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 (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, like hamburger buns; very large ones are shaped like parachutes. On average, raindrops are 1-2 mm in diameter. The biggest raindrops on Earth were recorded over Brazil and the Marshall Islands in 2004 - some of them were as large as 1cm. The large size is explained by condensation on large smoke particles or by collisions between drops in small regions with particularly high content of liquid water.
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.