For transparent clouds, the radiance recorded by the satellite sensor is a combination of the radiance of the cloud, and the radiance of the underlying surface. For meteorological clouds, the absorption of the of energy from the underlying surface is greater in band 5 than in band 4, due to a larger refractive indices for water and ice particles in the band 5 region than in the band 4 region. This results in a positive band 4 minus 5 brightness temperature difference.
This page maintained by Dave Schneider (djschnei@mt