Rain
From Encyclopediak
Rain, falling drops of water, the descent of which is due to natural causes. For the formation of rain the water vapor of the atmosphere must undergo condensation. This water vapor is derived through evaporation from moist surfaces of the land, but chiefly from the ocean. Evaporation is continually going on. A given volume of air can contain only a certain amount of this water vapor at a given temperature, and when this amount is present the volume of air is said to be saturated. If the temperature of such a volume of saturated air falls to the dew point, the vapor is condensed or changed back to water in the form of minute particles constituting clouds. When the tiny globules of water which form the cloud unite, the weight of the drop causes it to fall, unless upheld by strong air currents. Although water vapor is chemically pure, rain water, because it has been condensed upon particles of solid matter, contains quantities of soot, dust, pollen of plants and numerous other impurities. Rain water also contains a small percentage of ammonia, carbonic acid gas and nitrogen. See Dew; Cloud.
RAINFALL. The size of raindrops varies according to conditions of the atmosphere. If rain falls through dry air the drops may be greatly reduced in size by evaporation before they reach the surface, but if the air is saturated at the time of the rainfall the drops are apt to promote condensation upon their surfaces in their downward passage and thus greatly increase their size. A. fall of droplets smaller than one-twentieth of an inch in diameter is commonly called a mist. Numerous instances of abnormal rainfall have been recorded. It sometimes happens that rain fails to reach the ground because of the excessive dryness of the atmosphere. In such cases the moisture is evaporated and changed back to vapor in mid-air. These ‘suspended” showers are a common phenomenon in the desert areas of Arizona and in parts of other Western States. When water vapor is condensc1 at a rapid rate near the surface or above moist air, a heavy rainfall often results in which the water appears to conic down in sheets. This is known as a cloudburst. Although rain commonly falls from clouds, small quantities of rain have been observed to fall from a cloudless sky. This occurs when the temperature of air nearly free from dust falls to the dew point and there is a large quantity of water vapor to be condensed. As moisture condenses only upon a solid substance, each of the few solid motes becomes covered with a heavy coating of water, and drops are formed instead of cloud particles, and these drops fall as rain.
DISTRIBUTION. The rainfall of the earth is very unequally distributed, with the result that some localities are continually drenched while others are desert wastes. The geographical distribution of rain depends upon the distance from the sea, winds and the irregularities of the land. All coast lands visited regularly by winds blowing from the sea have heavy rainfall. The reason is obvious. The windward side of high mountains usually have heavy rainfall, while the opposite slopes are dry. This is due to the fact that when rain-bearing winds traveling horizontally reach the mountains they invariably ascend their slopes and are chilled and condensed, giving rise to a heavy downpour. The heaviest rainfall in the world, on the southern s1opes of the Himalayas, is due to the combination of the three powerful rain-producing agencies. The monsoon winds blowing from the Indian Ocean are intercepted by the mountains and deposit their moisture in destructive abundance. The average precipitation is 500 inches a year in the areas of greatest rainfall. In general, the rainfall is heavier in tropical regions than elsewhere, because of the greater evaporation. Here the annual variation in humidity is greater than that of temper- attire, and causes periodic or seasonal rains. In the United States the regions of the greatest rainfall are: the northwest coast, which averages 100 inches, the northeastern shore of Mexico, the southeast coast of Florida and a portion of the North Carolina coast. The region of least rainfall is the southwest part of Arizona. See Climate; Humidity.
