Weather satellites have been used by the National Weather Service since the sixties to map clouds, sea surface temperatures, and vertical temperature and moisture distributions. These data are supplemented by crucial ground observations, and are subsequently input to numerical models which try to predict the short-term evolution of the weather. The results are then made available to the public, most directly in the form of composite weather maps. The rain shown on these maps comes mostly from ground weather radars.

Unfortunately, the crucial ground observations are often simply not available because these regions are typically inaccessible. In addition, the large-scale numerical models are still woefully inaccurate over the tropics, mainly because we do not yet have a solid understanding of the often severe dynamics which govern the circulation in the warm moist tropical atmosphere, and which have a determining effect on the weather far away from the tropics. The unprecedented precision of former TRMM and now GPM measurements helps replace the missing ground measurements over the tropics, and helps fill the gap in our understanding of the processes which start around the equator but affect the weather all over the globe.