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3 - DEFINITIONS 
Various definitions of "canopy temperature" can be explored and calculated with an appropri 
ate soil-plant-atmosphere model such as Cupid. The following temperature definitions can be 
classified into three different categories: Kinetic temperature, radiative temperature and aero 
dynamic temperature. The kinetic temperature, which we might refer to as the "true" temper 
ature, is defined thermodynamically in terms of the kinetic energy of molecules. The radiative 
temperature usually is derived from a measurement of thermal radiance or flux density and 
represents a balance of radiative fluxes; it can be closely related to the true temperature of the 
radiating surface but is influenced by surface properties and external radiation sources. This 
radiative temperature may depend on wavelength considerations of the sensor and directional 
ity (directional versus hemispherical). The aerodynamic temperature is a fictitious temperature 
that is derived from a surface energy balance or extrapolation of the air temperature profile 
to the canopy displacement height, and it may not actually exist or be measurable except for 
smooth surfaces. 
Aerodynamic Temperature A temperature obtained by extrapolating the air temperature pro 
file to an apparent canopy height given by the displacement 
height plus the roughness length, which is typically about 3/4 
the canopy height. If the roughness length for momentum is 
used, then the extrapolated temperature is a momentum aero 
dy namic temperature and if the roughness for heat is used, then 
the extrapolated temperature is a thermal aerodynamic tempera 
ture . 
A general nonspecific term referring to aerodynamic temperature 
or directional infrared temperature usually made from a view 
angle oblique to the surface so that vegetation dominated the 
IRT field-of-view minimizing the effect of soil. Use of this 
term usually implies that directional and hemispherical, broad 
band and narrowband infrared temperatures and aerodynamic 
temperature are all equal. 
A temperature that assumes the surface to be a Lambertian Black 
Body. It is obtained by equating the measured radiance with the 
integral over wavelength of the Planck’s Black Body function 
times the sensor response, then inverting this equation for the 
temperature in the Planck’s function. This requires specification 
of wavelength interval, direction and whether the observation is 
immediately above the surface or above the atmosphere from a 
satellite. 
Emissivity of a surface from a particular view direction, which 
may be estimated as the ratio of the actual radiance from the 
surface to the radiance that would occur if all conditions were 
the same except that the elements of the surface were Black 
Bodies. This is appropriate for satellite observations (usually 
10-12 pm) and ground observations (usually 8-14 pm), but care 
must be taken in comparing these because of atmospheric effects 
and wavelength differences. 
Canopy Temperature 
Directional Brightness 
Temperature 
Directional Emissivity