered land and when the ground is dry than on wet,
ground.
On mountain tops temperature is lower than on flat surface. This is because solar radiation escapes to
space faster on mountain tops and temperature decreases with altitude. (Houze, 2001)
On latitude 23½o
S (tropic of Capricorn) in the southern hemisphere on December 21st
the sun is

80
overhead marking summer solstices. The duration of day lengths are longer hence intense solar
insolation isrecorded. (Evans, 1999).
Temperature in the higher latitudes 66½o
N and S which is characterized by four distinct seasons,
winter, summer, spring and autumn is marked by three months variations. Temperature varies within
the four different seasons.
Variation in Day Length
Meaning
Day lengths are the duration of the hours of day light. This varies with seasonal changes.
Location
The tropics are marked by variation in day length as the season changes. The summers in the two
tropics (Cancer in the northern hemisphere and Capricorn in the southern hemisphere) are
characterised by longer days. The length of days increase continually to the higher altitude (Lat.
66½o
N and S). Summer solstice in the northern hemisphere is in June 21when the sun is overhead at
latitude 23½o
N. While the summer solstice in the southern hemisphere is in December 21 when the
sun is overhead on Latitude 23½o
S.
Winters are characterized in both hemispheres by shorter length of days and longer nights. During
winter, the length of nights become longer with increase in latitudes to 66½o
N and S. December 21
in the north and
June 21 winter solstice in the southern hemisphere. The equator which isthe lower latitude (Lat. 0o
)
is marked by equal length of days and night twice a year that is March 21 (spring equinox) and
September 23 (autumn equinox). The length of days is calculated as a function of latitude and
declination angle.
Variation in Radiation
Meaning
The intensity of solar insolation that the earth receives. The intensity of the solar radiation is
inversely proportionate to the square of the earth- to-earth distance. Solar radiation receives on the
earth’s surface varies with seasons. More solar energy are accumulated during the longer days of
summer than shorter days of winter.

81
Location
The solar radiation reaches its pole when the sun is overhead directly at noun between latitude
23½o
N & S. On June 21 northern hemisphere and December 21 in the southern hemisphere. The
earth receives about 6.7%more radiation at perihelion. The total energy the earth receives from the
sun luminosity is 3.827x1026
watts. Winters are marked by less solar radiation in both hemispheres.
Causes
Differences in solar radiation can be caused by annual variation in the angle of the sun’s rays
(cloudiness of the atmosphere) which affects the rays that pass through the thick cloud and the nature
of the surface as higher insolation is received on the flat surface than on highlands (altitude). In the
temperate and polar regions seasons are marked by changes in intensity of sunlight that reaches
the earth surface. As aresult, the regions have four calendar seasons – spring, summer, autumn and
winter. Pyranometer is the instrument for measuring the intensity of solar radiation striking the
horizontal surface.
Variation in Rainfall
Meaning
Rainfall is the condensed water vapours. It is the product of watervapours that enter the atmosphere
through the surface evapotranspiration. Seasonal precipitation patterns are strongly influenced by
seasonal variations in quassi-stationary pressure system, regional convergence zones and
monsoonal circulations.
Location
Tropical precipitation is highest during the summer and lower during thewinter. The annual rainfalls
in the tropics vary from zero to 10000mm. The wettest regions of the tropics are the maritime
content, inter tropicalconvergent zone (ITCZ). Precipitation over the equator has the largest annual
range. Stations close to the equator have small seasonal variations.
Characteristics
The amount and distribution of rainfall is characterized by four climates:
i. Rainy climate

82
ii. Seasonal and monsoon climates
iii. Dry climates
iv. Tropical desert
Factors
The annual amount of rainfall is affected by;
a. relief of the area (topography)
b. amount of cloud condensation nuclei
c. trade winds
d. latitudes
e. nature of cloud
f. vegetation cover
Variation in evapotranspiration
Meaning
Evaporation is the release of water from the surfaces of water and landto form the atmospheric
vapour. While transpiration is the release of water vapour through the stomata in leaves of plants
from therefore, evapotranspiration means the release of water from water surfaces and water vapour
from stomata in leaves of plants respectively into theatmosphere. Evapotranspiration is an important
part of water cycle. Evaporation accounts for the amount of air from soil, canopy interception, and
water bodies while transpiration accounts for the amount of water within the plants and the
subsequent loss of water as vapour through the stomata of plants.
Location
Evaporation is less over land than ocean. Its distribution plays a vitalrole in the initiation and
evaluation of convective weather system. Regions between the equator latitude 0o
to let 30o
N or S
have much higher evaporation rates than the higher latitudes.
The tropical forested regions are significant sources of water vapour.The tropics drive the global
energy and water cycle since the oceans receive most of the surplus radiative heating.
Evaporation is low along the equator as solar heating is at its maximum and deep convective cloud
reduces the amount of solar radiation. The low wind speed in the equatorial ocean reduces the
evaporation rates.

83
The highest evaporation rate occurs along the western side of the sub- tropical oceans during the
winter when cold, dry continental air flows over warmer ocean currents. Evaporation is increased by
wind speed, inflow areas of hurricane and storm.
Factors
Factors that affect evapo-transpiration are;
a. plant growth and type
b. soil cover
c. wind
d. solar radiation
e. humidity
Evapotranspiration rate is relatively low where the surface to atmosphere moisture gradient is weak
and relatively high where the gradient is strong.
Instrument
Evaporation pans and lysineaters are two methods used to measure the potential
evapotranspiration. (Houze, 2001, Yu, 2008).

84
EQUIPMENT AND MAINTENANCE OFSTANDARD METEOROLOGICAL STATIONS
EQUIPMENT IN A METEOROLOGICALSTATION
1.0 INTRODUCTION
This unit will describe equipment found in a meteorological station and how they are used for
measuring weather elements. These elements include; rainfall, temperature, wind, pressure,
humidity and sunshine.
Meteorological Station
Meteorological station is a facility located either on land or sea, with instruments and equipment for
measuring atmospheric conditions, whereobservations are undertaken on surface weather conditions.
Themeasurements taken include temperature, pressure, humidity, wind speed, wind direction and
precipitation amount. Wind measurements are taken with as much obstructions as possible, while
temperature andhumidity measurements are kept from direct solar radiation or insolation.
Observations are taken at least once daily (manually) whole automated measurements are taken at
least once an hour. (Blij, Muller, Williams, Comrad and Long, 2005).
Instruments in a Meteorological Station
A typical meteorological station has the following instruments: thermometers, barometers,
hygrometers, anemometers, wind vane, rain gauge and sunshine recorder.
Thermometer
This is a narrow glass tube containing mercury of alcohol. Thermometer is used for measuring
temperature either at the air or sea surfaces. It is usually graduated in degree centigrade (o
C) or
Fahrenheit (o
F).Thermometer measures or records temperature at its peak (highest attained during
the day). This type of thermometer is called maximum thermometer while the minimum
thermometer measures or records the lowest temperature attained during the day. Both maximum
and minimum thermometers are jointed together in a u-shape and both are read at different times of
the day. Thermometers are kept in a wooden cabinet called “Stevenson screen” to protect them
thermometer from the effect of sun and rain in order to get accurate readings.

85
Barometer
This is used for measuring atmospheric pressure. Barometer is a glass tube containing mercury of
alcohol connected to a container at its base. The mercury in the container supports a column of
mercury about
760mm high or below depending on the condition at that particular time with a vacuum of air.
Hygrometer
This consists of wet and dry bulbs thermometers usually used for measuring humidity, either through
transpiration, evaporation or evapotranspiration.
Anemometer
Is a cup-like instrument used for measuring wind speed.
Wind Vane
This is an instrument with a framework of four cardinal points connected with a pole and an
indicator (arrow) used for measuring wind direction.
Rain Gauge
Is a metal container with a metal jar or glass bottle and metal funnel usually sunk into the ground at
least 30cm above the ground level. Rain gauge is used for measuring liquid precipitation over a
set period oftime.
Sunshine Recorder
This is an instrument used for measuring sunshine.
In addition, at certain automated airport weather station, additional instruments may be employed,
these include; present weather/precipitation identification sensor for identifying falling precipitation.

86
For instance, Disdrometer is used for measuring drip size distribution; transmitter is used for
measuring visibility; ceilometers for measuring cloud ceiling. More sophisticated stations may also
measure the ultraviolet index, solar radiation, leaf wetness, soil moisture, soil temperature, water
temperature in ponds, lakes, creeks or rivers and occasionally other data as described in Wikipedia,
2015. Except for those instruments requiring direct exposure to the elements (anemometer, rain
guage, wind vane), the instruments should be sheltered in a vented box, usually a Stevenson screen,
to keep direct sunlight off the thermometer and wind off the hygrometer. The instrumentation may
be specialized to allow for periodic recoding otherwise significant manual labor is required for
record keeping. Automatic transmission of data in a format like METAR, is also desirable as
many weather station’s data is required for weather forecasting (Waugh, 2000)
UNIT SIX LAYOUT OF A METEOROLOGICAL STATIONCONTENTS
1.0 INTRODUCTION
This unit will examine the layout of a standard meteorological station and how equipments used for
measuring weather elements are positioned for proper functioning without obstruction for adequate
utilization.
Positioning of Instruments
Generally, a meteorological station must be placed in a location whereno shading can occur. It is
important to remember that shade patterns vary with the season due to changes in earth-sun
geometry. It is therefore best to place the station well away from large obstacles if possible. An
open location is also necessary to measure wind speed and where the station is hidden from view
behind an out-building or a solid wall will not accurately represent the wind speed over more open
areas such as fairways or sports fields. Weather stations should be isolated from large obstacles
such as fences, trees or buildings by a distanceequal to 7-10cm times the height of the obstacle.
Using this rule, one should place a station 70-100m away from a 10m high building toensure proper
wind flow at the site. The terrain surrounding the weather
station should be relatively level if possible. The ideal situation wouldbe to centrally locate the
station in a large, well-watered turf area that is a considerable distance from objects that might
disrupt wind flow or shade the station.

87
Except for those instruments requiring direct exposure to the elements (anemometer, rain gauge,
wind vane and sunshine recorder), the instruments should be sheltered in a vented box, usually a
Stevenson screen, to keep direct sunlight off the thermometer and wind off the hygrometer. The
instrumentation may be specialized to allow for periodic recordings otherwise significant manual
labour is required for record keeping. Space is generally required within the station to permit free
movement. A normal station should have the Stevenson screenlocated at 1.5m away from the wire
gauge used as fence or boundary of the station. Same measurement should equally be used for
positioning wind vane and anemometer. Rain gauge should be placed at 1.2m away from the
boundary. A 2m pillar should be used to install or mount the sunshine recorder at a of evaporation
should be placed close to wind vane since d height has no significant impact in obstructing wind
flow.
Advantages of Good Positioning
To determine atmospheric conditions, weather elements are instrumentally observed and to achieve
desired results it depends greatlyon the positioning of the instruments in the meteorological station.
Understanding the layout of a standard meteorological station is essential if the weather station is to
provide the data necessary to estimate weather condition in a consistent and reliable manner. The
following are advantages of good positioning of instrument in a meteorological station.
1. The positioning of thermometer in a Stevenson screen is to protect the thermometer against the
effects of sunshine and rainso as to get accurate temperature of the day.
2. Keeping rain gauge in an open space free from trees, grasses and buildings enables the rain gauge to
collect rain water directly and to ensure that no drops from roof or trees enter the funnel afterthe
rain has stopped. When this is done, records are not overestimated but accurately done.
3. Accurate measurement of pressure and humidity are obtainedwhen barometer and hygrometer are
properly positioned thereby providing and obtaining accurate and reliable information concerning
weather conditions.
4. Reliable data is obtained on wind direction and speed.
5. All activities conducted in the station yield results with complete absence or minimum errors.
Taking Records of Atmospheric Condition

88
1. Thermometer
This is used for measuring temperature. Records are taken on daily basis in degree centigrade or
Fahrenheit. Thermometers are often read atdifferent times of the day to find out the maximum and
minimum temperatures of the day. Temperature is often designated on maps by a line drawn to join
places having the same amount of temperature known as isotherm. The freezing or cooling point for
centigrade scale is 00
and 320
for Fahrenheit scale. The boiling point for centigrade is 1000
and 2120
for Fahrenheit. There are two types of thermometer namely; maximum thermometer which records
the highest temperature attained during the day and minimum thermometer which records the lowest
temperature during the day.
2. Rain Gauge
This is used for measuring rainfall. When taking records of rainfall, the rain gauge must be examined
every day and daily records taken. The instrument should be sunk into the ground such that 30cm of
it is above
the ground level and should be held firm in position. A line used on mapto connect two places of
equal average annual rainfall is known asisohyet.
3. Barometer
This is an instrument used for measuring the atmospheric pressure of an area. For the barometer to
work well, Place the barometer and scale in a shaded location free from temperature changes (i.e.
not near a windowas sunlight will adversely affect the barometer's results). In your notebook or the
table, record the current date, time, the weather conditions, and air pressure (i.e. the level where the
end of the straw measures on the scale). Continue checking the barometer twice a day (if possible)
each day over a period of several weeks.
4. Hygrometer
This is an instrument used for measuring the humidity of an area at a given time. Some hygrometers
have internal data logging. In other cases they are read using a computer (by connection, or even
wirelessly). Otherwise, records depend on the person reading and writing downresults.
Always record the humidity value and units. For relative humidity measurements, temperature is
usually essential. Pressure must be knownfor psychrometers, and sometimes for other cases such as
measurements in compressed air lines especially if planning to convert to equivalent at atmospheric
pressure. (Stephanie 2011)

89
As with all measurements, it is also good practice to record the date, time, place, method, operator,
and anything else that allows the measurement to be understood later. Measuring humidity correctly
takessome skill and judgment.
5. Anemometer
We use an instrument called an Anemometer to measure wind speed.The cup anemometer is the
simplest type. It consists of four hemispherical cups mounted on the end of four horizontal arms. The
speed at which the cups rotate is proportional to the speed of the wind.
Therefore, by counting the number of turns over a set time, we can workout the average wind speed.
Place the anemometer outside to see if the wind will spin it around. Using the watch, count the
number of times the marked cup spinsaround in one minute. Repeat this every day for a month.
Record thedata on the notepad or work book.
Choose one month in winter and one in summer to show differences. After a month of recording,
draw a graph to represent the data. Plot the days along the horizontal axis and the wind speeds (turns
per minute) along the vertical axis and Join the dots. The experiment in different locations to record
and compare the wind speed, will surely be different.Try to explain why there might be variations.
6. Wind Vane
Wind vane is used for measuring wind direction. It has the four cardinal points mounted on a pole.
When recording the direction of wind, places the instrument outside and position the instrument far
from obstacles such as buildings and trees. Observe the changing direction of the cardinal points at
intervals of an hour or two then record the observation including the date and time the observations
were made.
7. Sunshine Recorder
This is an instrument used for measuring sunshine. Sunshine recorder essentially consists of a glass
sphere mounted in a spherical bowl and a metallic groove which holds a record card. Sun's rays are
refracted and focused sharply on the record card beneath the glass sphere, leaving burnt marks on the
card. As the sun traverses, continuous burnt marks will appear on the card. Observers can measure
the sunshine duration based on the length of the burnt marks (according to LAM Hok-yin (2013)).
To obtain uniform results for observation of sunshine duration with a sunshine recorder, the
following points should be noted when reading records:
(a) If the burn trace is distinct and rounded at the ends, subtract half of the curvature radius of the trace’s
ends from the trace length at both ends. Usually, this is equivalent to subtracting 0.1 hoursfrom

90
the length of each burn trace.
(b) If the burn trace has a circular form, take the radius as its length. If there are multiple circular burns,
count two or three as a sunshine duration of 0.1 hours, and four, five or six as 0.2 hours. Count
sunshine duration this way in increments of 0.1 hours.
(c) If the burn trace is narrow, or if the recording card is only slightly discolored, measure its entire
length.
(d) If a distinct burn trace diminishes in width by a third or more, subtract 0.1 hours from the entire
length for each place of diminishing width. However, the subtraction should not exceed half the total
length of the burn trace.
MAINTENANCE OF A METEOROLOGICAL STATION
INTRODUCTION
For effective utilization and adequate results from the instruments positioned in a weather station,
there is need for proper maintenance of the station. This unit will explain the maintenance of a
standard meteorological station with focus on the maintenance of equipment, local chores and
technical maintenance.
• Maintenance of Different Instrument
A meteorological station like any other piece of equipment, requires regular maintenance if it is to
perform its assigned function correctly. These maintenance chores should be performed by local
grounds maintenance personnel’s since they access the station on a regular basis. Other maintenance
work should be performed by trained meteorological technicians. Meteorological station is designed
to monitor fire meteorological parameters; solar radiation, wind, temperature, humidity, and
precipitation. (Wikipedia, 2016).
• Maintenance of Thermometer and Hygrometer
These should be housed in a naturally ventilated radiation shield thatwill prevent direct sunlight
from reaching them. This is because a platinum resistance thermometer sensor exhibit a change in

91
electrical resistance in response to changes in temperature and humidity. (It is measured with sensors
that generate a change in electrical resistance or capacitance with changes in humidity).
• Maintenance of Windvane and Anemometer
The main problem that could arise from using windvane and anemometer is a guiding sound which
may result in poor rotation at low speed. This is caused by poor bearings and should be replaced as
soon aspossible by a trained technician.
Maintenance of Raingauge
1. Keep the gauge on a ground level and the collection funnelconstantly clean
2. Wipe out dirt and debris from the tipping bucket mechanism ifrequired.
Maintenance of Power Supply
Power failure causes loss of data. A solar panel may provide power to weather stations located
away from a reliable source of air condition
A.C. power. Dust and debris should be removed weekly to maintain proper output from the panel.
Bird droppings should be removed as soonas possible to ensure optimal panel performance and also
charging circuit can be repaired by trained technicians in the case of batteryusage.
Mechanical Maintenance of a Standard MeteorologicalStation
Technical maintenance should be carried out whenever routine maintenance reveals a problem.
Therefore, it is suggested that there should be a technical representative to check the system once
every year
even when there is no problem observed during the routine local maintenance. Ensure that the
following are regularly done.
1. Anemometer bearings should be replaced every 12 months to ensure proper measurement of wind
speed.
2. Thermometer, hygrometer in an automated station and raingauge should be checked and examined
regularly by the trained technical representative. Temperature and humidity sensors should be
replaced every 24 months.

92
Technical Maintenance
This is best performed by a trained representative of the company that supplies the instruments.
Technical maintenance is an essential aspect ofoperating a meteorological station, and turf facilities
should be wary of suppliers that do not provide both telephone and on-site technical assistance. If the
supplier does not provide on-site technical service, theyshould be able to train a third party who
can. Doing this will improve the longevity and performance of instruments found in the
meteorological station (National weather services, 2015).

93
THE TROPICAL CLIMATE
1.0 INTRODUCTION
The tropics have been described as the Firefox of the atmospheric engine. Most of the solar radiation
is absorbed in the tropics and energy transferred into the cooler, energy-poor latitudes. Thus, transfer
is brought about by wind systems and ocean currents. Simple approach to climate in the tropics is
distinguished in four main areas;
a) the equatorial trough zone (inter-tropical convergencezone)
b) the sub-tropical highs
c) the trade wind areas
d) the monsoons and tropical cyclones
2.0 OBJECTIVES
• identify the four main climatic zones (areas) in the tropics
• state the characteristics of the equatorial trough
• describe the features of the sub-tropical highs
• differentiate between trade wind and the monsoon.
MAIN CONTENT
Equatorial Trough
It is the equatorial trough area that most closely meets people’s idea of atropical climate. During the
day, clouds build up massive cumulonimbusdisplays. Rainfall is frequent and abundant, temperature

94
and humidity acting together, resulting to tropical rainforests. The tropical rainforest climate is
located within 5o
N&S of the equator. This area lies within the Amazon basin in South America,
Zaire (Central Africa) and the coast of West Africa. The climate is constantly warm with temperature
of 26o
C and no winter, recording >18o
C as the coldest month. The rainfall distribution is relatively
high approximately 2,400mm and there is no month with rainfall less than 250mm. Double
maximum rainfall pattern is experienced in the area with great intensity often accompanied by
thunder storm and lightning.
The equatorial trough has many different forms of climatic conditions. Itrepresents the area of low
pressure somewhere near the equator towards which the trade winds blow. The precise form it takes
will dependsignificantly upon the stability of the trades, their moisture content and the degree of
convergence and uplift. For instance, the Brazilian Amazonia records a mean monthly temperature
variation by 2.8o
C over the year and a mean monthly minimum by only 0.6o
C. Extremes are rareand
insignificant by temperate latitude standards. Mean annual rainfall is high with 1811mm, though
even in this zone there is a drier seasonwhen rain days are fewer. This is applicable to most of
the equatorial through zone, though variation exists in terms of intensity and duration of the dry
season. Only few areas have no dry season. For instance in Indonesia, Padang in Sumatra, an area
located 7m above sea-level receives an average rainfall of 4427mm, only one month has less than
250mm. The driest season occur when the trough move pole wards in response to continental heating
in the summer hemisphere. As one moves further away from the equatorial trough zone, the dry
season lengthens reaching the monsoon or trade wind areas (Briggs and Smithson, 1985).
The multitude of names which have been used for the area gives some idea of its variety; the
doldrums, intertropical front, intertropical convergence zone, intertropical trough, equatorial trough
or intertropicalconfluence zone. For simplicity all are referred to as equatorial trough although it
does extent towards the sub-tropics, and it is quite variable in features (Blij, Muller, Williams,
Conrad and Long, 2005).
Sub-Tropical Highs
This pressure zone acts as the meteorological boundary between the tropical and temperate latitudes.
The dominant air movement is usually away from the highs; the circulation is maintained by the
subsiding air from the Hadley cell. Because the air is subsiding, it tends to be warm and dry. An
inversion develops in the lower atmosphere and so these sub-tropical highs are generally cloud-free
and deficient in rain. Where the highs remain fairly constant in position, the main desert areas of the
world are found; Sahara, Kalahari, and the great Australian Desert.
Within this system, there is often low pressure area which result from intense heating of the ground

95
during the cloudless days, taking temperatures above 40o
C in summer. The air becomes less dense
and thermal lows form. They tend to be fairly shallow and are replaced by high pressure by the
850mb level.
Climate of this zone can be characteristerised by little rain and extremes of temperature. In mid-
summer, the mean maximum temperatures are 42o
C but in winter the minimum temperatures are
only 8o
C and frost canoccur occasionally. The dry atmosphere helps by allowing long wave radiation
from the ground to escape to space with little counter – radiation from water vapour or clouds.
Precipitation is negligible, rainfalls is experienced for about 10 days per year giving a total of about
75mm. Most of it falls in winter and spring when temperate latitude depressions extend their effects
far south and do give occasional rain (Evans, 1999).
In some of the sub-tropical high pressure belts, additional factors reduce the likelihood of rain. On
the west coast of Sahara, Kalahari and Atacama deserts cold ocean currents flow offshore, prevailing
wind and mountain barrier. They cool the air and make it even more stable. Mist and fog may be
frequent but rain is rare. The result of these factors acting against the mechanisms of rainfall
generation is produced on the driest parts of the earth, as seen in Africa and Atacama desert of Chile.
Trade Winds
Trade winds blow away from the sub-tropical anticyclones of each hemisphere; north easterlies in
the northern hemisphere and south easterlies in the southern hemisphere. The trade winds of the
world can be some of the most constant and reliable winds of the world.
Around the tropics, the trade winds are very stable, being affected by subsidence so the moist-layer
near the surface is thin. The sudden rise oftemperature and drying of the air at about 900mls pressure
surface is called Trade wind inversion. In the north east of the Atlantic and pacific

96
oceans, it may be only a few hundred metres above the ground, effectively preventing rainfall over
the oceans. When Canary Islands, rise through the inversion, the lower windward slopes may be
moist due to cloud and some rain, but above the mean level of the inversion and on the leeward
slopes, desert are formed.
Trade winds gradually pick up moisture as they blow away from their source areas, the anticyclone
normally noticed in the shape of the trade winds cumulus clouds. They are visible signs that moisture
is being evaporated from the seas and partly condensing as clouds. With more moisture being added
and the influence of the anticyclones weakening, the intensity of the trade winds inversion weakens
and its gets higher. Rainfall is likely to occur as seen on the western side of the Atlantic Ocean with
much moist climate, although with a distinct wet and dry season.
Monsoons and Tropical Cyclones
In some parts of the world, the wind system appears to experience a seasonal reversal; they may
be blowing from the south-west in oneseason; in the other season they are from the north-east. A
large area of the tropics is affected by seasonal reversal in areas where trade windsare dominant.
Seasonal reversal is linked to the position of the continents in the northern hemisphere. During
summer in the northern hemisphere, surface heating of the continental landmasses is intense. A
shallowsurface low pressure centre forms over the Sahara, India and Central Asia. The equatorial
trough moves northwards allowing an inflowing of moist south-west to give the wet season in west
Africa, India and some parts of Asia. In winter, the continents cool down, high pressure becomes
established at the surface and winds between north-east and north predominate. This is the cool dry
season for the monsoon areas of the northern hemisphere.
In the southern hemisphere, land masses are smaller and only Australia develops the semblance of a
monsoon, though its influence does not extend very far inland. Over East Africa, set aside the
equator, a seasonal reversal occurs, but the winds tend to be blowing parallel to the coast. As a
result, the rainy season is between the main monsoon flows, rather than during one of them as in
most of the other regions. The monsoon climates is characterized by 1,500mm annual rainfall

97
distribution concentrated during the wet or rainy season, temperature is high about 27o
C with an
average of 5o
C. The climate is associated with alternating wet and dry seasons. Monsoons are
usually found 10o
and 35o
N&S of the equator.
The major disturbance of tropical latitude is the cyclone. Its main features affect regional distribution
of climates. It is apparent that cyclones only develop over warmer parts of the ocean, in each
hemisphere; cyclones are most likely to strike in the summer and autumn. Along the Atlantic and
Gulf coasts of the USA, the normal hurricane season is from June to November. Early in the season,
storms develop in the Gulf of Mexico with progressive eastward movement from their starting points
until September when they may reach as far east as the Cape Verde Islands of West Africa. There
may be a shiftback towards the Gulf of Mexico after September (Opeke, 2005).
The zone of recurvature may be affected by another seasonal change. Most storms initially move
westwards but at some stage may begin a curving track towards north and then north-east. The
average latitudes of recurvature is at its northernmost position in August and furthest South in
November. Many storms could reach hurricane intensity and decay without being recorded by the
global observing network. Pacific Ocean has the most hurricanes but the fixtures are difficult to
compare.
The mean rainfall of areas affected by tropical cyclones in summer and autumn reflect the vast
amounts of water which a hurricane can release. Not all tropical areas are affected by tropical storms
or easterly waves. However, other less organized disturbances give appreciable precipitation. For
instance in Mozambique and parts of Brazil. A few areas miss major disturbances altogether,
anomalous dry zones occur in north-east Brazil where annual rainfall of less than 500mm is
found.Less than 250mm as mean annual precipitation is experienced in Somalia and aridity prevails,
though the area is only just north of the equator.
RELATIONSHIP BETWEEN AGRICULTURE AND CLIMATE WITH REFERENCE TO
CROPS, LIVESTOCK, IRRIGATION, PEST AND DISEASE
INTRODUCTION
Climate is the most important of the factors which determine the type of agricultural practices of the
world. The type of climate and fluctuations in climate from time to time and year to year may cause
radical changes in species composition, disease spread and type of agriculture practiced.
No doubt, climate plays a significant role to agriculture. Most or virtually all agricultural processes
depend to a greater degree on climatic factors especially rainfall, temperature, wind systems,

98
humidity and sunlight.
This unit explains the identified relevant concepts in agriculture, stating their relationships to
different climatic conditions with reference to crops, livestock, irrigation, pest and disease.
Concepts Related to Agriculture
Agriculture and Climate in Focus
Agriculture is the science, art or occupation concerned with the cultivation of land for crop
production and rearing of livestock of various types. In its broaden sense, it is the cultivation of
crops, raising and breeding of livestock, processing, storage, distribution and marketing of
agricultural products. On the other hand, weather is defined as the atmospheric condition of a place
over a short period of time while climate is the average weather condition of a place measured over a
long period of time, relatively 35 years. A branch of meteorology which studies weather condition as
it affects agriculture is identified as agrometeorology. This discipline studies the meteorological,
climaticand hydrological conditions as they relate to agricultural production. It is closely allied to
biology, soil science, geography and the agricultural science. (Cayan, Maurer, et al, 2008)
Climate affects the distribution of crops as it determines the type of cropproduced in a season and the
location suitable for such production. For instance, tree crops thrive better under moist soil, rich in
organic matter. This can be obtained in areas of high temperature and rainfall distribution. The
leaves of plants around form the greater proportion of organic matter used by plants or crops. Tree
crops and tuber crops are likely found in areas of sufficient rainfall and temperature. Such areas
often have rich soil for crop production.
On the other hand, the vegetation of an area is determined by the climateof such area. Where the
temperature is high and rainfall distribution is equally adequate, it is believed to have a luxuriant
vegetation and gallery of forests with tall grasses and shrubs. But, when the climatic condition tends
to be different with the stated condition above, grasslandvegetation is likely to be formed. A typical
example of grassland vegetation is seen in the northern part of Nigeria. Such vegetation only
supports animal rearing in vast proportion and production of cereal crops (mostly grains). The
grasses serve as feeds for cattle, sheep and goat. Changes in weather leads to the occurrence of
seasons, leading to loss of available feeds for livestock due to shortage of water from the ultimate
source (precipitation) through withering of plant leaves. This makes farmers from the north where
such practices are dominant to the south in search of pasture. During the wet season, these herdsmen
move their cattle northwards while southward in the dry season. This systemof movement with

99
changes in season is known as pastoral nomadism.
Also the movement can be up and down the plateau because valleys are then infested with tsetseflies
during the wet season. In the dry season when the valley is free from tsetseflies and the plateau is
dry, they move their flocks down the valley. This system of moving livestock up and down the
highland by the herdsmen is known as transhumance.
Crops and Livestock
Crops refer to the yield or cultivated produce of the land while growing or gathered by the farmer
while livestock are animals such as goats, cattle, sheep and other useful animals raised by farmers
for either personal or commercial purpose. Crops produced by farmers can be annual (rice, maize,
groundnut, soybean, etc) or biannual (ginger, etc) or perennial (mangoes, pineapple, pea, banana,
etc).
Pests and diseases often attack and effect changes to both crops and livestock in areas where
temperature is high and rainfall equally is equally high, frost are to emerge. Such areas are not
conducive for livestock breeding as tsetseflies are predominantly habitat in such areas. The tsetse fly
bite causes infection to animals especially cattle resulting in trypanosomiasis which is also injurious
to man. This serve as indicator as to why domestic animals are not present in the south asmuch
as in the northern part of Nigeria. The northern part is devoid of tsetseflies thereby making it safe for
the animals to survive. (Cayan, Maurer, et al, 2008).
Irrigation
Irrigation is often regarded as dry season farming or farming that takes place in areas that experience
deficit in rainfall distribution. It is the artificial application of water to land in order to improve the
moisture content of soil and meet up with plants or crop water need. Climateplays a significant
role in these operations. When the climatic condition is favourable i.e. temperature and rainfall are
adequate throughout the year. Without deficit in rainfall, there will be no need for irrigation as
farmers will cultivate under natural supply of rainfall and improved soil moisture condition. The
changing nature of weather as experienced in the tropical region led to irrigation. During dry
season, rainfall is absent,making it difficult to cultivate the land and plant seeds. During wet season,
constant increase in the rate of evapotranspiration may force farmers to utilize streams, rivers,
ponds, wells and other sources ofwater to produce crops in order to meet up with food demand.
Pest and Disease: Spread and Control

100
A pest is an insect or any other organism that harms or destroys garden plants. The presence of pests
could result to pathogens that cause continuous irritation in plants. Disease on the other hand, means
a malfunctioning process caused by continuous irritation. Plants develop diseases when they catch
the pathogens. Plant diseases can be attributed to several factors in the environment which could be
physical, chemical or biological. Diseases caused by biotic agents such as viruses, bacteria, fungi,
nematodes and parasitic flowering plants are called infectious diseases while those caused by
physical or chemical factors such as air pollutants, water, frost, nutrition, etc are called non-
infectious diseases. Unlike the non-infectious diseases, the infectious diseases show a sigmoid curve.
This means that they first increase as the biotic agent reproduces and later diminish on the non-
availability of hosts.
It is estimated that between 10% and 16% of the world’s crops are lostto disease outbreaks. The
spread of pests and diseases could be traced to climate change. It is believed that global trade in
crops is mainly responsible for the spread of pests and pathogens from country to country. Increase
in temperature contributes to a pole ward movement ormigration of many organisms and results in
higher rate of growth and reproduction in insect herbivores. Cold winter temperature has helped to
keep pest and disease life cycles at a minimum and that wise delay the growth and dispersal of pest
organisms. Crop diseases are often spread through an insect vector. This can be achieved by wind
dispersal either through spores carried by wind or an increase in severe weather event such as
hurricanes.
A significant number of measures can be advanced to address the aforementioned challenge. This
includes a combination of farming strategies, biological control agents and appropriate pesticide and
herbicide using a variety of methods. (Blijand Smithson, 1985;Oluwafemi, 1998).
RELATIONSHIP BETWEEN CLIMATE ANDCROP DISTRIBUTION
1
.
0
INTRODUCTION
Crops make up a greater proportion of food consumed by humans. It is evident that the production and
distribution of crops depend on climatic factors which to a greater degree influence the distribution of crops
on the basis of location suitable for them. This unit explains the relationship between climate and crop
distribution with focus on the effects of rainfall, temperature, sunlight, wind and humidity on crop growth and

101
distribution.
Effects of Rainfall on Crop Growth and Distribution
Rainfall distribution varies according to seasons and the lengths of seasons differ according to
latitudinal location. This determines the growing seasons and nature of crop production.
The amount of rainfall determines the growth and distribution of crops. This is because some crops
require high rainfall for greater productivity
e.g. root and tuber crops, while others require little or moderate amount of rainfall supply across
different regions e.g. cereal and legumes. Rainfall decomposes organic matter; increases soil fertility
and dissolve minerals in soil for plant use. (Oluwafemi, 1998).
Temperature Wind, Humidity and Sunlight as Factors inCrop Growth and Distribution
Temperature as a Factor in Crop Growth and Distribution
Temperature variation has great influence on the growth and distributionof crops.Some crops require
high temperature while others require cold temperature to grow favourably. Temperature contributes
greatly in the decay of organic matter which in turn improves soil organic content.
Wind as a Factor in Crop Growth and Distribution
The variations in wind speed and direction play an important role in the growth and distribution of
crops in different regions or areas. Wind prevents frost by disrupting a temperature invasion at
different locations, movement of pollen grains to ensure fertilization and distribution of energy
during photosynthesis by bringing carbon dioxide for plants utilization and oxygen for animal use.
Humidity as a Factor in Crop Growth and Distribution
Humidity plays a significant role in crop growth and production; it strongly determines the crop
grown in a region. Internal water potentials, transpiration and water requirement for the growth of

102
plants and crops depends on humidity though extremely high humidity is harmful as it enhances the
growth of saprophytes and parasitic fungus, bacteria and pests while very low humidity reduces
yield.
Sunlight as a Factor in Crop Growth and Distribution
Crops depend on sunlight. However, photoperiod varies greatly at different latitudes; therefore,
many plants cannot be successfully moved from one latitude to another even though environmental
and other cultural factors are compatible. Rainfall distribution varies according to seasons, and the
length of seasons differs on latitudinal basis, thus determining the growing seasons and the nature of
crops produced.
The hours or duration of sunlight also determines the growth and distribution of crops in different
regions. This is due to variation inhours of sunlight in the various regions. This affects the growth
and distribution of crops. Sunlight serves as the energy source forphotosynthesis. (Blij and Smithson,
1985)
RELATIONSHIP BETWEEN CLIMATE ANDAGRICULTURE
1.0 INTRODUCTION
This unit describes the relationship between agriculture and climate with focus on the effects of
rainfall, temperature, wind, and humidity on livestock and crop growth and distribution; moisture on
irrigation, temperature and humidity on pest and disease spread.
MAIN CONTENT
Effects of Rainfall, Temperature, Wind, Humidity andSunlight on Agriculture
Rainfall and Temperature on Livestock Growth andDistribution
Rainfall distribution and temperature are major climatic elements that have significant impact on

103
human activities. They influence the growth and distribution of livestock because the presence of
these elements when favourable provides viable grazing reservoir for livestock which provide the
essential nutritional needs of livestock for proper growth anddevelopment. The amount of rainfall
received provides a good source of drinking water which maintains the internal or optimum
temperature of the animal body. Combined effects of these elements enhanced livestock growth
(Jules, Robert, Frank and Vernon, 1981).
Moisture on Irrigated Agriculture
Irrigation becomes necessary when natural precipitation and moistureare absent either due to excess
evapotranspiration or deficit in rainfall. Crops need water in the soil to help mix up minerals which
they absorb through their root hair for metabolic system to be complete and where the water is
absent, it signifies that moisture is poor and crops finds it difficult to thrive. Low water application
levels and less irrigation frequencies reduces crop growth as well as low crop yield due to low or
poor soil water.(Schneider, Hollier, et al, 2005).
Temperature and Humidity on Pest Distribution
Changes in climate resulting to increased temperature could impact crop pest-insect population in
several complex manners. Increased temperature can potentially affect insect’s survival,
development,geographical range and population size. Temperature can impact insect physiological
development directly or indirectly through the physiology or existence of the host depending on the
development strategy of insect species.
The nature of humidity depends on the rate of evapotranspiration and the rate of humidity
determines the extent and magnitude of rainfall. The rate of evapotranspiration renders most
excess moisture-loving organisms to survive due to dryness of the environment. In this case, pest-
insect, feeds population will reduce when the rate of evapotranspiration is high. High humidity
increase the rate of rainfall
and excess rainfall is detrimental to the survival of less moisture-loving organisms. (Parmesan,
2006).
Temperature, Wind and Humidity on Disease Spread
Weeds, diseases and insects pest benefit significantly from warming andwill require more attention
as climate changes; these changes are due to increase in temperature which is advantageous and
facilitates the spread of pests and diseases.
Increase Co2 concentration reduces land’s ability to supply adequate livestock feeds as increased

104
heat, disease and extreme weather conditionreduces livestock production and increases the spread of
pests and diseases. Global warming is a major cause of increase rainfall in some area which could
lead to an increase in atmospheric humidity and the duration of wet season. These conditions favour
the development of fungal disease. Similarly, because of higher temperature and humidity, there
could be an increased spread of diseases.
Sunlight on pest and disease spread
Sunlight supplies not only energy to plants but all forms of animals including pests. Pest attack
causes diseases to plants and spreads widely. Some pest thrives better under hot conditions while
others under cold conditions. The hotness and coldness depends on the duration of light and
magnitudes which also depends greatly on latitudinal location. The disease in most cases under cold
conditions favour some disease causingagents and increase its spread while the reverse is the case
when the condition changes. The magnitude of the effects of each disease dependson the location
and the disease causing agent e.g. fungi that causedisease on sunflower.
Generally pests are considered as pathogens, predators and weeds, which can cause diseases and
damage to both plants and animals. When sunlight and other environmental factors are in place, crop
productivity level will be high and the effects of pests may be reduced. Crops suffer from disorders
caused by climatic conditions. When sunlight is high and moderately distributed, the process of
photosynthesis will be active and most pests will be comfortable and multiply fast produce
fruits, e.g.fungi which depend on green plants for their food. Wind systems do not distribute only
moisture and energy but also aid the spread of pathogens and diseases. The stronger the wind, the
more likely the rate of spread ofdiseases. When the wind system is less, the magnitude of spread
would be low. This is possible because most disease causing agents travel faster in air.
(Prospero, Grunwald, Winton and Hansen 2009).

105
CLIMATE CHANGE ISSUES IN
AGRICULTURE AND VARIOUS METHODS OF AMELIORATION
1.0 INTRODUCTION
In the past, when climatology was still at an infant stage, it was acommon belief that climate
was a non-changing feature of the environment. By averaging climatic data over a sufficiently long
period of time (30-50 years), it was assumed that true climate would be determined. However, it was
rather established that climate fluctuates allthe time giving rise to conditions affecting agriculture,
environment, human health etc. This unit will describe the concept of climate change and resultant
effects on agriculture.
MAIN CONTENT
The Concept and Meaning of Climate Change
Climate change is a change in the statistical distribution of weather patterns which may last for an
extended period of time (i.e. decades to millions of years). It is a change in average or longer
weather conditions (i.e. more or fewer extreme weather events). Climate change is caused by
factors such as biotic process, variations in solar radiation receivedby earth, plate tectonics and
volcanic eruptions. Certain human activities have also been identified to play a significant role in
recent causes of climate change which to a greater degree is called anthropogenic causes of climate
change often referred to as global warming.
To understand past and future climate, observations and theoretical models has been actively used by
climatologists. A record extending deep into the earth’s past has been gathered and continues to be
built up based on geological evidence from borehole temperature profiles, cores removed from deep
accumulations of ice, flora and fauna records, glacial and periglacial processes, stable isotope and
other analysis of sediment layers, and records of past sea levels. More recent data are provided by
instrumental records. General circulation models, based on the physical sciences, are often used in
theoretical approaches to match past climate data, make future projections and link causes and

106
effects of climate change.
Fluctuations over periods shorter than a few decades do not represent climate change. The term
sometimes is used to describe climate change caused by human activity, as opposed to changes in
climate that may have resulted as part of earth’s natural processes. In environmentalpolicy context,
the term climate change has become synonymous with anthropogenic global warming. Global
warming scientifically refers to surface temperature increase while climate change includes global
warming and everything else that increasing greenhouse gas levels will affect. (Wikipedia 2015).
Causes of Climate Change
The rate at which energy is received from the sun and the rate at which itis lost to space determine
the equilibrium temperature and climate of the earth. Energy is distributed around the globe by wind
system, ocean currents and other mechanisms to affect climatic condition of different locations.
Factors that can shape climate are called climate forcing or forcing mechanisms. These
include processes such as variations in solar radiation, earth orbit, albedo or reflectivity of the
continents and oceans, mountain-building and continental drift and changes in greenhouse gas
concentrations. A variety of climate change feedbacks can either amplify or diminish the initial
forcing; there are also key thresholdfactors which when exceeded can produce rapid change. Forcing
mechanisms can either be internal or external.
Internal Forcing Mechanisms
Internal forcing mechanisms are natural processes within the climate system itself. Natural changes
in the climate system result in internal climate variability. Examples include the type and distribution
of speciesand changes in ocean currents.
Ocean Variability
The ocean is a fundamental part of the climate system; some changes in it occurring at longer time

107
scales than in the atmosphere, gatheringhundreds of times more and having very high thermal inertia.
Short termfluctuations such as the ElNino-Southern oscillation, the pacific decadal oscillation, the
north Atlantic oscillation and the arctic oscillation, represent climate variability rather than
climate changes on a longer time scale. Alterations to ocean processes such as thermohaline
circulation play a significant role in redistribution of heat by carrying out a very slow and
extremely deep movement of water and the long term redistribution of heat in the oceans.
Species (Life)
Life affects climate through its significance in carbon and water cycles and through such
mechanisms as albedo, evapotranspiration, cloud formation and weathering. Examples:
• Glaciation 2.3 billion years ago triggered by the evolution of oxygenic photosynthesis
• Glaciation 300 million years ago ushered in by long term burial of decomposition resistant detritus
of vascular land-plants forming coal
• Termination of the Paleocene-Eocene-Thermal maximum 55 million years ago by flourishing marine
phytoplankon.
• Reversal of global warming 49 million years driven by the expansion of grass-grazer ecosystems.
External Forcing Mechanisms
External forcing mechanism can either be natural (changes in solar output) or anthropogenic
(increased re-emissions of greenhouse gases).
Orbital Variations
Slight differences in earth’s orbit leads to changes in the seasonal distribution of sunlight reaching
the earth’s surface and how it is distributed across the globe. Little change occurs in the area,
averaged annually, sunshine has a significant effect in the geographical and seasonal distribution.
Three types of orbital variations exist; variationsin earth’s eccentricity, changes in the tilt angle of
earth’s axis of rotation, and precession of earth’s axis. Combine together, these produce
Milankovitch cycles which have a large impact on climate and arenotable for their correlation to
glacial and interglacial periods, advance and retreat of the Sahara and for their appearance in the
stratigraphic record.
Milankovitch cycles drove the ice age cycles, CO2 followed temperaturechange with a lag of some
hundreds of years, and that as a feedback amplified temperature change. Ocean depths have a long

108
time in changing temperature (thermal inertia on such scale). Temperature change upon sea water,
the solubility of CO2 in the oceans changed as well as other factors impacting air – sea CO2
exchange.
Solar Output
On earth, the sun is the predominant source of energy, both long and short term variations in solar
intensity affect global climate. Solar outputalso varies on shorter time scales, including the 11 years
solar cycle and longer term modulations. Solar intensity variations possibly as a resultof the wolf,
sporer and maunder minimum are considered to have been influential in triggering the little ice age
and some of the warmingobserved from 1900 – 1950. Therefore, variation in solar output increases
from cyclical sunspot activity affecting global warming and climate may be influenced by the sum of
all effects (solar variation, anthropogenic radiative forcing).
Volcanism
Volcanic eruptions capable of affecting the earth’s climate on a scale of more than 1 year are the
eruptions that inject over 100,000 tons of SO2 into the stratosphere. This is due to the optical
properties of SO2 and sulfate aerosols, which strongly absorb or scatter solar radiation, creating a
global layer of sulfuric acid haze. Such eruption on an average
occur several times per century, and cause cooling by partially blocking the transmission of solar
radiation to the earth’s surface for a period of years though not much.
Small eruptions with injections of less than 0.1m+ of sulfur dioxide (SO2) into the stratosphere,
impact the atmosphere only partly, as temperature changes are comparable with natural variability.
Smaller eruptions occur at a much higher frequency, they have a significant impact on earth’s
atmosphere.
Volcanoes are also part of the extended carbon cycle because the releasecarbon dioxide (CO2) from
the earth’s crust and mantle, counteracting the uptake by sedimentary rocks and other geological
carbon dioxide sinks. A study by the US geological survey estimates that, volcanic emissions are at a
much lower level than the effects of current human activities which generate 100-300 times the
amount of carbon dioxide emitted by volcanoes.
Plate Tectonics
The motion of tectonic plates reconfigures global land and ocean areas and generates topography.
This can affect both the global and local patterns of climate and atmospheric ocean circulation.
The position of the continents determines the geometry of the oceans and therefore influences

109
patterns of ocean circulation. The locations of the seas are vital in controlling the transfer of heat and
moisture across the globe and thereby determine the global climate systems. The size of the
continents is also significant in stabilizing effects of the oceans temperature. Annual temperature
variations are generally lower in coastal areas than they are inland. A larger super continent will
therefore have more area inwhich climate is strongly seasonal than will several smaller continents
or islands.
Human Influences
Anthropogenic factors are human activities which affect the climatic systems. The scientific
consensus on climate change is that, the climate is changing and that these changes are in large
proportion caused by human activities and it is irreversible to a greater extent.
The most concern in these anthropogenic factors is the increasing rate ofCO2 level due to emissions
from fossil fuel combustion followed by aerosols (particulate matter in the atmosphere) and the CO2
released by cement manufacture. Others include land use, ozone depletion, animal agriculture and
deforestation which are of great concern for their significant impact on climate, micro climate and
measures of climatic variables. (Wikipedia, 2015).
Evidences of Climate Change
Evidence for climate change is taken from a variety of sources that can be used to reconstruct past
climates. Complete global records of surface temperature are available beginning from the mid-late
19th
century. For early periods, most of the evidence is indirect, climate changes arereferred from
changes in proxies, indicators that reflect climate such as vegetation, ice cores, dendrochronology,
sea level change and glacial geology.
Temperature Measurements and Proxies
Instrumental temperature record from surface stations was supplemented by radio sound balloons,
extensive atmospheric monitoring by the mid- 20th
century and from the 1970s on, with global
satellite data as well. The 18
O/16
O ratio in calcite and ice core samples used to deduce ocean
temperature in the distant past is an indication of a temperature proxy method, likewise other
climatic metrics observed in subsequent categories.
Historical and Archaeological Evidence
Climate change in recent past may be observed by changes in settlementand agricultural patterns.
Archaeological evidence, oral history and historical documents can offer insights into past changes

110
in the climate. Climate change effects have been linked to the collapse of variouscivilizations.
Glaciers
Are considered as the most sensitive indicators of climate change, their size is determined by a mass
balance between snow input and melt output. As temperatures warms up, glaciers retreat unless
snow precipitation increases to make up for the additional melt; the converseis also true.
Arctic Sea Ice Loss
The decline in sea ice both in extent and thickness over the last decades is a further evidence for
rapid climate change. Sea ice is frozen sea water that floats on the sea surface covering over
millions of square miles in the polar regions with differences on the basis of seasons with little ice
remaining but southern ocean or Antarctic sea ice melts away and reforms annually.
Vegetation
A change in vegetation distribution may occur given a change in the climate. Some changes in
climate may result to precipitation increaseand warmth, resulting to improve plant growth and the
subsequent sequestration of airborne Co2. Warmth increase in a location will give rise to earlier
flowering and fruiting times, driving a change in thetiming of life cycles of dependent organisms.
Plants bio-cycles may be affected by cold faster; this may result in vegetation stress, plant loss
and desertification in certain circumstances.
Pollen Analysis
The study of fossils palynomorphs, including pollen is called palynology. This is used to describe
geographical distribution of plants species which vary under different climatic conditions. Different
groups of plants have pollen with distinctive shapes and surface textures and since the outer surface
of pollen is composed of a very resilient material,they resist decay. Changes in the type of pollen
found in different layers of sediment in lakes, bogs or river deltas indicate changes in plant
communities. These changes are often a sign of a changing climate. Palynological studies have been
used to track changing vegetation patterns through the quaternary glaciating.
Cloud Cover and Precipitation
Precipitation can be estimated in the modern era with the global network of precipitation gauges.
Surface coverage over oceans and remote areas is relatively sparse but reducing reliance on
interpolation, satellite clouds and precipitation data has been available since 1970s. Quantification
of climatological variation of precipitation in prior centuries and epochs is less complete but
approximated using processes such as marine sediments, ice cores, cave stalagmite and tree rings.

111
Climatological temperatures substantially affect cloud cover and precipitation. Estimated global land
precipitation increased by approximately 2% over the course of the 20th
century, though the
calculated trend varies if different time end points are chosen, complicated by oscillations
including greater global land cloud cover precipitation. Slight overall increase in global river runoff
and in average soil moisture has been perceived.
Dendroclimatology
Is the analysis of tree ring growth patterns to determine past climate variations, wide and thick rings
indicate a fertile, well-watered growing period, whilst thin, narrow rings indicate a time of lower
rainfall andless than ideal growing conditions.
Animals
Remains of beetles are common in fresh water and land sediments.Different species of beetles
tend to be found under different climatic conditions. Given the extensive lineage of beetles whose
genetic makeup has not altered significantly over the millennia. Similarly the historical abundance of
various fish species has been found to have a substantial relationship with observed climatic
conditions. Changes in the primary productivity of autotrophs in the oceans can affect marine food
webs.
Change in Sea Level
Global sea level change from much of the last century has generallybeen estimated using tide
gauge measurements collated over long periods of time to give a long term average. More recent
altimetermeasurements in combination with accurately determined satellite orbitshave provided an
improved measurement of global sea level change.
To measure seal levels prior to instrumental measurements, scientisthave dated coral reefs that
grow near the surface of the ocean, coastal sediments, marine terraces, zooids in limestones and near
shore archaeological remains. The predominant dating methods used are uranium series and
radiocarbon, with cosmogenic radio-nuclides being sometimes used to date terraces that have
experienced relative sea level fall.

112
CLIMATE CHANGE ISSUES INAGRICULTURE
1.0 INTRODUCTION
Climate change and agriculture are interrelated processes, both of which take place on a global scale.
Climate change affects agriculture a numerous ways, which include variation in average temperature,
rainfalland climate extremes e.g. heat waves; changes in pest and diseases; changes in atmospheric
carbon dioxide and ground-level ozone concentrations; changes in the nutritional quality of some
foods and changes in sea level.
Climate change is already affecting agriculture with effects unevenly distributed across the world.
Future climate change will likely negatively affect crop production in low latitude countries, while
effects in northern latitudes may be positive or negative. Climate change will probably increase the
risk of food insecurity for vulnerable group such as the poor. Agriculture contributes to climate
changes by the following:
i. anthropogenic emissions of greenhouse gases (GHGS)
ii. conversion of non-agricultural land into agricultural land.
Agriculture, forestry and land-use change contributed around 20 to 25% to global annual emission
as stated in 2010. There are range of policies
that can reduce the risk of negative climate change impacts onagriculture, and to reduce GHG
emissions from the agriculture sector.
2.0 OBJECTIVES
• explain the impact of climate change on agriculture
• discuss the potential effects of climate change on pests, diseasesand weeds
• discuss the effects of glacier retreat and disappearance onagriculture
• explain the effects of temperature on growing period
• explain the effects of elevated carbon dioxide on crops.
MAIN CONTENT

113
Impact of Climate Change on Agriculture
Technological advancement has improved varieties, geneticallymodified organisms and irrigation
system but weather and climate are still key factors in agricultural production, as well as soil
properties and natural communities. The effect of climate change on agriculture is related to
variabilities in local climates rather than in global climate pattern. Agricultural on the other hand, has
grown in recent years, and provides significant amount of food, as well as comfortable income to
exporting ones.
Climate change induced by increasing greenhouse gases is likely to affect crops differently
from region to region. More favourable effects tend to depend to a larger extent on realization of the
potentially beneficial effects of carbon dioxide on crop growth and increase of efficiency in water
use. Decrease in potential yields is likely to becaused by shortening of the growing period, decrease
in water availability and poor vernalisation.
In the long run, the climatic change could affect agriculture in thefollowing ways.
i) Productivity, in terms of quantity and quality of crops
ii) Agricultural practices, through changes in water use (irrigation) and agricultural inputs such as
herbicides, insecticides and fertilizers
iii) Environmental effects, particularly in the frequency and intensity of soil drainage leading to nitrogen
leaching, soil erosion, reduction of crop diversity.
iv) Rural space, through the loss and gains of cultivated lands, land speculation, land renunciation and
hydraulic amenities.
v) Adaption, organisms may become more or less competitive, as well as human may develop urgency
to develop more competitiveorganisms such as flood resistant or salt resist and varieties of rice.
Agronomists believed that agricultural production will be mostly affected by the severity and the
pace of climate change. If change is gradual, there may be enough time for biota adjustment. Rapid
change in climate could harm agriculture in many countries, especially those that are already
suffering from rather poor soil and climate conditions because there is less time for optimum natural
selection and adaptation. (Briggs and Smithson, 1985).
Observed Impact of Climate Change on Agriculture

114
Change in crops phenology provides significant evidence of the response to climate change.
Phenology is the study of naturalphenomena that reoccur periodically and how these phenomena
relate toclimate and seasonal changes. Phenology has been observed significantly in agriculture and
forestry in large parts of the northern hemisphere.
Secondly, droughts have been occurring more frequently because ofglobal warming and they
are expected to become more frequent and intense mostly in some parts of Africa, and other parts of
the world continents. The impacts are aggravated because of increased water demand, population
growth urban expansions, and environmental protection efforts in many areas. Drought results in
crop failure and loss of pasture grazing land for livestock.
According to IPCC forth assessment report, on impacts of climate change on food security, there
could be large decreases in hunger globally by 2080, compared to the previous one experience in
2006. Reduction in hunger was driven by projected socio-economic development. In Africa, 70% of
the populations rely on rain-fed agriculture for their livelihoods; therefore the tendency of food
insecurity may likely be in upward projection.
Potential Effects of Temperature on Growing Period
Duration of crop growth cycles are above all, related to temperature. An increase in temperature will
speed up development. For instance, annual crops duration of sowing and harvesting will
shorten, especially for
maize between one and four weeks. The shortening of such cycle may have an adverse effect on
productivity.
Effects of Elevated Carbon Dioxide (CO2) on Crops
In the process of photosynthesis, carbon dioxide is essential to plant growth. Rising CO2
concentration in the atmosphere can have bothpositive and negative consequences. Increased carbon
dioxide CO2 is expected to have positive physiological effects by increasing the rate of
photosynthesis. This is known as carbon fertilization. Currently, the amount of CO2 in the
atmosphere is 380 parts per million. In comparison, the amount of oxygen is 210, 000 ppm, this
means that, plants may be starved of carbon dioxide as the enzymes that fixes CO2 also fixes oxygen
in the process called photorespiration. The effects ofan increase in CO2 would be higher on C3
crops (such as wheat) than C4 crops (such as maize) because maize is more susceptible to carbon
dioxide shortage.
Effects of Climate Change on Quality of AgriculturalProducts

115
The importance of climate change impacts on grain and forage quality emerges from new research.
For grains such as rice, the amylose contentof the grain which is the major determinant of cooking
quality may increase under elevated CO2. Cooked rice grain from plants grown in high CO2
environments would be firmer than that from present plants. However, iron and zinc concentrations
will be lower which are important for human nutrition.
The protein content of grains decreases under the combined increase of temperature and CO2
because increase in CO2 leads to decreased concentrations of micronutrients in crop plants. This may
have effect on other parts of the ecosystems as herbivores will need to eat more food togain the same
amount of protein. Higher level of CO2 leads to reduced plant uptake of nitrogen resulting in crops
with lower nutritional value. This would impact primarily on populations in poorer countries less
ableto compensate by eating more food, more varied diets or possibly taking supplements.
Reduced nitrogen content in fields meant for grazing has also beenshown to reduce animal
productivity especially in sheep, which depends on microbes in their gut to digest plants which
in turn depends onnitrogen intake.
Impact of Climate Change on Agricultural Surfaces
Increase in arable land in high-latitude region may be experience by reduction of the amount of
frozen lands. However, an impact of global warming on agriculture indicates a conflicting effects on
extension of arable and farmable lands with possible productivity losses and increased risk of
drought. Low land meant for rice production maylikely experience loss in productivity too, if a rise
in sea level is experience. But any rise in sea level of no more than a meter will drown several km2
of rice paddies, rendering the location incapable of producing its main staple and export of rice.
a) Climate Change on Erosion and Fertility
Erosion and soil degradation as environmental problem may likely occurwhen there is increase in
atmospheric temperature leading to more vigorous hygrological cycle, with extreme rainfall. This
will affect soil fertility. The ratio of carbon and nitrogen is suppose to be constant butin situation
where carbon is higher a storage of nitrogen in soils as nitrate may equally be higher, thus
providing fertilizing elements for plants and providing better yields. Extreme climate could result to
increase in precipitation rate resulting to erosion and as well provide soilwith better hydration, due to
the intensity of rain. Temperature would increase the rate of production of essential minerals thereby
reducing thecontent of soil organic matter and atmospheric CO2 concentration wouldtend to increase.
b) Climate Change on Pests, Diseases and Weeds
Most weeds are C3 plants that would undergo same acceleration of life cycle as cultivated crops with

116
increase temperature resulting from climate change effect and also benefit from carbonaceous
fertilization. They are likely to compete even more than C4 crops such as maize.
Global warming causes increases in rainfall in some locations which would in turn increase
atmospheric humidity and the duration of wet seasons. Combined with temperature, these could
result to a favourable condition for the development of fungal diseases. Also, high temperatureand
humidity could increase pressure from insects and disease vectors.
c) Glacier Retreat and Disappearance
Retreat of glaciers have a number of different quantitative impacts on agriculture. Areas dependent
on heavily water run-off from glaciers that melt during the warmer summer months, a retreat will
eventually depletethe glacial ice and substantially reduce or eliminate run off. A reduction in runoff
will affect the ability to irrigate crops and will reduce summer stream flows necessary to keep dams
and reservoirs replenished.
d) Ozone and Ultraviolent Radiations B (UV-B)
According to some scientists, agriculture could be affected by any decrease in stratospheric ozone
which could increase biologically dangerous ultraviolent radiation B. Excess UV-B can affect plant
physiology directly and cause massive amounts of mutations and indirectly through changed
pollinator behavior though such changes are not easy to quantify. Possible effect of increased
temperature in significantly higher levels of ground level ozone, would substantially lower crop
productivity.
e) ENSO Effects on Agriculture
ENSO is an acronym for EL Nino Southern oscillation. This climatic situation will affect monsoon
patterns more intensely in the future as climate change warms up the ocean’s water. Crops that lie on
the equatorial belt or render the tropical worker circulation, such as rice, will be affected by
varying monsoon patterns and more unpredictable weather. Planting scheduled and harvesting based
on weather patterns will become less effective. As climate change affects ENSO and consequently
delays planting, harvesting will be late and in drier conditions, resulting in less potential yields and
productivity. (Wikipedia, 2015).
Impact of Agriculture on Climate
The agricultural sector is a driving force in the gas emissions and land use effects which is thought
to cause substantial change in climate. Agriculture contributes directly to greenhouse gas

117
emissions through practices such as rice and maize production and raising of livestock. Fossil fuels,
land use and agriculture are considered the main causes of increased greenhouse gases observed over
the years.
Land use Practices
Agriculture contributes to greenhouse gas increases through land use in four main ways:
i) carbon dioxide (Co2) releases linked to deforestation
ii) methane releases from rice cultivation
iii) methane releases from enteric fermentation in cattle
iv) nitrous oxide releases from fertilizer application
These processes together comprises 54% of methane emissions, roughly 80% of nitrous oxide
emissions and virtually all CO2 emissions tied to land use. Deforestation has been identified as the
earth’s major changes to land cover. When forests woodlands are cleared to make room for
fields and pastures, the albedo of the affected area increases which can result as either warming or
cooling effects depending on localconditions.
Deforestation also affects carbon dioxide uptake, which can result in increased concentrations of
CO2, the dominant greenhouse gas. Land clearing methods such as slash and burn compounds
directly releases greenhouse gases and particulate matter such as soot into the air.
Livestock
Livestock – related activities such as deforestation and increasingly fuel intensive farming practices
and livestock rearing on grazing lands are responsible for over 18% of human-made greenhouse gas
emission including:
i) 9% of global carbon dioxide emissions
ii) 35-40% of global methane emissions mainly due to enteric fermentation and manure
iii) 64% of global nitrous oxide emissions chiefly due to fertilizer use. Livestock activities also
contribute disproportionately to land
– use effects, since crops such as maize and alfalfa are cultivated in order to feed the animals.
Livestock production occupies 70% of all land used for agriculture or 30% of the land surface of the
earth.
UNIT SEVEN METHODS OF AMELIORATION

118
1.0 INTRODUCTION
Global warming will have profound effects on where and how food is produced, and also, to a
reduction in the nutritional properties of some crops, all of which has policy implications for the
fight against hunger and poverty and for the global food trade. The growing threat of climate change
to global food supply and the challenges it poses for food security and nutrition, requires urgent
concerted policy responses. This unit will examine some methods of amelioration of climate change
towards sustaining agricultural productivity.
2.0 OBJECTIVES
• mention various methods of amelioration of climate change
• discuss mitigation and adaptation in developing countries
• explain crop development model as a method of reducing theeffects of climate change.
MAIN CONTENT
Mitigation and Adaptation to Climate Change
The Inter-governmental Panel on Climate Change (IPCC) has reported that agriculture is responsible
for over a quarter of total global greenhouse gas emissions. Given that agriculture’s share in global
gross domestic product (GDP) is about 4 percent: These figures suggest that agriculture is highly
greenhouse gas intensive. Innovative agricultural practices and technologies can play a role in
climate mitigation and adaptation. (IPCC AR5WG3, 2014).
The adaptation and mitigation potential is nowhere more pronounced “than in developing countries
where agricultural productivity remains low; poverty, vulnerability and food insecurity remains high;
and the direct effects of climate change are expected to be especially harsh. Creating the necessary
agricultural technologies and harnessing them to enable developing countries to adapt their
agricultural systems to changing climate will require innovations in policy and institutions which are
considered important in multiple scales.
Six (6) policy principles of mitigation and adaptation to climate change were suggested by Travis
Lybbert and Daniel Suminer; these include;
i) The best policy and institutional responses will enhance information flows, incentives and flexibility
ii) Policies and institutions that promote economic development and reduce poverty will often improve

119
agricultural adaptation and may also pave the way for more effective climate change mitigation
through agriculture
iii) Business as usual among world’s poor is not adequate
iv) Existing technology options must be made more available and accessible without overlooking
complementary capacity and investments
v) Adaptation and mitigation in agriculture will require local responses, but effective policy response
must also reflect global impacts and inter-linkages.
vi) Trade will play a significant role in both mitigation and adaptation, but will itself be shaped
importantly by climate change.
Agricultural Best Practices
Models are suggested for adaptation of the changing climate to suit agricultural production. Models
for climate behavior are frequently inconclusive. In order to further study effects of global warming
on agriculture, other types of model such as crop development models, yield prediction,
quantities of water or fertilizer consumed, can be used. Such models condense the knowledge
accumulated of the climate, soil, and effects observed from the results of various agricultural
practices. They thus could make it possible to test strategies of adaptation tomodifications of the
environment.
These models are necessarily simplified natural conditions often based on the assumption that weeds,
diseases and insect pests are controlled. Itis not clear whether the results obtained have an in-field
reality. However, some results are partly validated with an increasing number ofexperimental results.
Other models such as insect and disease development models based on climate projections are also
used; for instance simulation of aphid reproduction or septoria (cereal fungal disease development).
Scenarios are used in order to estimate climate change effects on crop development and yield. Each
scenario is defined as a set meteorological variables based on generally accepted projections. For
instance, many models are running simulations based on doubled carbon dioxide projections,
temperature raise ranging from 1o
C-5o
C and with rainfall level an increase or decrease of 20%. Other
parameters may include humidity, wind and solar activity. Scenarios of crop models are testing farm-
level adaptation such as losing data shift, climate adapted species (vernalisation need, heat and cold
resistance), irrigation and fertilizer adaptation, resistance to disease. Most developed models are
about wheat, maize, rice and soybeans. (Lobell, 2008).

Agroclimatology for agronomy

More Related Content

What's hot

Similar to Agroclimatology for agronomy

Recently uploaded

Agroclimatology for agronomy