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The necessary equations are presented for obtaining the noise temperature due to thermal radiation which would be sensed by a receiver with an antenna located at some altitude above the earth. Emission and absorption of radiation by the atmosphere is considered. Calculated over-all absorptivities and apparent atmospheric temperatures are given as a function of antenna observation angle for beam paths through the atmosphere. Six wavelengths in the microwave band and three types of weather conditions were chosen for the calculations. Some typical antenna temperatures are presented as examples of the magnitudes of the effects to be expected as a function of the type of surface being viewed, the weather, and polarization.