CAMBRIDGE AS GEOGRAPHY REVISION: ATMOSPHERE AND WEATHER - 2.3 WEATHER PROCESSES AND PHENOMENA

CAMBRIDGE AS GEOGRAPHY REVISION: ATMOSPHERE AND WEATHER - 2.3 WEATHER PROCESSES AND PHENOMENA

• 1.
  ASGEOGRAPHYREVISION ATMOSPHEREANDWEATHER 2.3 WEATHERPROCESSES ANDPHENOMENA
• 2.
  KEYTERMSANDDEFINITIONS Adiabatic lapse rateis the observed change of temperature caused by the air changing internally, i.e. by rising and falling in height. Adiabatic processes refer to the change in temperature of a parcel of air caused by its ascent or descent (i.e. there is no external source of heating or cooling). All changes are internal. Advection fog is fog formed by warm air flowing over a cold surface, being chilled, reaching dew point, and condensation taking place, close to the ground’s surface. Air mass is a large body of air with relatively similar temperature and humidity characteristics.
• 3.
  KEYTERMSANDDEFINITIONS Altitude is themeasurement of height, usually given in metres above sea level. Temperature declines on average 1 °C for every 100 m (and therefore rises with every decrease in altitude of 100 m). Air also becomes thinner (lower pressure) with increased altitude. Clouds are water vapour condensed into millions of minute water particles that float in masses in the atmosphere. Clouds are formed by the cooling of air containing water vapour, which generally condenses around tiny dust or ice particles.
• 4.
  KEYTERMSANDDEFINITIONS Condensation level isthe altitude at which relative humidity is 100 per cent, hence clouds form. Condensation nuclei are tiny particles (aerosols) that moisture is able to condense onto, e.g. salt, dust. Conditional instability occurs when the ELR (environmental lapse rate) is lower than the DALR (dry adiabatic lapse rate) but higher than the SALR (saturated adiabatic lapse rate). The air is unstable on the condition that dew point is reached, and it cools at the SALR.
• 6.
  KEYTERMSANDDEFINITIONS Convectional rainfall israinfall associated with hot climates, resulting from the rising of convection currents of warm air. Air that has been warmed by the extreme heating of the ground surface rises to great heights and is cooled quickly. The water vapour carried by the air condenses and rain falls heavily. Convectional rainfall is usually associated with a thunderstorm. Depression is a region of low atmospheric pressure. In mid latitudes a depression forms as warm, moist air from the tropics mixes with cold, dry polar air, producing warm and cold fronts and unstable weather. The warm air, being less dense, rises above the cold air to produce the area of low pressure on the ground.
• 7.
  KEYTERMSANDDEFINITIONS Dew is precipitationin the form of moisture that collects on the ground. It forms after the temperature of the ground has fallen below the dew point of the air in contact with it. As the temperature falls during the night, the air and its water vapour become chilled, and condensation takes place on the cooled surfaces. Dew point is the temperature at which relative humidity is 100 per cent. Environmental lapse rate (ELR) is the normal decline of temperature with altitude; usually about 6 °C / 1000 m.
• 8.
  KEYTERMSANDDEFINITIONS Evaporation is theprocess by which a liquid is transformed into a gas. Evapotranspiration is the combined losses of transpiration and evaporation. Fog is cloud at ground level, composed of water vapour that has condensed on condensation nuclei such as dust in the atmosphere. Cloud and fog are both caused by the air temperature falling below dew point. The thickness of fog depends on the number of water particles it contains. Officially, fog refers to a condition when visibility is reduced to 1 km or less, and mist or haze to that giving a visibility of 1– 2 km. Front is a boundary between a warm air mass and a cold air mass, resulting in frontal (depressional or cyclonic) rainfall.
• 9.
  KEYTERMSANDDEFINITIONS Humidity is ameasure of the amount of moisture in the air. Absolute humidity tells us how much moisture is in the air (g/m3), whereas relative humidity expresses this amount as a percentage of the maximum that air of a certain temperature could hold. Instability refers to unstable atmospheric conditions (rising air), likely to cause cloud formation and precipitation. Lapse rate is the rate of change of atmospheric factors (e.g. temperature and humidity) with changing altitude. Mist is low cloud caused by the condensation of water vapour in the lower part of the atmosphere. Mist is less thick than fog, visibility being 1–2 km.
• 11.
  KEYTERMSANDDEFINITIONS Orographic (relief) rainfallis rainfall that occurs when an airstream is forced to rise over a mountain range. As the air rises, it becomes cooled. The amount of moisture that air can hold decreases with decreasing temperature. So the water vapour in the rising airstream condenses, and rain falls on the windward side of the mountain. The air descending on the leeward side contains less moisture, resulting in a rain shadow where there is little or no rain.
• 13.
  KEYTERMSANDDEFINITIONS Precipitation is theconversion and transfer of moisture in the atmosphere to the land. It includes all forms of rainfall, snow, frost, hail and dew. Radiation fog is fog that forms on clear, calm nights when the land surface loses heat rapidly (by radiation); the air above is cooled to below its dew point and condensation takes place. Rainfall is a form of precipitation in which separate drops of water fall to the Earth’s surface from clouds. The drops are formed by the accumulation of fine droplets that condense from water vapour in the air. The condensation is usually brought about by rising and subsequent cooling of air. Rain can form in three main ways – frontal (or cyclonic) rainfall, orographic (or relief) rainfall, and convectional rainfall.
• 14.
  KEYTERMSANDDEFINITIONS Rain shadow isthe leeward side of hills, which has a relatively light rainfall compared to the windward side. Stability refers to pressure conditions; air is unable to rise above a low level; calm, dry conditions, limited cloud formation. Thunderstorms are usually caused by the intense heating of the ground surface during summer. The warm air rises rapidly to form tall cumulonimbus clouds with a characteristic anvil-shaped top. Electrical charges accumulate in the clouds and are discharged to the ground as flashes of lightning. Air in the path of lightning becomes heated and expands rapidly, creating shock waves that are heard as a crash or rumble of thunder. Water vapour is water in gaseous form, especially when evaporated below boiling point.
• 16.
  TOPICSUMMARY Atmospheric moisture existsin all three states – vapour, liquid and solid. Energy is used in the change from one phase to another, for example between a liquid and a gas. In evaporation, heat is absorbed. By contrast, when condensation occurs latent heat locked in the water vapour is released, causing a rise in temperature. Saturated air is air with a relative humidity of 100 per cent. The term ‘precipitation’ refers to all forms of deposition of moisture from the atmosphere in either solid or liquid states. It includes rain, hail, snow and dew.
• 17.
  TOPICSUMMARY Adiabatic processes arethose which relate to the rising and sinking of air. This means that the temperature of the air is changed internally i.e. without any other outside influence. Adiabatic cooling and warming in dry (unsaturated) air occurs at a rate of approximately 10 °C/km. This is known as the dry adiabatic lapse rate (DALR). Air in which condensation is occurring cools at the lower saturated adiabatic lapse rate (SALR) of 4–9 °C/km. Instability occurs when a parcel of air is warmer and therefore less dense than the air above, causing rising and expansion. Stable air conditions (stability) exist when ELR < DALR and the SALR.
• 18.
  TOPICSUMMARY There are threemain types of rainfall – convectional, frontal (depressional) and orographic (relief). Thunderstorms are special cases of rapid cloud formation and heavy precipitation in unstable air conditions. Radiation fog is formed in low lying areas during calm weather, especially during spring and autumn. Advection fog is formed when warm moist air flows over cooler land or sea surface horizontally.
• 19.
  ADDITIONALWORK 1. How doesan understanding of lapse rates help explain weather phenomena? 2. What are the conditions necessary for the formation of rain? 3. Explain the formation of fog.
• 20.
  SUGGESTEDWEBSITES www.uwsp.edu/geO/faculty/ritter/geog101/textbook/atmospheric_ moisture/lapse_rates_1.html for lapserates and temperature change. www.enotes.com/earth-science/atmospheric-lapse-rate for notes on lapse rates. www.s-cool.co.uk/alevel/geography/weather-conditions/lapse-rates- and-microclimate.html for lapse rates and microclimates. www.forestencyclopedia.net/p/p775 for lapse rates and whether smoke rises or not.