In an El Nino, warm water and its coupled cloud systems spread eastward from Indonesia toward South America. Weather conditions associated with El Nino include wetter than normal weather in California, wetter and colder winters than normal in the eastern United States and dryer summer monsoon seasons across the southern hemisphere. With temperature, rainfall is perhaps the most important factor in defining climate. Excess rainfall can cause flooding and enormous property and crop damage. Still, a deficiency of rainfall can cause drought and crop failure.
Rainfall is also the major source of energy that drives the circulation of the atmosphere.
As water changes from liquid to vapor and back to liquid, heat is stored and then released. This latent heat can intensify some weather systems, making tropical rainfall an important determinant of atmospheric circulation and short-term climate changes. Major advances are occurring in the modeling of rainfall and rain-related cloud processes. The response of cloud systems to their environment is an important link in a chain of climate variations. Scientists are developing cloud models to convert TRMM's signals into rainfall rates.