Precipitation plays a major role in determining the climate of an area. Rainfall is critical, in particular, for it replenishes ground water and supplies both natural watershed systems and irrigation channels. Worldwide rainfall averages vary greatly from region to region. Areas that receive less than 250 mm (10 in) of rain each year are considered deserts, while regions receiving more than 2000 mm (80 in) are usually termed equatorial or tropic. Average rainfall is determined by the depth of water falling on a flat surface and is measured by a rain gauge.

Rain is precipitation of liquid drops of water. Raindrops generally have a diameter greater than 0.5 mm (0.02 in). They range in size up to about 3 mm (about 0.13 in) in diameter, and their rate of fall increases, up to 7.6 m (25 ft) per sec with their size. Larger drops tend to be flattened and broken into smaller drops by rapid fall through the air. The precipitation of smaller drops, called drizzle, often severely restricts visibility but usually does not produce significant accumulations of water.

Amount or volume of rainfall is expressed as the depth of water that collects on a flat surface, and is measured in a rain gauge to the nearest 0.25 mm (0.01 in). Rainfall is classified as light if not more than 2.5 mm (0.10 in) per hr, heavy if more than 7.50 mm (more than 0.30 in) per hr, and moderate if between these limits.

Air masses acquire moisture on passing over warm bodies of water, or over wet land surfaces. The moisture, or water vapor, is carried upward into the air mass by turbulence and convection. The lifting required to cool and condense this water vapor results from several processes, and study of these processes provides a key for understanding the distribution of rainfall in various parts of the world.

In the U.S. the heaviest average rainfall, up to 1778 mm (70 in), are experienced in the Southeast, where air masses from the tropical Atlantic and Gulf of Mexico are lifted frequently by cyclones and by convection. Moderate annual accumulations, from 762 to 1270 mm (30 to 50 in), occur throughout the eastern U.S., and are caused by cyclones in winter and convection in summer. The central plains, being farther from sources of moisture, have smaller annual accumulations, 381 to 1016 mm (15 to 40 in), mainly from summer convective storms.

The southwestern U.S. is dominated by widespread descent of air in the subtropical Pacific anticyclone; average rainfall is light, less than 254 mm (less than 10 in), except in the mountainous regions. The northwestern states are affected by cyclones from the Pacific Ocean, particularly during the winter; but rainfall is moderate, especially on the westward-facing slopes of mountain ranges.

The world's heaviest average rainfall, about 10,922 mm (about 430 in) per year, occurs at Cherrapunji, in northeastern India, where moisture-laden air from the Bay of Bengal is forced to rise over the Kh?si Hills of Assam State. As much as 26,466 mm (1042 in), or 26 m (87 ft), of rain have fallen there in one year. The rainfall data indicate that  other extreme rainfall include nearly 1168 mm (nearly 46 in) of rain in one day during a typhoon at Baguio, Philippines; 304.8 mm (12 in) within one hour during a thunderstorm at Holt, Missouri; and 62.7 mm (2.48 in) in over a 5-min period at Portobelo, Panama.