Rain and snow are key elements in the Earth's water cycle, which is vital to all life on Earth. Rainfall is the main way that the water in the skies comes down to Earth, where it fills our lakes and rivers, recharges the underground aquifers, and provides drinks to plants and animals. Water vapor condenses and returns to Earth as precipitation, once again replenishing reservoirs, lakes, rivers, underground aquifers, and other sources of water and providing the moisture required by plants and animals in an endless water cycle. This web page provides an overview of USGS's research about rain and precipitation. It includes related research, multimedia, and a collection of data and tools (Introduction).