Knowledge of surface emission efficiency (emissivity) over land and ocean can be used to retrieve information on surface type, snow and ice cover, as well as properties such as ocean surface roughness (used in the retrieval of ocean surface wind speed).

The emissivity over land can vary strongly with surface type and frequency in the microwave between 10 and 100 GHz. Emission also varies over the ocean (water vs. sea ice), but spatial changes at any given frequency tend to be small relative to changes over land. Since window channel brightness temperatures are directly related to surface emissivity, microwave brightness temperature observations can be used to quantify emissivity and extract further hydrometeorological information.