RAINFALL. Rainfall or precipitation, the generic term applied to the condensation of aqueous vapor into cloud, fog dew, rain, snow, frost and hail, varies greatly in the several zones and in the successive seasons and at dif ferent altitudes. Temperature, by which is meant the temperature of the air at a given place and elevation, is one of the controlling physical factors in determining its annual rain fall or precipitation. The atmosphere, con sisting according to Prof. Julius Hann of the University of Vienna of the following elements and proportions, nitrogen 78.03, oxygen 20.99, argon 0.94, carbon dioxide 0.03, hydrogen 0.01, neon. 0.0015 and helium 0.00015 parts and sub ject to rapid and continual changes in tem perature, largely conditions precipitation over i the earth, itself, owing to its varying topog raphy, one of the controlling factors in the problem. The vast expanse of oceans, seas, gulfs and inland waters and the uplift and ex tent of its various mountain ranges and the expanse and location of its continents in their relation to oceans and to the torrid, temperate and frigid zones, together with the rotation of the earth on its axis and its revolution around the sun, produce different atmospheric conditions, conducive to great variation in pre cipitation. Furthermore the atmosphere rapidly decreases in density upwards from the surface of the earth and solar radiation likewise de creases in its intensity. The sun's rays pene trate the atmosphere at various angles and these are continually changing so that the amount of radiant energy is neither constant nor uni form in the different latitudes and seasons. 'Owing to these physical conditions and the inclination of the earth's axis to the plane of the ecliptic, producing unequal day and night in the four seasons, there are ceaseless move ments in the atmosphere itself, such as currents, winds and storms, that affect its humidity and the amount of rainfall. Both terrestrial and solar disturbances affect the atmosphere and the amount of rainfall. In some localities periodic winds prevail with such constancy that they are denominated by such terms as 'trade winds,' 'monsoons," and sea breezes* and some ocean currents are no less constant. All these and other physical phenomena must be considered in accounting for precipitation and its variation in different localities.
Evaporation is the physical transformation of solids and liquids into gases, due to the kinetic energy of their molecules to diffuse themselves through space. Heat increases that energy of the molecules of water in whatever form it may be, though the coefficient of dif fusion for aqueous vapor is greatest at the earth's surface. Professor Cleveland Abbe of the United States Weather Bureau has said that 'the vapor constituent of the atmosphere is not distributed according to the law of gaseous diffusion, but like temperature and the ratio between oxygen and nitrogen, is controlled by other laws, prescribed by the winds and currents, namely— convection.' That may be
either horizontal; due to the winds, or vertical, due to upward or downward currents. Tem perature is also one of the most important factors in the problem of evaporation and in the distribution of aqueous vapors. Atmos pheric pressure is another important factor in the problem. Atmospheric gases retard the dif fusion of aqueous gases and act independently of each other. Professor Adolph F. Meyer of the University of Minnesota states that (water in the gaseous state has a specific gravity of .622, as compared with dry air, and that `the pres sure of water vapor at a given temperature is greater than the pressure of an equal amount of dry air at the same temperature,' so that when dry air is displaced in part by aqueous vapor, the weight of the cubic content is reduced. Professor Meyer maintains that when water at 212° F. passes into a gaseous state, it increases in volume 1,658 times and stores up 970 British thermal units of heat for each pound of water so transformed. The United States Weather Bureau gives the elastic pres sure of saturated vapor at 32° F. as 0.180 of an inch of barometric pressure, at 68° F. as 0.684 of an inch of barometric pressure, at 100° F. as 1.916 inches of barometric pressure and at 110° F. as 2.576 inches of barometric pressure. It thus appears that vapor pressure in creases much more rapidly than does tempera ture. Professor Meyer also states that eat ordinary open air temperatures, the elas tic pressure" (also known as 'vapor ten sion" and °gaseous pressure') °of saturated water vapor is substantially doubled for every 20° F. increase in temperature,' and oat a given temperature and pressure only a certain definite amount of water can oc cupy a given space and as soon as the space is saturated with moisture, dew will be de posited.' Then heat is liberated, which tends to retard a further fall in the temperature. The higher the temperature at sea-level, the greater may be the amount of aqueous vapor and stored up heat and the slower will be the condensation and cooling. In the heat of summer, the air rises, expands, cools and as it does so, it loses its capacity to hold vapor and condensation may ensue. Professor Meyer says that 'If the temperature of the vapor is 68° F. it will cool in rising 425 feet.' The heat so liberated warms the surrounding atmosphere and tends to prevent further condensation, so summer showers are often of short duration.