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A Note on Brightness Temperature at Millimeter Wavelengths

The first-order correction to the approximate proportionality between the brightness temperature and the specific intensity of radiation which is usually assumed to hold in the near earth environment at microwave frequencies is examined. Corrections as large as 7°K in the vicinity of 300 GHz are shown to be required. It is shown that the influence of this correction can be incorporated into solutions of the low frequency form of the radiative transfer equation simply by a change in the boundary conditions describing the downward flowing flux at the top of the earth's atmosphere.