RAIN (AS. regn, run, Goth. rign, OHG. vegan, Ger. Regen, rain; connected with Lat. rigarc, Gk. PpOzetv, breehein, to wet). Drops of water formed in the atmosphere by the condensa tion of its aqueous vapor and falling rapidly by virtue of their weight: the very small drops that fall slowly are spoken of as mist, cloud, o• fog. The largest drops of rain that have been'Ineas fired are as much as 0.25 to 0.30 inch in diameter and fall at the rate of from 15 to 25 feet per second. The smallest drops that are likely to be spoken of as rain are about one-twentieth of an inch in diameter and fall at the rate of about five feet per second. As rain water is condensed vapor that had previously been evaporated from distant water surfaces, there fore, in accordance with the laws of evaporation, it world he chemically pure water were it not for a small percentage of foreign substance which it gathers to itself from the atmosphere. Rain water washes down out of the air dust, soot, pollen, spores of fungi. and many other solid substances: Ordinary rain water contains an appreciable percentage of dissolved oxygen, nitro gen. ammonia, and carbonic acid gas, and in spe cial ca-ses it is found to contain nitric acid, sulphuric acid, and other components of the impure air of cities. The acid and alkaline im purities generally increase the power of the rain water to dissolve the mineral constituents of the earth's crust; the gases make it possible for plants and animals to live in rivers and ponds, which would be impossible if the water were chemically pure. Rain water only becomes whole some potable water for man's use after it has been thoroughly filtered through the earth, whence it issues as springs of pure water.
Up to the middle of the nineteenth century rain was supposed to be naturally formed by the mixture of cold and warm masses of moist air, but the publication of Espy's Philosophy of Storms (Boston, 1842), and his life-long conten tion that cloud and rain are not due to cooling by mixture o• by radiation, but are a consequence of the cooling of the atmosphere by virtue of the work done in expansion, supported as he was by Professor Joseph Henry, Sir William Thom son (Lord Kelvin), and other physicists, finally led meteorologists to study the great thermodynamic problems of the atmos phere. When air is forcibly compressed, the work done by compression is repre sented by the increase in temperature of the confined air; .rice versa when the air expands
by diminution of pressure, the work done in expansion is represented by the heat abstracted from the expanding air, which therefore ex periences a corresponding cooling. The laws of convective equilibrium governing the temperature and the volume of a unit mass of rising air were first expressed in the exact language of mathe matical physics by Sir William Thomson in 1861. Graphic methods of treating the complex meteorological problems were de vised by Hertz in 1884, and improved by Von Bezold in 1888 and Neuboff in 1900. The analytical treatment of the subject is given by F. H. Bigelow with convenient tables in his report of 1900 Oa the International Observations of Clouds. When warm moist air ascends from near the earth's surface it cools by expansion: if no heat is added or subtracted, it is said to cool adiabatically, and does so at the rate of about one degree Centigrade per 99 meters of ascent, or one degree Fahrenheit for 185 feet, until it reaches an altitude at which its tem perature is the same as the temperature of the dew point of the original air. At and above this elevation cloud is formed as the air ascends. If the rise continues until the air has cooled to the temperature of freezing point of water, then the watery cloud particles begin to change to ice, giving out a little of their latent heat as they do so. When in the course of its further ascent all the cloud particles have become ice, then any additional rise will he accompanied by the formation of snow crystals. This latter condition would continue to exist throughout the further ascent of the air were it not that in these higher regions the formation of snow is very slight. if the sun is shining upon the clouds, the process ceases to be adiabatic, and the particles of water o• ice may be immediately evaporated hack into vapor. Owing to the resist ance of the air, the cloud particles fall very slowly to the ground, o• may, in fact, be upheld in definitely by a gentle ascending current. But if numerous small particles combine together into drops of water, the latter may fall rapidly to the ground as rain. The above paragraph cor rectly explains the formation of cloud by cooling clue to expansion, but nothing is as yet known satisfactorily as to the process by which large raindrops are formed from the minute cloud particles.