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Formation of Raindrops
Measurement of Rainfall
Distribution of Rainfall
One of the primary elements in climate and a factor of tremendous importance in the distribution of plant and animal life, rainfall varies from less than an inch annually in a desert 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 doldrums; polar front), 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 latitudes) 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 weather.
Rain and Religion
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.