The atmospheric pressure at the level of the sea is 14.7 pounds per square inch, which is equivalent to a column of pure water at F. 34 feet in height. That pressure decreases one pound for the first 1,880 feet, and two pounds for the first 3,900 feet, and three pounds for the first 6,080 feet above the level of the sea, and so on until it becomes negligible. The United States Weather Bureau has compiled statistics giving the weight of a cubic foot of aqueous vapor of different percentages of saturation and at various temperatures at the level of the sea. At 32° F. a cubic foot of aqueous vapor ranges from 211 grains with 10 per cent saturation to 2.113 grains with 100 per cent saturation. At 68° F. a cubic foot of aqueous vapor ranges from .748 grains with 10 per cent saturation to 7.480 grains with 100 per cent saturation. At 100° F. a cubic foot of aqueous vapor ranges from 1.977 grains with 10 per cent saturation to 19.766 grains with 100 per cent saturation. At 110° F. a cubic foot of aqueous vapor ranges from 2.611 grains with 10 per cent saturation to 26.112 grains with 100 per cent saturation. Vapor is saturated, when it is at the point of condensation and evapora tion goes on as long as there is any deficit be low 100 per cent of such saturation. From the foregoing meteorological compilation, it will ap pear that the point of saturation and weight of aqueous vapors per cubic foot vary greatly at different temperatures. The matter under this sub-title is involved in the problems of WATER SUPPLY and will receive further con sideration under that title in a succeeding volume of this encyclopedia.
Controlling Factors in RainfalL—At sea level in the tropics over such large bodies of water as the Indian Ocean, the amount of aqueous vapor moving vertically upward may exceed the quantity in a column of air over the Arabian and Sahara deserts. Still the vapor contents of the air over Siberian and Lybian deserts average nearly as much as that of the air over Vienna and Paris. The heat prevents its condensation. Were it cooled, it would produce normal rainfall. Humidity is the amount of aqueous vapor in the atmosphere, as compared with the amount of such aqueous vapor in it, when the atmosphere at a given temperature is 100 per cent saturated. Hu midity, therefore, is relative and in sonic zones and localities it varies inversely, as the tem perature, especially where there may be a lack of moisture as there is east of the Rocky Mountains and over the desert regions of the earth. When water is vaporized, pressure is exerted on atmospheric gases. Thus both verti cal or convective currents may be generated, clouds may be formed and heavy rainfall may ensue, as a result of the cooling of the satu rated water vapor in the upper strata of the atmosphere. The heat, liberated in the cooling process, retards the condensation and checks the rainfall. A rising barometer indicates the absence of vapor pressure, the expansion of dry air, slight evaporation and no rainfall. Humidity, temperature, topography and the physical phenomena already mentioned are fac tors more or less controlling, in the problem of rainfall over a given territory. There are others,
such as the contour of the territory, its ele vation above the level of the sea, the extent of its forests, the configuration of its mountains, the influence of continents upon humidity and character of its seasons. Notwithstanding the complexity of the phenomena, conditioning rain fall or precipitation, the same has been observed over long periods and reduced to mean annual tabulations. There have also been deduced formula to ascertain the presence and amount of evaporation, which, to slight extent only, indicates the amount of rainfall or precipitation, for the former may be both a contributing cause and one of the results of the latter. Dalton, Bige low, Russell, Meyer and others have proposed formula to determine evaporation various conditions of humidity, tempera ture, wind velocity, vapor tension and baro metric pressure. The results obtained from the application of such formula are not identi cal, nor do they always conform to actual measurements, which are difficult to make.
The United States Weather Bureau and the climatological stations of some other na tions have compiled statistical tables of evapo ration over various land and inland water areas, but not over oceans, seas and gulls, which altogether cover three-fourths of the surface of the earth. From all such bodies of water evaporation is continuous and the greatest. These great fountains of the deep supply the vapor-laden clouds, which are swept landward and release their waters in refreshing rains, wintry snows or in some other form of precipi tation. Frequently such vapor-bearing clouds are swept against mountain ranges, as those rising from the Mediterranean Sea are swept against the Alps and those rising from the Pacific Ocean against the Andes Mountains. In some regions precipitation is much greater than it is in others, and over the same region it varies greatly in different years. That is due to the operation and effect of the physical conditions already stated. However, over cy cles of years it is quite uniform, as will here inafter appear. Deforestation decreases and reforestation increases the general average. Forests promote rainfall, retard evaporation and run off and store up precipitation in pools, ponds and lakes, some of which are the sources of streams and rivers. The United States and many other nations have bureaus, or depart ments devoted to scientific forestry, one of the most important and necessary functions of government, if the habitable areas of the earth are to be preserved. Deforestation and conse quent lack of rainfall have rendered many once populous areas now uninhabitable, as in dicated in this article. This and other genera tions ought not to neglect a matter of such vital importance to themselves and to succeed ing generations, but ought to enter upon the systematic reforestation of all properly avail able areas.