Solar heating of the Earth occurs mostly in the tropics, much of which is covered by ocean. Oceanic surface currents, such as the Atlantic Gulf Stream and the Pacific Kuroshio current, transport some of that heat away from the tropics to influence the climate at mid-latitudes. Oceans store heat in the summer and release it during the remainder of the year, so that oceanic heat moderates land temperatures, especially at mid-latitudes. When ocean surface currents fluctuate, as occurs during El Niño events, the climatic effects can be disastrous and widespread. The amount and rate of heat transferred between the Earth and atmosphere is determined by both conduction, which contributes about 2/3 of the total incident solar energy, and by evaporation which accounts for the remaining third. Water vapor, having absorbed heat from the evaporative process, can be transported far from the site of its origin.

Upon cooling, when moisture laden air is saturated and the vapor that it contained condenses, rain is produced and the heat that was originally used to evaporate the water from the Earth's surface is released into the atmosphere. The rate of energy release for each mm/hour of rainfall is three times as great as the solar energy ( ~350 Watts/m2) that falls on the same surface area. Thus the precipitation process concentrates heat that was used to evaporate moisture from large expanses of the tropics by factors of ten to a hundred into those regions where rain occurs. While solar heating of the atmosphere takes place mainly at the surface, the heat released by condensation occurs at high altitudes where it has a greater impact on the atmosphere's large scale circulation. Averaged over the entire Earth the heating released by precipitation is about five times greater than that produced by variations in surface heating.

Before TRMM's launch measurements of the global distribution of rainfall at the Earth's surface had uncertainties of the order of 50% and the global distribution of vertical profiles of precipitation was far less well determined. TRMM is providing some of the first spaceborne rain radar and microwave radiometric data that will measure the vertical distribution of precipitation over the tropics in a band between 35 degrees north and south latitudes. Such information will greatly enhance our understanding of the interactions between the sea, air and land masses which produce changes in global rainfall and climate. TRMM observations will also help improve modeling of tropical rainfall processes and their influence on global circulation leading to better predictions of rainfall and its variability at various time scales.