Rain is a form of precipitation, as are snow, sleet, hail, and dew. Rain forms when separate drops of water fall to the Earth's surface from clouds. Not all rain reaches the surface, however; some evaporates while falling through dry air, a type of precipitation called virga.
Rain plays a major role in the hydrologic cycle in which moisture from the oceans evaporates, condenses into clouds, precipitates back to earth, and eventually returns to the ocean via streams and rivers to repeat the cycle again.
The amount of rainfall is measured using a rain gauge. It is expressed as the depth of water that collects on a flat surface, and is measured to the nearest 0.25mm.
Falling raindrops are often described as "tear-shaped", round at the bottom and narrowing towards the top, but this is incorrect. Small raindrops are nearly spherical. Larger ones become increasingly flattened, like hamburger buns; very large ones are shaped like parachutes.
Several cultures have developed means of dealing with rain and have developed numerous protection devices such as umbrellas and raincoats. Many people also prefer to stay inside on rainy days, especially in tropical climates where rain is usually accompanied by thunderstorms.
Generally, rain has a pH slightly under 6, simply from absorption of atmospheric carbon dioxide, which dissociates in the droplet to form minute quantities of carbonic acid In some desert areas, airborne dust contains enough calcium carbonate to counter the natural acidity of precipitation, and rainfall can be neutral or even alkaline. Rain below pH 5.6 is considered acid rain.
