Precipitation may be measured operationally on a local or regional basis using active microwave sensors. For example, Nexrad Weather Service in the United States provide excellent real-time precipitation information. Similar systems function in many other country. However, the majority of the world’s land surface do not enjoy such coverage by weather radars or even rain gauge networks, and will not for the foreseeable future. Meteorological satellite are the only systems capable of acquiring rainfall data over vast distances. Unfortunately, remote sensing methods must rely on using indirectly remotely sensed data on cloud reflectance, cloud-top temperature, and/or the presence of frozen precipitation aloft to estimate the surface rain rate. Petty (1995) reviews the various satellite-based precipitation estimation techniques.