"On a Clear Day You Can See Forever"

"On a Clear Day You Can See Forever"

Almost without exception early motion pictures about the French Foreign Legion featured troops marching over endless rolling sand dunes. From this misleading view of the desert has evolved the impression that such topography is typical of all deserts. Actually sand dunes cover only a small portion of the deserts of the world. They make up a mere 3 percent of the great American deserts, and even in the vast Sahara only 30 percent is actually composed of dunes. The remainder, and by far the greater portion of these deserts, consists of rocky or mountainous terrain.

Anyone traveling across extensive desert areas immediately notices that the air is remarkably clear. The desert traveler can see, usually without difficulty, all the way to the horizon. The major reason for the clear atmosphere is the low moisture content of the air.

Arid regions tend to have brighter skies than humid regions because water vapor absorbs some of the light passing through it. Therefore, with the scarcity of water in arid lands, it is not uncommon to see mountains more than forty miles away quite distinctly. Since such remote terrain appears quite clear and offers an unbroken view, distances are often misleading, and what appears to be only ten miles away may in reality be two or three times that distance from the viewer.

The atmosphere of the desert does, of course contain moisture, but the proportion is relatively low compared with humid environments. Much of this moisture returns to the parched land in the form of dew.

Recently scientist have discovered that dew, the result of condensation of moisture from air, is an important source of desert water. It can often equal as much as ten inches of rainfall annually. Except for the driest of deserts, there is almost as much dew in arid lands as there is in the humid coastal regions! As a matter of fact, desert plants, as well as desert animals, supplement much of their meager ration of water with morning dew.

From the book: 
Our Fascinating Earth