![Albedo
Fraction of incidental radiation that is
reflected back is Albedo.
Albedo = Reflected radiant energy
Incidental radial energy
Energy balance of earth
Value ranges from 0 to 1 [value zero=100% absorption
(black body), Value 1= 100% reflection(absolute white surface)]
Larger the albedo , cooler the soil
Albedo high for light col. Soils.
Albedo low for dark colored soils, wet soils](https://image.slidesharecdn.com/enrgy-trnsfr-and-thrml-prpts-of-soil-97-2003-160218145310/85/Methods-of-heat-transfer-and-thermal-properties-of-soil-14-320.jpg)
Methods of heat transfer and thermal properties of soil
- 1. Methods of heat transfer and thermal properties of soil DATHAN C S
- 2. Heat Heat is a form of energy. The physical cause due to which hotness or coldness is felt is termed as heat. Heat energy is the kinetic energy (Energy of motion) due to vibration of molecules. Its defined as the kinetic energy of random motion of particles with which the material bodies are composed of. Unit of heat: Calorie- The quantity of heat required to raise 1gm of pure water through 10 C.
- 3. Temperature It is the measure of hotness of body. Defined as thermal condition of a body which determines whether it will receive or communicte heat to another body, if two bodies having different temperatures are put together for thermal communication
- 4. Heat and temperature HEAT TEMPERATURE 1 Form of energy Indicates thermal condition of body 2 Amount of heat may vary at same temps Temp of two bodies can be the same but Quantity of heat can vary 3 Heat flow unaffected by quantity of heat Heat Flow b/w bodies depend only on temp difference 4 Heat flow from High to low temps irrespective of quantity of heat When temps become same no further transfer of heat 5 Unit- Calorie Unit-degree centigrade or degree Fahrenheit
- 5. Latent heat (Hidden heat) Heat for which there is external manifestation such as rise or fall. Defined as quantity of heat required to change the state of matter without changing its temperature. Two types 1. Latent heat of fusion- 2. Latent heat of vapourisation
- 6. 1.Latent heat of fusion Latent heat of fusion of a solid is the quantity of heat required to change unit mass of the substance from solid to liquid at its melting point withiut changing the temp. Example: Latent heat of fusion of ice 80 calories. This means that 80 calories of heat req. to change 1gm of ice at 00 c into 1gm of water at 00 C
- 7. 2.Latent heat of Vaporization Latent heat of vaporozation of a liquid is the quantity of heat required to change unit mass of the substance form liquid to vapor state at its boiling point without changing the temp. Example: Latent heat of vaporization of water at 1000 C is 537 calories. This means that 537 calories of heat req. to change 1gm of water at 1000 c into steam at same temp.
- 8. Modes of heat transmission 1.Conduction- Process by which heat transf. between hotter to cooler substances or parts of the same substance by internal molecular motion without any transference of material particles. -Caused by transf. of kinetic energy of numerous inter molecular collisions - Agency of a natural medium necessary -- In solids
- 9. 2.Convection - Process by which heat transf. from one point to another by actual movnt. of heated particles from a place of higher temp to lower temp. -In liquids and gases. -Molecules of the hotter Body transfer heat continuously Towards the colder parts of the body Modes of heat transmission
- 10. Modes of heat transmission 3. Radiation- Heat transmitted from hotter to colder body without intervention of of any material medium -Heat reaches to earth from sun through vaccum -Radiation from sun occurs as short wane radiation(0.3 to 5 microns). Soil and atms. Re-radiates this as long wave radiation(6.8 to 100 microns) -Radiation measured as heat energy/unit area/unit time. (cal/cm2min)
- 11. Modes of transmission Conduction Convection Radiation Mode of transmission Through mat. particles Through mat. particles Through vaccum Movt. of heat Hotter part to cooler part of body Particles move Wave motion Meduim solids Liquids and gases Empty space
- 13. Partitioning of solar radiation Absorbed by Atms Scattered and diffused by small particles in air 45% 11% 16% 28%
- 14. Albedo Fraction of incidental radiation that is reflected back is Albedo. Albedo = Reflected radiant energy Incidental radial energy Energy balance of earth Value ranges from 0 to 1 [value zero=100% absorption (black body), Value 1= 100% reflection(absolute white surface)] Larger the albedo , cooler the soil Albedo high for light col. Soils. Albedo low for dark colored soils, wet soils
- 15. Albedo values of natural surfaces
- 16. Factors affecting albedo • Seasonal and diurnal radiation changes • Vegetation cover- forest vegetation, albedo less • Moisture content-High moisture content, low albedo • Surface color- high OM content, low albedo • Surface roughness- Fine textured dry soils, high albedo
- 17. Soil temp. and diurnal variation Intensity of heat in soil is soil temperature. •Surface of soil heated by radiation and heat moves downward by conduction •At any time, temp varies from layer to layer •Heat taken in or given out=Mass x Sp. Heatx Diff. in temp
- 18. Day time heat balance
- 19. Soil Temp. regimes • Soil temp. regimes depend on heat flow into soil and heat transfer processes in the soil and processes between soil and air • Study of STR helps in • Thermal characteristics of soil and their influe. On soil phy, chem, and biol processes in soil • Defining soil classes in soil taxonomy • Soil temp regimes recorded at depth of 50cm or at lithic or paralithic contact(Boundary between soil and coherent underlying material)
- 20. Soil Temp. regimes • Pergelic: Mean annl. ST less than 00 C • Cryic: More than 00 C but less than 80 C • Frigid: Mean annl. ST less than 80 C and the diff between mean winter and mean summer ST is more than 50 C • Mesic: Mean annl. ST between 80 C and 150 C and the diff between mean summer and mean winter ST is more than 50 C • Thermic: 150 C or more but less than 220 C and the diff between mean summer and mean winter ST is more than 50 C • Hyperthermaic: Mean annl. ST more than 220 C
- 21. Factors affecting soil temp. Temp. of soils in the field is affected by three factors. 1.Absorption of net amount of heat energy 2.Amount of heat energy required to bring about a change in soil temp 3.Amount of heat energy req. for processes like evpn
- 22. 1.Absorption of net amount of heat energy • 99% of incoming solar radiation as short wave- between 300 um and 4000 um • Soil re-radiates this as long wave radiation (WL b/w 4000 and 10 000 um) • Amount of heat that reaches earth is 2 gm cal/cm2 or 2 Langley/min. • Sunlight- 30-40% -on cloudy humid regions. 70-80% on bright days in arid regions . Global average 50%. • Absorption of heat influenced by • Exposure and slope of land • Soil water content • Surface cover • Vegetation • Plant residue • Om content • Soil color • Surface roughness and stoniness
- 23. 2.Amount of heat energy required to bring about change in soil temp. It depends on •Thermal properties of soil •Radiation exchange over the soil surfaceTC •Nature of soil surface Black colored soils good absorbersC Dry soils good absorbers (sp.heat of water higher( 1 cal/gm) than dry soil particles(0.2 cal/gm) Compacted soils & naturally aggregated soilshave higher thermal conductivity Soils having blocky and platy structure, high TC Soils having high mineral fraction, more TC than Org. soils
- 24. 3. Chemical or physical processes • Evaporation of water from soil requires large amount of heat energy(580 kcal/kg of water). • Evaprn has cooling effect on soils
- 25. Specific heat Ratio of quantity of heat required to raise a little mass of a substance to a range of temp, to the qnty. of heat required to raise the same mass of water to same range of temp. Sp. Heat= Heat req. to raise m g of sub. From 15-160 0C Heat req. to raise m g of water From 15-160 0C = Heat req. to raise 1 g of sub. Through 10 C 1 cal Unit= cal gm-10 C-1 or J gm-10 C-1
- 26. Volumetric heat capacity Heat capacity of a body is the quantity of heat required to raise the temp. of a unit volume of body by 10 C Defined as change in heat content of a unit volume of soil per unit change in temp -Denoted by Cv. -Unit = cal cm-30 C-1 -It is equal to product of Density and sp. heat
- 27. Sp. Heat and Vol. heat capacity of some soils and minerals
- 28. Conduction of heat in soil under steady state condition 1. Thermal conductivity It’s the ability of a substance to transfer heat from a hotter molecule to a cooler molecule. The thermal conductivity is a physical property of the solid. It the a measure of the materials ability to conduct heat.
- 29. Thermal properties of different soil constituents and ice at 200C and 1 aatm. pressure
- 30. • Typical values of conductivity for common rock types are: • sandstone k = 1.5 - 4.2 J s-1m -1K -1 • gneiss k = 2.1 - 4.2 J s-1m -1K -1 • granite k = 2.4 - 3.8 J s-1m -1K -1 • salt k = 5.4 - 7.2 J s-1m -1K -1 • iron k = 73 J s-1m -1K
- 31. Fouriers first law of conduction •In 1822 Fourier postulated that the rate of heat transfer is proportional to the temperature gradient present in a solid, assuming that the heat flow is unidirectional. •He found out that the total quantity of heat(Qh) flowing through a soil column is directly proportional to the •Cross section area of soil(A) •Diff in temp(T1-T2), to the time ‘t’ secs and inversely proportional to the distance or thickness between two points assuming that heat flow is unidirectional and there is no heat loss. •The laws governing conduction of heat are very similar to Ohm’s Law, which governs electrical conduction.
- 32. Fouriers law of heat conduction
- 33. Fouriers law of conduction
- 34. 2. Thermal diffusivity • When soil receives heat temp rises. The rate of change is dir. propotional to (Kh) thermal conductivity and inversely propnl. to heat capacity on volume basis ie. Volumetric heat capacity. The ratio of Kh/Cv is constant which determines the rate of rise of temp in a given soil. This constant is Thermal diffusivity(Dh). Dh=kh/Cv • Unit- cm2/sec. • A measure of a material’s ability to respond to changes in its thermal environment. • It measures the ability of a material to conduct thermal energy relative to its ability to store thermal energy • It expresses the rate at which a body warms up under a given heat gradient
- 35. Factors affecting thermal conductiity and diffusivity of soil 1. Composition of soil 2. Soil texture 3. Water content 4. Om content 5. compaction