Rain is liquid precipitation. On Earth, it is the condensation of atmospheric water vapor into drops heavy enough to fall, often making it to the surface. 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. However, not all rain reaches the surface; some evaporates while falling through dry air. This is called virga, a phenomenon often seen in hot, dry desert regions. The METAR code for rain is RA.

Rain is also known or suspected on other worlds. On Titan, infrequent methane rain is thought to carve that moon's numerous surface channels. On Venus, sulfuric acid virga evaporates 25 km from the surface. There is likely to be rain of various compositions in the upper atmospheres of the gas giants, as well as precipitation of liquid neon and helium in the deep atmospheres.

Formation

Rain plays a role in the hydrologic cycle in which moisture from the oceans evaporates, condenses into drops, precipitates (falls) from the sky, and eventually returns to the ocean via rivers and streams to repeat the cycle again. The water vapor from plant respiration also contributes to the moisture in the atmosphere.

A major scientific explanation of how rain forms and falls is called the Bergeron process. More recent research points to the influence of Cloud condensation nuclei released as the result of biological processes.

Human influence

The fine particulate matter produced by car exhaust and other human sources of pollution forms cloud condensation nuclei, leads to the production of clouds and increases the likelihood of rain. As commuters and commercial traffic cause pollution to build up over the course of the week, the likelihood of rain increases: it peaks by Saturday, after five days of weekday pollution has been built up. In heavily populated areas that are near the coast, such as the United States' Eastern Seaboard, the effect can be dramatic: there is a 22% higher chance of rain on Saturdays than on Mondays.

Classifying the amount of rain

When classified according to the rate of precipitation, rain can be divided (although any such classification scheme is necessarily arbitrary) into:

• Very light rain — when the precipitation rate is < 0.25 mm/hour
• Light rain — when the precipitation rate is between 0.25 mm/hour - 1.0 mm/hour
• Moderate rain — when the precipitation rate is between 1.0 mm/hour - 4.0 mm/hour
• Heavy rain — when the precipitation rate is between 4.0 mm/hour - 16.0 mm/hour
• Very heavy rain — when the precipitation rate is between 16.0 mm/hour - 50 mm/hour
• Extreme rain — when the precipitation rate is > 50.0 mm/hour

Precipitation is measured using a Rain gauge.