The blanket of water vapor that forms over a large body of water prevents the wind from exercising its full effect in promoting evaporation, and measurements made with isolated pans in the same desert give values nearly twice as great. Even more remarkable are the measurements made under the almost cloudless skies of the upper Nile Valley, in Egypt, where, at Wadi Haifa, the depth of water evaporated amounts to more than 19 feet a year! It is common to think of an arid region as one having merely a deficient rainfall, but it is the prevailing dryness of the atmosphere that gives to such regions some of their most striking characteristics.
The typical vegetation of a dry climate is conspicuously adapted to preventing harmful loss of water by evaporation; leaves are often small or lacking, or they have thick tissues for storing water, protected by a leathery cuticle. Of some other effects of aridity Dr. Billings says, in describing the climate of Yuma, Arizona: "The air is so dry that the skin becomes dry and hard, and the hair crisp. The furniture falls to pieces. Newspapers have to be handled with the greatest care, and a No. 2 Faber pencil leaves no more trace on paper than a piece of anthracite."
Rivers flowing into deserts wither away under the effects of evaporation. As water vapor is a great absorber of radiation, this gas serves to temper the ardor of the sun's rays by day and to check the loss of heat from the earth by night, and its deficiency, in a dry climate, results in a large diurnal range of temperature. In the southern Sahara, which is a veritable furnace in the daytime, it is not uncommon to find water at the surface of the ground frozen in the early morning.
Lastly, dry air contains few microorganisms and thus acts as an antiseptic; whence it happens that "jerked beef" keeps indefinitely in the southwestern United States and that the Egyptian and Peruvian mummies have been preserved through the centuries. In recent years we have heard a great deal about the "desert" air found in American schoolhouses and other buildings in winter, and its harmful effects upon human health and efficiency. While it is true that the air in question is as dry as that of a desert, the conclusions drawn from this fact are probably misleading. In a desert the drying effect of the atmosphere is greatly increased by the winds; the temperatures are, at times, much higher than those prevailing in our buildings; and there is often a large amount of dust in the air to irritate the breathing passages. Moreover, notwithstanding these disadvantages of the desert as compared with the furnace-heated schoolroom, a typical desert climate is not unhealthful. If it were, Egypt and Arizona would hardly enjoy their present reputations as resorts for the victims of pulmonary tuberculosis, bronchitis and other ailments.
