The lower microwave frequencies -- less than 50 GHz -- lie within an atmospheric window region and are primarily sensitive to emission by water vapor, clouds, precipitation, and surface features. Thus the lower frequencies are very useful for discrimination of surface type, liquid water in clouds, and moderate to heavy precipitation composed of liquid water.

Notice that a weak absorption region due to water vapor is present near 22 GHz. Frequencies within this absorption region are used by the AMSU and SSM/I to help infer total precipitable water (TPW).