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Measurements of thermal radiation at infrared wavelengths (7-14 mu m) yield much information about the land surface. The primary use of these observations is for surface temperature determination as the emissivity is usually close to one. For this purpose it is fortuitous that the peak in the thermal emission occurs in an atmospheric transmission window. In additions there are variations in the emissivity of minerals and soils in the 7-14-mu m region which can be interpreted for identification purposes, The emissivity for vegetative canopies has been found to be close to one with little spectral variation. Applications of the derived surface temperature to study the surface energy balance and to estimate the energy fluxes from the land surface are discussed. The basic concepts of the energy balance at the land surface are presented along with an example of how remotely sensed surface brightness temperatures can be used to estimate the sensible heat and to estimate plant water use. The example is from the Monsoon 90 experiment conducted over an arid watershed in the state of Arizona in the United States. In this case, surface temperatures derived from an aircraft thermal infrared sensor and vegetation and land use characteristics derived from a Landsat TM image were used in a two-source model to predict the surface heat fluxes. The agreement with ground measurements is reasonably good for the 3 days of observations. ((C) Inra/Elsevier, Paris.).