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rain,precipitation in liquid form. It consists of drops of water falling from clouds; if the drops are very small, they are collectively termed drizzle. Rain plays a key role in the hydrologic, or water, cycle in which moisture from the oceans evaporates, condenses into clouds, precipitates back to earth, and eventually returns to the ocean via runoff into streams and rivers to begin the cycle again.
Formation of Raindrops
Clouds contain huge numbers of tiny droplets of moisture. Raindrops are formed when these tiny droplets are enlarged, first by moisture from the surrounding air condensing on them and then by coalescing with other droplets during their descent. Raindrops vary in size from about 0.02 in. (0.5 mm) to as much as 0.33 in. (8 mm) in thunderstorms. From the time they leave the bottom of the cloud, evaporation takes place and, if the cloud is high, the air warm and dry, and the raindrops small, so that they fall slowly, they may evaporate completely before they reach the earth. If they do so, the drops are called virga.
Measurement of Rainfall
There are thousands of stations throughout the world where rainfall observations and records are made. Included in such records is the fall of snowsnow,
precipitation formed by the sublimation of water vapor into solid crystals at temperatures below freezing. Sublimation resulting in the formation of snow takes place about a dust particle, as in the formation of raindrops.
..... Click the link for more information. , reduced to its equivalent in rain. Rainfall is measured, in terms of inches or millimeters of depth, by means of a simple receptacle-and-gauge apparatus or by more complex electrical or weighing devices placed where eddies of air will not interfere with the normal fall of the raindrops. In addition to the daily, monthly, and annual totals, the depth of individual rainfalls and their intensity (amount of rain falling during a specific period of hours or minutes) and other pertinent facts are recorded.
Distribution of Rainfall
One of the primary elements in climateclimate,
average condition of the atmosphere near the earth's surface over a long period of time, taking into account temperature, precipitation (see rain), humidity, wind, barometric pressure, and other phenomena.
..... Click the link for more information. and a factor of tremendous importance in the distribution of plant and animal life, rainfall varies from less than an inch annually in a desertdesert,
arid region, usually partly covered by sand, having scanty vegetation or sometimes almost none, and capable of supporting only a limited and specially adapted animal population.
..... Click the link for more information. to more than 400 in. (1,000 cm) where the monsoons strike the Khasi hills in Assam, India, and on the windward slopes of Hawaiian mountains. In the United States the range is from less than 2 in. (5 cm) in Death Valley, Calif., to more than 100 in. (250 cm) on the coast of Washington state; in most of the country the average rainfall is between 15 and 45 in. (38 and 114 cm) annually.
Factors controlling the distribution of rainfall over the earth's surface are the belts of converging-ascending air flow (see doldrumsdoldrums
or equatorial belt of calms,
area around the earth centered slightly north of the equator between the two belts of trade winds. The large amount of solar radiation that arrives at the earth in this area causes intense heating of the land and ocean.
..... Click the link for more information. ; polar frontpolar front,
zone of transition between polar and tropical air masses. Its average position during the winter is at about 30° lat. and during the summer at about 60° lat.
..... Click the link for more information. ), air temperature, moisture-bearing winds, ocean currents, distance inland from the coast, and mountain ranges. Ascending air is cooled by expansion, which results in the formation of clouds and the production of rain. Conversely, in the broad belts of descending air (see horse latitudeshorse latitudes,
two belts of latitude where winds are light and the weather is hot and dry. They are located mostly over the oceans, at about 30° lat. in each hemisphere, and have a north-south range of about 5° as they follow the seasonal migration of the sun.
..... Click the link for more information. ) are found the great desert regions of the earth, descending air being warmed by compression and consequently absorbing instead of releasing moisture. If the temperature is low, the air has a small moisture capacity and is able to produce little precipitation. When winds blow over the ocean, especially over areas of warm water (where evaporation of moisture into the air is active) toward a given coastal area, that area receives more rainfall than a similar area where the winds blow from the interior toward the oceans. Areas near the sea receive more rain than inland regions, since the winds constantly lose moisture and may be quite dry by the time they reach the interior of a continent.
The windward slopes of mountain ranges generally receive heavy rainfall; the leeward slopes receive almost no rain. The southwest coast of Chile, the west coast of Canada, and the northwest coast of the United States receive much rain because they are struck by the moisture-bearing westerlies from the Pacific and are backed by mountains that force the winds to rise and drop their moisture. The territories immediately east of the regions mentioned are notably dry. See weatherweather,
state of the atmosphere at a given time and place with regard to temperature, air pressure (see barometer), wind, humidity, cloudiness, and precipitation. The term weather
..... Click the link for more information. .
Rain and Religion
The need for rain at a particular time and the dangers attendant upon droughtdrought,
abnormally long period of insufficient rainfall. Drought cannot be defined in terms of inches of rainfall or number of days without rain, since it is determined by such variable factors as the distribution in time and area of precipitation during and before the dry
..... Click the link for more information. brought rain prominently into the religion of most agricultural peoples. Rain-gods and thunder-gods are more prominent in many mythologies than sun-gods, and they have been propitiated in various ways in different cultures. The rain dances of the Native Americans may, however, be said to be generally typical of all in the elaborate symbolic gestures and patterns and in the extensive use of drums and rattles (presumably sympathetic magic by imitation of the sounds of thunder and showering rain). Because the purpose is to make the fields bear crops, the connection of such rites with those of fertility is obvious.
See J. Burton and K. Taylor, The Nature and Science of Rain (1997); J. Williams, The Weather Book (2d ed. 1997).
atmospheric precipitation falling from clouds in the form of droplets of water measuring from 0.5 to 6-7 mm in diameter. Liquid precipitation with droplets of smaller diameter is called drizzle. Droplets larger than 6-7 mm in diameter break up into smaller drops in falling. The intensity of rain varies from 0.25 mm per hour (very light rain) to 100 mm per hour (heavy rain).
As a rule, rain falls from mixed clouds (predominantly nimbostratus and altostratus) containing supercooled droplets and ice crystals at below-zero temperatures. The saturation pressure of water vapor over the droplets is greater than over the ice crystals at the same temperature. For this reason, a cloud, even one not saturated with water vapor with respect to the water droplets will be oversaturated with respect to the crystals. This leads to the growth of the crystals with the simultaneous evaporation of the droplets. In growing larger and heavier, the crystals fall from the cloud, crystallizing the supercooled drops onto them in the process. In entering the lower part of the cloud or below the cloud in layers with above-freezing air temperature, the crystals melt, turning into raindrops. A lesser role is played by the coalescence of cloud drops in the formation of rain.
What does it mean when you dream about rain?
Rain is a natural element of cleansing. Because it is essential to plant growth, rain is a symbol of fertility. In a dream, rain may indicate a new direction of thought and purpose—washing away the old and fertilizing the new. Alternatively, gray, dismal clouds and rain may indicate desolation or barrenness.