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precipitation Water that falls to the surface from the atmosphere in liquid, solid, or fluid form is called precipitation. Whether it falls as rain, drizzle, fog, snow, sleet, freezing rain, or hail, it is measured as a liquid-water equivalent. The types and amounts of precipitation in different parts of the world vary greatly, from places that have never had any measurable precipitation to places that regularly receive several to more than 10 feet (hundreds of centimeters) of rain per year. Precipitation is strongly seasonal in some places, with dry and wet seasons, and distributed more regularly in other climates.
Rain is liquid precipitation with droplets greater than 0.02 inches (0.5 mm) in diameter, whereas drizzle has droplets between 0.008-0.02 inches (0.2-0.5 mm) in diameter. Fog is a cloud whose base is at the surface and has smaller particles that only truly become precipitation when wind drives them against surfaces or the ground. Freezing rain and drizzle both fall in liquid form but freeze upon hitting cold surfaces on the ground, creating a frozen coating known as glaze. Sleet consists of frozen ice pellets less than 0.2 inches (5 mm) in diameter, and hail consists of larger transparent to opaque particles that typically have diameters of 0.2-0.8 inches (5-20 mm) but sometimes are as large as golf balls or rarely even grapefruit. snow is frozen precipitation consisting of complex hexagonal ice crystals that fall to the ground.
In tropical regions and temperate climates in warmer parts of the year, most precipitation falls as rain and drizzle. Heavy rain is defined as more than 0.16 inches (4 mm) of precipitation per hour, moderate rain falls between 0.16-0.02 inches (4 mm and 0.5 mm) per hour, and light rain (commonly called drizzle) is less than 0.02 inches (0.5 mm) per hour. Frequent and steady rains characterize some regions; others are characterized by infrequent but intense downpours, including thunderstorms that may shed hailstones. At high elevation, high latitudes, and in midlatitudes during colder months, most precipitation falls as frozen solid particles. Most frozen precipitation falls as snow that typically has a water equivalent of one-tenth the amount of snow that falls (i.e., 10 cm of snow equals 10 mm of rain). More freezing rain and sleet than snow characterize some regions, particularly coastal regions influenced by warm ocean currents.
Uplift within clouds or larger-scale systems are generally necessary to initiate the formation of water droplets that become precipitation. Convection cells in thunderheads, air forced over mountains, zones of convergence along fronts, and cyclonic systems can all produce dramatic uplift and induce precipitation. In order to form precipitation, the small water (or ice) droplets that are separated by very wide
Dark storm clouds at beach (Javarman, Shutterstock, Inc.)
spaces must coalesce into particles large enough to fall as precipitation. Additionally, the particles must overcome the forces of evaporation as they rise or fall through unsaturated air in order to make it to the ground. Rapid lateral and vertical motions in clouds, leading to collisions between particles, aid the coalescence of particles, and gravity then accelerates particles to the ground with larger particles initially falling faster than smaller ones since they are less affected by updrafts. Large particles therefore tend to collide with and incorporate smaller particles. After frozen particles form in the upper levels of vertically extensive cloud systems, they alternately fall into and rise out of lower levels, where they partially melt, grow, and rise on updrafts. Such cycling produces relatively large particles that may fall as precipitation.
See also climate; climate change; clouds; hurricanes.
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