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What is temperature?
- Measure of the average random kinetic energy of the molecules of a substance
- Physical property that determines the direction of heat flow between two substances in thermal contact
- T ≠ Heat Content
- Heat Content = ( T - Tref ) x C heat capacity – we’ll get to it later. docsity.com
Soil Temperature
- Critical factor that influences important physical, chemical, and biological processes in soil and plants: - Soil hydraulic properties - Organic matter decomposition and mineralization - Biodegradation of pesticides - Seed germination - Plant growth
Radiation
- All objects at a temperature T > 0 K
emit radiation: energy in the form of electromagnetic waves
- Total amount of radiation emitted
strongly depends on T.
Jt : total energy emitted, W m -
ε: emissivity (unitless)
= 1 for a “black body”; 0.9 to 1.0 for soil
σ: Stefan-Boltzmann constant
= 5.67 x 10 -8^ W m -2^ K-
T : temperature of the emitter (K) Note: by convention, little t is time, big T is Temperature.
Radiation
4
J t = ε T σ
λ m is the wavelength of maximum radiation
intensity
- λ m is inversely proportional to temperature
Radiation
T
m K m
⋅
μ λ
2900
- Wien’s law
- Sun Surface Temperature ≈ 5760 K
- Earth Surface Temperature ≈ 288 K
- λ m for the sun is about 0.5 μm. 99% of solar radiation is in
the wavelengths 0.3 - 4.0 μm and is called shortwave
- λ m for the earth is about 10 μm ( range of 3 - 50 μm), and is called infrared or long-wave radiation
Radiation
T
m K m
⋅
μ λ
2 9 0 0
Net radiation at the soil surface
- The global solar radiation as it reaches the land is further partitioned
- Net radiation at the soil surface: Jn = (J (^) s + Ja) (1-α)+ Jil - Jo J (^) n = net radiation J (^) s = direct beam incoming short-wave J (^) a = diffuse incoming short-wave α = albedo: the fraction of incoming short- wave radiation reflected by the surface J (^) il = incoming long-wave J (^) lo = outgoing long-wave
Albedo (α)
- Shortwave reflectivity
- For soils, α varies from 0.1 - 0.4 (unitless)
- Depends on:
- Soil color
- Surface roughness
- Sun angle (latitude, time of day)
- Soil wetness
- Slope and aspect
Surface energy balance
Heat flow in soil
- Radiation is how energy gets from the sun to the soil surface.
- Convection and conduction are the two most important mechanisms of heat transport under normal conditions
- A third mechanism, latent heat movement, will show up in the next lecture.
