Precipitation in meteorology refers to all forms of liquid or solid water particles that form in the atmosphere and then fall to the earth's surface. Types of precipitation include hail, sleet, snow, rain, and drizzle. Frost and dew are not classified as precipitation because they form directly on solid surfaces.
The formation of precipitation may occur at temperatures above or below freezing. Precipitation that is formed in temperatures entirely above freezing is called warm precipitation; cold precipitation involves ice at some stage of the process.
Nearly all precipitation begins with condensation of water vapor about small (diameters between 0.00001 to 0.0001 cm) particles in the air called cloud condensation nuclei. Condensation may occur at relative humidities less than 100 percent for hygroscopic particles (those having an affinity for water) or may be delayed until the relative humidity exceeds 100 percent if the particles are hydrophobic (lacking an affinity for water). Sea-salt particles left behind when sea spray evaporates are particularly effective nuclei.
Saturation of air occurs when rising air currents cool adiabatically (that is, without loss of heat) by expansion. Because the saturation vapor pressure of water decreases exponentially with decreasing temperature (a property often summarized by the statement "cold air can hold less water vapor than warm air"), cooling of a moist air mass by lifting is an efficient mechanism for producing saturation and condensation.
The condensation processes are efficient in producing only cloud drops that are too small (diameters between 0.0001 and 0.005 cm) to have an appreciable fall velocity relative to the air (terminal velocity). In order to produce precipitation particles that are heavy enough to fall to the surface, a cloud drop with a radius of 0.001 cm must increase its radius by a factor of 10 and its volume by a factor of 1,000. In clouds with temperatures above freezing, the growth occurs by coalescence, which is simply the merging of water drops that collide. This merging is facilitated when an electric field is present. Laboratory experiments show that drops will bounce off one another in the absence of an electric field.
Whereas collision and coalescence are efficient means for producing precipitation in the warm, humid tropical regions, the formation of precipitation in middle latitudes usually involves ice. Because the vapor pressure at saturation is less over ice than over water, ice crystals will grow at the expense of water drops when both exist together in a supercooled cloud (which contains liquid drops at temperatures below freezing). This mechanism, named the Bergeron-Findeisen process for the scientists who studied it in the 1930s, is a very efficient mechanism for growing ice crystals to a size big enough to fall to the surface.
Although most precipitation in middle latitudes begins as snow at altitudes above the freezing level (about 3 km/1.8 mi.), the form of the precipitation reaching the surface depends on the temperature structure of the atmospheric layers through which the precipitation falls. If the temperature near the ground is warm enough, the snow has time to melt and reaches the ground as rain. A warm layer aloft and a subfreezing layer at the surface may produce sleet (ice pellets) or freezing rain (rain that freezes immediately upon contact with surface objects). Hail occurs when alternating strong updrafts and downdrafts cause ice crystals to pass repeatedly through layers that contain supercooled water. The frequent passage through these layers allows the water to freeze around the growing hailstone and to accumulate in one layer after another. Strong updrafts with velocities of 10 to 30 m/sec (22 to 67 mph) are necessary to prevent the growing hailstone from falling to the surface.
Precipitation is produced whenever moist air rises sufficiently to produce saturation, condensation, and the growth of the precipitation particles. Thunderstorms, with updraft wind components of more than 30 m/sec (67 mph), carry moist low-level air to the tropopause, the upper boundary of the troposphere, and hence can produce heavy precipitation in short amounts of time. Rainfall rates of 10 cm/hr (4 in./hr) are not uncommon in thunderstorms.
Less intense rainfall occurs over much larger scales (horizontal distances of 1,000 km/620 mi. or more) when warm, moist air is lifted over a frontal surface or put in circulation around extratropical cyclones (those poleward of the tropical easterlies). In the Northern Hemisphere the favored location for this frontal type of precipitation is north of the surface warm front and north of the track of an eastward-moving cyclone. Orographic precipitation is produced when air flowing over mountainous terrain is forced to rise. An example is the heavy precipitation that falls on the Sierra Nevadas in western North America. Moist Pacific air forced over the mountains produces up to 250 cm (100 in.) of liquid precipitation annually. As the air descends over the lee slopes and warms adiabatically, the rainfall decreases abruptly. This rain shadow effect can produce extreme variability in rainfall over short horizontal distances.
Because the annual distribution of precipitation depends on small-scale factors such as orographic variability as well as the vertical branches of the average global circulations (the general circulation), the geographic distribution of precipitation is not simple. In general, the average rising branch of the general circulation that occurs in a band between latitudes 15° S and 15° N results in heavy equatorial rainfall. Sinking motion occurs on the average over a band approximately 15° of latitude wide centered at 30° north and south, and rainfall is at a minimum near these latitudes. A second rainfall maximum occurs between 40° and 50° latitude because of the high frequency of traveling cyclones there. Finally, the annual precipitation drops off sharply toward both poles because the cold air cannot contain enough moisture for heavy precipitation.
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