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. The saturation pressure of water vapor over the droplets is greater than over the ice crystals at the same temperature.
The intensity of rain varies from 0.25 mm per hour (very light rain) to 100 mm per hour (heavy rain).Īs a rule, rain falls from mixed clouds (predominantly nimbostratus and altostratus) containing supercooled droplets and ice crystals at below-zero temperatures. Droplets larger than 6-7 mm in diameter break up into smaller drops in falling. Liquid precipitation with droplets of smaller diameter is called drizzle.
Rain and ReligionĪtmospheric precipitation falling from clouds in the form of droplets of water measuring from 0.5 to 6-7 mm in diameter. The territories immediately east of the regions mentioned are notably dry. 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 windward slopes of mountain ranges generally receive heavy rainfall the leeward slopes receive almost no rain. 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. 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. If the temperature is low, the air has a small moisture capacity and is able to produce little precipitation. 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.
Ascending air is cooled by expansion, which results in the formation of clouds and the production of rain. (38 and 114 cm) annually.įactors 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. (250 cm) on the coast of Washington state in most of the country the average rainfall is between 15 and 45 in. (5 cm) in Death Valley, Calif., to more than 100 in. In the United States the range is from less than 2 in. (1,000 cm) where the monsoons strike the Khasi hills in Assam, India, and on the windward slopes of Hawaiian mountains. 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. 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. 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. Included in such records is the fall of snow, reduced to its equivalent in rain.
There are thousands of stations throughout the world where rainfall observations and records are made.
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