Rain

water falling in drops through the atmosphere. Air is capable of containing and carrying considerable quantities of moisture. When it has all the moisture it can carry, it is said to be saturated. The warmer the air, the more moisture it can carry, and the more it takes up by evaporation from the surface of open water, moist land, and everything green. Whenever moisture-laden air is cooled until it cannot hold all its moisture, the water settles down in the form of dew, mist, rain, snow, or hail. Whenever hot air, heavily laden with moisture, is cooled by contact with a cold wind, a shower or fall of rain is the inevitable result. The so-called cloudbursts which sometimes occur, especially in mountainous countries, with great loss of life and property, are caused by the coming together of a body of hot air containing a large amount of moisture and a chill wind.

The rainfall in different localities varies greatly. More rain is to be expected in the tropics, because evaporation is greater. The air naturally takes up more moisture from open water than from land. Other conditions being equal, localities near the sea and large lakes have a heavier rainfall than those which are inland. A forest or lake region in which evaporation takes place rapidly may be expected to have more rainfall than a dry or desert region. For this reason chopping away forests and turning grass lands into bare fields diminishes the supply of rain. As moisture has weight, even when it is invisible, low localities are more apt to have an abundant rainfall than mountain heights.

Winds have a great influence on the distribution of rain. A wind that sweeps over a moist country carries a supply of moisture to distant regions. A mountain barrier is a great obstacle to the distribution of rain. In passing over mountain ranges, winds are chilled and drop their moisture, so that the farther side of the range receives sometimes none at all. There is a region on the west side of the Andes, lying near the Pacific Ocean, which has almost no rainfall, because the prevailing winds blow from the east. Moisture from the Atlantic is deposited on the Andes, while moisture from the Pacific is carried away westward, so that this region gets almost none at all. A study of temperature, winds, and mountain ranges will show that the deserts of the world are occasioned either by distance from the sea or by intervening mountain ranges. In either case, the air loses its moisture before it arrives, or else it is so heated that it never cools below the dew point. The cool Mediterranean winds that blow over the Sahara are not devoid of moisture; but they grow hot as they travel southward, and have no opportunity to deposit their moisture.

So far as known, the greatest annual rainfall occurs on certain southern slopes of the Himalayas, where the moisture-laden winds from Bengal Bay strike the mountains. The month of December is almost rainless, but during the summer the rainfall averages ten inches a day with a total rainfall for the year of over 500 inches. In one unusual season enough rain fell in the locality named to have formed a lake sixty-seven feet deep, had it not run off through the Ganges. As might be expected, such a torrent of rain sweeps the mountains bare of soil.

It is believed that there are no regions absolutely rainless. The average amount of rainfall the world over, counting both sea and land, is estimated variously at from forty to sixty inches. The evaporation requisite to maintain this amount of rain would dry up the ocean in from 10,000 to 15,000 years, if water did not find its way back again.

Rain making, or the causing of rainfall by artificial means, has long been a subject both of superstition and of scientific investigation. Most savage people whose living was at all dependent on what they raised had their rain dances and rain doctors, whose prayers, incantations, shaking of mysterious ingredients in a sack, beating of tom-toms, and the like, were supposed to have an influence on the clouds. More than one rain doctor's life has been saved and his reputation made by the timely appearance of a cloud; and very likely these pretended makers of rain knew enough about the approach of rainy seasons to time their efforts wisely. A theory was long prevalent that the heavy concussions accompanying the cannonading of battles caused rain; but it has been abandoned. It is still acknowledged that heavy rainfall has sometimes followed heavy cannonading, but there is no apparent explanation for the fact that the discharge of artillery is followed more often by dry weather. In 1891 Congress expended an appropriation through the department of agriculture in making experiments in Texas. Rain certainly followed the discharge of cannon, and a favorable official report was sent in to the department; but those who were in a position to know say that it had begun to sprinkle before the cannon were fired.

The following table gives the mean annual rainfall in inches for a number of representative localities:

Someone with a gift for figures has computed the entire rainfall of the United States at 215,000,000,000,000 cubic feet yearly. Of this vast amount one-half evaporates, one-third flows to the sea, one-sixth sinks into the soil.

See Dew; Hail; Frost; Snow; Rain Gauge