Precipitation

In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull.

[2] Moisture that is lifted or otherwise forced to rise over a layer of sub-freezing air at the surface may be condensed by the low temperature into clouds and rain.

A stationary front is often present near the area of freezing rain and serves as the focus for forcing moist air to rise.

Eventually, the cloud droplets will grow large enough to form raindrops and descend toward the Earth where they will freeze on contact with exposed objects.

In mountainous areas, heavy precipitation is possible where upslope flow is maximized within windward sides of the terrain at elevation.

On the leeward side of mountains, desert climates can exist due to the dry air caused by compressional heating.

Saturn's largest satellite, Titan, hosts methane precipitation as a slow-falling drizzle,[6] which has been observed as rain puddles at its equator[7] and polar regions.

[14] The dew point is the temperature to which a parcel of air must be cooled in order to become saturated, and (unless super-saturation occurs) condenses to water.

[18] An elevated portion of a frontal zone forces broad areas of lift, which form cloud decks such as altostratus or cirrostratus.

[20] The air can rise due to convection, large-scale atmospheric motions, or a physical barrier such as a mountain (orographic lift).

[29] As these larger water droplets descend, coalescence continues, so that drops become heavy enough to overcome air resistance and fall as rain.

[37] Like other precipitation, hail forms in storm clouds when supercooled water droplets freeze on contact with condensation nuclei, such as dust or dirt.

These large crystals are an efficient source of precipitation, since they fall through the atmosphere due to their mass, and may collide and stick together in clusters, or aggregates.

[35] Occult deposition occurs when mist or air that is highly saturated with water vapour interacts with the leaves of trees or shrubs it passes over.

[52] A wide variety of weather can be found along an occluded front, with thunderstorms possible, but usually their passage is associated with a drying of the air mass.

Moisture is removed by orographic lift, leaving drier air (see katabatic wind) on the descending and generally warming, leeward side where a rain shadow is observed.

Local climates vary considerably on each island due to their topography, divisible into windward (Koʻolau) and leeward (Kona) regions based upon location relative to the higher mountains.

[58] In South America, the Andes mountain range blocks Pacific moisture that arrives in that continent, resulting in a desertlike climate just downwind across western Argentina.

[59] The Sierra Nevada range creates the same effect in North America forming the Great Basin and Mojave Deserts.

[60][61] Extratropical cyclones can bring cold and dangerous conditions with heavy rain and snow with winds exceeding 119 km/h (74 mph),[62] (sometimes referred to as windstorms in Europe).

[65] Because of this temperature difference, warmth and moisture are transported upward, condensing into vertically oriented clouds (see satellite picture) which produce snow showers.

[67] The wet, or rainy, season is the time of year, covering one or more months, when most of the average annual rainfall in a region falls.

[78] On the large scale, the highest precipitation amounts outside topography fall in the tropics, closely tied to the Intertropical Convergence Zone, itself the ascending branch of the Hadley cell.

[81] An exception to this rule is in Hawaii, where upslope flow due to the trade winds lead to one of the wettest locations on Earth.

Thermal infrared (IR) sensors record a channel around 11 micron wavelength and primarily give information about cloud tops.

[99] The humid subtropical climate zone is where winter rainfall (and sometimes snowfall) is associated with large storms that the westerlies steer from west to east.

While a regular rain pattern is usually vital to healthy plants, too much or too little rainfall can be harmful, even devastating to crops.

Drought can kill crops and increase erosion,[106] while overly wet weather can cause harmful fungus growth.

Partly as a result of this warming, monthly rainfall is about 28% greater between 32 and 64 kilometres (20 and 40 mi) downwind of cities, compared with upwind.

[116] Starting in the mid to late 1990s, QPFs were used within hydrologic forecast models to simulate impact to rivers throughout the United States.