Air temperature is significantly related to crop development and conditions. Operational crop and soil moisture models require daily minimum and maximum shelter temperature and dew point temperature. Canopy (or skin) temperature may be more directly related to growth and evapotranspiration than the shelter temperature. The difference between the two is a measure of crop stress. The ability to observe canopy temperature directly is an advantage of satellite observations. However, it is to be noted that the satellite derived "skin temperature" and "crop canopy temperature" are equivalent only when a satellite field of view (FOV) is filled with vegetation. If FOV constitutes a mixture of bare soil, water bodies, etc. the relation between the two becomes complex. Satellite observations in the thermal IR window (10-12 ^m) are used to obtain estimates of canopy or skin temperature. Price (1984) found that surface temperatures over vegetated land can be estimated with an accuracy of 2-3o C using AVHRR split window technique. The errors can largely be attributed to imperfect knowledge about atmospheric water vapor content and wavelength dependent surface emissivity. Clouds pose a serious problem in the estimation of surface temperature by infrared techniques.
Satellite surface temperature estimates have been used to delineate areas of freezing for frost warning and for monitoring freezing events that can affect food production. In U.S., the infrared observations from geostationary satellite GOES are used for freeze forecasting by following the diurnal progression of the freeze line. Accurate freeze forecasts permit farmers to protect crops only when there is a significant freeze threat.
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