Abstract This paper describes a method to combine geosynchronous IR and low-orbit microwave data to estimate mean monthly rainfall useful for climate studies. The IR data have the advantage of high time resolution (important for rapidly changing precipitation patterns and for the detection of diurnal signals) but lack a strong physical connection between the remotely sensed signal and the surface rainfall. The microwave data provide a stronger relation between the radiance and the rainfall but provide poor time sampling of the rainfall signal. The microwave technique uses the brightness temperature at 37 and 86 GHz from the Special Sensor Microwave/Imager instrument on board the Defense Meteorological Satellite Program (DMSP) satellite to define raining areas over water and land and uses the 86-GHz scattering signal to assign rain rate based on cloud model-microwave calculations. The microwave results are generally good for both individual swaths and monthly totals, except for a glaring underestimation of...