Precipitation types
Precipitation types
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Precipitation types

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Precipitation types

In meteorology, the different types of precipitation often include the character, formation, 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 and condenses on the slope, such as a mountain.

Precipitation can fall in either liquid or solid phases, is mixed with both, or transition between them at the freezing level. Liquid forms of precipitation include rain and drizzle and dew. Rain or drizzle which freezes on contact with a surface within a subfreezing air mass gains the preceding adjective "freezing", becoming the known freezing rain or freezing drizzle. Slush is a mixture of both liquid and solid precipitation. Frozen forms of precipitation include snow, ice crystals, ice pellets (sleet), hail, and graupel. Their respective intensities are classified either by rate of precipitation, or by visibility restriction.

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

The parenthesized letters are the shortened METAR codes for each phenomenon.

Precipitation occurs when evapotranspiration takes place and local air becomes saturated with water vapor, and so can no longer maintain the level of water vapor in gaseous form, which creates clouds. This occurs when less dense moist air cools, usually when an air mass rises through the atmosphere to higher and cooler altitudes. However, an air mass can also cool without a change in altitude (e.g. through radiative cooling, or ground contact with cold terrain).

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 winds and atmospheric dynamics force them to move over each other. Orographic precipitation is similar, except the upwards motion is forced when a moving air mass encounters the rising slope of a landform such as a mountain ridge or slope.

Convection occurs when the Earth's surface, especially within a conditionally unstable or moist atmosphere, becomes heated more than its surroundings and in turn leading to significant evapotranspiration. Convective rain and light precipitation are the result of large convective clouds, for example cumulonimbus or cumulus congestus clouds. In the initial stages of this precipitation, it generally falls as showers with a smaller area and a rapidly changing intensity. Convective precipitation falls over a certain area for a relatively short time, as convective clouds have limited vertical and horizontal extent and do not conserve much water. Most precipitation in the tropics appears to be convective; however, it has been suggested that stratiform and convective precipitation often both occur within the same complex of convection-generated cumulonimbus.

Graupel and hail indicate convection when either or both are present at the surface. They are indicative that some form of precipitation forms and exists at the freezing level, a varying point in the atmosphere in which the temperature is 0°C. In mid-latitude regions, convective precipitation is often associated with cold fronts where it is often found behind the front, occasionally initiating a squall line.

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