Precipitation types

In meteorology, "precipitation types" can include the character or phase of the precipitation which is falling to ground level. There are three distinct ways that precipitation can occur. Convective precipitation is generally more intense, and of shorter duration, than stratiform precipitation. Orographic precipitation occurs when moist air is forced upwards over rising terrain, such as a mountain.

Precipitation can also fall in either liquid or solid phases, or transition between them. Liquid forms of precipitation include rain and drizzle. Rain or drizzle which freezes on contact within a subfreezing air mass gains the preceding adjective "freezing", becoming known as freezing rain or freezing drizzle. Frozen forms of precipitation include snow, ice needles, sleet, hail, and graupel. Intensity is classified either by rate of fall, or by visibility restriction.

Precipitation falls in many forms, or phases. They can be subdivided into:

The parenthesized letters are the METAR codes for each phenomenon.

Precipitation occurs when the air becomes saturated with water vapor, and can no longer hold all of it in vapor form. This occurs when moist air cools, usually when an airmass rises through the atmosphere. However, an airmass can also cool (e.g. through radiative cooling, or ground contact with cold terrain) without a change in altitude.

Convective precipitation occurs when air rises vertically through the (temporarily) self-sustaining mechanism of convection. Stratiform precipitation occurs when large air masses rise diagonally as larger-scale atmospheric dynamics force them to move over each other. Orographic precipitation is similar, except the upwards motion is forced when a moving airmass encounters the rising slope of a landform such as a mountain ridge.

Convection occurs when the Earth's surface, within a conditionally unstable or moist atmosphere, becomes heated more than its surroundings, leading to significant evaporation. Convective rain, or showery precipitation, occurs from convective clouds, e.g., 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. Most precipitation in the tropics appears to be convective; however, it has been suggested that stratiform precipitation thunderstorms.

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