Rain

Rain occurs when atmospheric water vapor condenses, falling down from clouds to the Earth's surface as separate drops of water. As in a closed circuit, the sun heats the Earth's surface; water on it evaporates; the warm air carries the vapor up into the sky where the vapor concentrates and becomes clouds. At high altitude temperature is much lower, and when it is low enough, the vapor condenses into water which falls down to the Earth's surface in drops as heavy rain or drizzle. Not all rain reaches the surface; some of it evaporates while falling through dry air. When none of it reaches the ground, it is called virga, a phenomenon often seen in hot, dry desert regions. Rain is the primary source of fresh water for most areas of the world, providing suitable conditions for diverse ecosystems, as well as water for hydroelectric power plants and crop irrigation.

Falling raindrops are often depicted in popular culture as "teardrop-shaped" — round at the bottom and narrowing towards the top — but this is incorrect. Only drops of water dripping from some sources are tear-shaped at the moment of formation. Small raindrops are nearly spherical. Larger ones become increasingly flattened on the bottom, like hamburger buns; very large ones are shaped like parachutes. The shape of raindrops was studied by Philipp Lenard in 1898. He found that small raindrops (less than about 2 mm diameter) are approximately spherical.