Rainfall

Rainfall 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.

Overview

The precipitation of rain is part of the water cycle. Water evaporates from the surface of the earth, rises and cools, condenses into clouds, and falls back to the surface as precipitation. The water that evaporates from the earth's surface becomes a gas, a process called evaporation. As the water vapor rises, it cools and condenses into water droplets and ice, forming clouds. This process is called condensation. When the cloud particles combine and grow larger, they fall from the sky as precipitation. This can be in the form of rain, snow, sleet, or hail.

Measurement

Rainfall is typically measured using a rain gauge. It is expressed in units of length per unit time, typically in millimeters per hour, or in countries where imperial units are more common, inches per hour. The "length", or amount, of rainfall is often reported as the depth of water that would accumulate on a flat, horizontal and impermeable surface during a given amount of time, typically an hour. One millimeter of rainfall is the equivalent of one liter of water per square meter.

Factors affecting rainfall

Rainfall is affected by a number of factors. The amount of moisture in the atmosphere, the amount of heat energy available to convert the liquid water into a gas, the prevailing wind directions, and the height of the land all affect the amount of rainfall that an area will receive.

Types of rainfall

There are three main types of rainfall: convective, orographic and frontal.

Convective rainfall

Convective rain, or showery precipitation, occurs from convective clouds, for example cumulonimbus or cumulus congestus. It falls as showers with rapidly changing intensity. Convective precipitation falls over a certain area for a relatively short time, as convective clouds have limited horizontal extent.

Orographic rainfall

Orographic precipitation occurs when moist air is forced upwards over rising terrain, such as a mountain.

Frontal rainfall

Frontal precipitation occurs when two air masses meet. When warm air meets cold air, the warm air rises over the cold air and forms clouds. Frontal precipitation can produce a wide band of precipitation over an area.

See also

• Drizzle
• Drought
• Flood
• Precipitation

Rainfall Resources
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