Snow cover is identified by the scattering of high frequency microwaves from ice particles. As in the case of precipitation, scattering reduces the high frequency brightness temperatures relative to the lower frequency brightness temperatures. The frequency vs emissivity curves shown above illustrate that as the scattering effect increases for snow cover, corresponding emissivity decreases. The only exception to this rule is melting or wet snow cover, whose emissivity remains high with increasing frequency. Special care must be taken to identify wet snow as it produces a signature similar to that of precipitation over land.

Much of the practical experience regarding snow cover is based on knowledge gained from using SSM/I measurements at 37 and 85 GHz. As such, the approach taken for the AMSU product is just the difference between the brightness temperature at the lowest frequency 23.8-GHz channel and the higher frequency 31.4- and 89-GHz channels.