230
Plants and Plant Communities
14.3 Radiometric Temperature of Plant
Canopies
Vegetative canopies are exceedingly complex, being composed of many
leaves, branches, stems, soil, etc. Even though the aerodynamic temperature can be defined by Eq. (14.8), its relation to true thermodynamic
temperature is virtually impossible to discover. However, another canopy
temperature is easily measured. This is the radiometric temperature. The
radiometric temperature of a blackbody (unity emissivity) surface (TBB)
is estimated from a direct measurement of thermal radiant flux density
(@(TBB)) by inverting an integral of Eq. (10.4) over the wavelength band
of sensitivity of the infrared radiometer. If a surface is not a blackbody,
then adjustments must be made for emissivity. Norman and Becker (1995)
discuss radiometric temperature and thermal emissivity in detail. Infrared
radiometers that are used to measure radiometric temperature are called
infrared thermometers. Because infrared thermometers are intended to
estimate the temperature of a surface and be minimally influenced by the
intervening atmosphere, usually they are sensitive only to wavelengths
where the atmosphere is relatively transparent (between 8 and 13 pm
wavelengths, see Fig. 10.6). From satellites, atmospheric influences of
3 to 10" C are not uncommon even in the most transparent wavelength
bands. We know that the integral of Eq. (10.4) over all wavelengths is
equal to GT;,. If we assume the radiant flux density in the 8 to 13 p m
wavelength band is proportional to T 4 (a good approximation, but not
perfect) we can work with T~ instead of complicated functions of the
blackbody integral. Unfortunately the thermal emissivity of natural surfaces between 8 and 13 p m may not be equal to the broad-band (4 to
80 pm) thermal emissivity (particularly for soils), and the 8 to 13 pm
emissivity must be known to obtain radiometric temperatures. Fortunately
most full-cover vegetative canopies have thermal emissivities in the 8 to
13 pm wavelength band of 0.98 to 0.99. The reason for this is discussed
near the end of Ch. 15.
Even for a blackbody, the radiometric temperature, the thermodynamic
temperature, and the aerodynamic temperature resulting from a surface
energy balance (Eq. (14.8)) will all be equal only if the surface and its
surroundings are in thermodynamic equilibrium (they have a constant,
uniform temperature). Since this rarely occurs in nature, in general these
temperatures are not expected to be interchangeable. The aerodynamic
temperature depends on the areodynamic conductance between the atmosphere and various parts of the surface that are at different temperatures.
The radiometric temperature depends on the fourth power weighting of
the absolute temperature of the parts of the surface that make up the
view of the infrared thermometer. Because radiometric temperame can
depend on radiometer view angle and aerodynamic temperature does
not, the two will generally be different. Consider a partial-cover canopy
with hot dry soil (50" C) and cool transpiring leaves (25" C), a common occurance. If an infrared thermometer pointed at this surface from
Plants and Plant Communities
14.3 Radiometric Temperature of Plant
Canopies
Vegetative canopies are exceedingly complex, being composed of many
leaves, branches, stems, soil, etc. Even though the aerodynamic temperature can be defined by Eq. (14.8), its relation to true thermodynamic
temperature is virtually impossible to discover. However, another canopy
temperature is easily measured. This is the radiometric temperature. The
radiometric temperature of a blackbody (unity emissivity) surface (TBB)
is estimated from a direct measurement of thermal radiant flux density
(@(TBB)) by inverting an integral of Eq. (10.4) over the wavelength band
of sensitivity of the infrared radiometer. If a surface is not a blackbody,
then adjustments must be made for emissivity. Norman and Becker (1995)
discuss radiometric temperature and thermal emissivity in detail. Infrared
radiometers that are used to measure radiometric temperature are called
infrared thermometers. Because infrared thermometers are intended to
estimate the temperature of a surface and be minimally influenced by the
intervening atmosphere, usually they are sensitive only to wavelengths
where the atmosphere is relatively transparent (between 8 and 13 pm
wavelengths, see Fig. 10.6). From satellites, atmospheric influences of
3 to 10" C are not uncommon even in the most transparent wavelength
bands. We know that the integral of Eq. (10.4) over all wavelengths is
equal to GT;,. If we assume the radiant flux density in the 8 to 13 p m
wavelength band is proportional to T 4 (a good approximation, but not
perfect) we can work with T~ instead of complicated functions of the
blackbody integral. Unfortunately the thermal emissivity of natural surfaces between 8 and 13 p m may not be equal to the broad-band (4 to
80 pm) thermal emissivity (particularly for soils), and the 8 to 13 pm
emissivity must be known to obtain radiometric temperatures. Fortunately
most full-cover vegetative canopies have thermal emissivities in the 8 to
13 pm wavelength band of 0.98 to 0.99. The reason for this is discussed
near the end of Ch. 15.
Even for a blackbody, the radiometric temperature, the thermodynamic
temperature, and the aerodynamic temperature resulting from a surface
energy balance (Eq. (14.8)) will all be equal only if the surface and its
surroundings are in thermodynamic equilibrium (they have a constant,
uniform temperature). Since this rarely occurs in nature, in general these
temperatures are not expected to be interchangeable. The aerodynamic
temperature depends on the areodynamic conductance between the atmosphere and various parts of the surface that are at different temperatures.
The radiometric temperature depends on the fourth power weighting of
the absolute temperature of the parts of the surface that make up the
view of the infrared thermometer. Because radiometric temperame can
depend on radiometer view angle and aerodynamic temperature does
not, the two will generally be different. Consider a partial-cover canopy
with hot dry soil (50" C) and cool transpiring leaves (25" C), a common occurance. If an infrared thermometer pointed at this surface from