For opaque volcanic and meteorological clouds, all of the emitted thermal energy from the surface underlying the cloud is absorbed by the cloud. The radiance recorded by the satellite sensor is only that which is emitted by the cloud. The band 4 radiance is greater than the band 5 radiance, which results in a positive band 4 minus 5 brightness temperature difference. Most of the earth's surface (water, land, snow, etc..) has a positive band 4 minus 5 temperature difference.
This page maintained by Dave Schneider (djschnei@mt