Boy, the air sure is dry today! Not surprising, since this is the desert. But what do we actually mean by "dry air"? Usually, we're really not talking about how much water is held by the air, but about relative humidity. That is, the amount of water vapor in the air compared to the amount it could hold before condensation and cloud formation begins. And that amount varies with temperature. The warmer the air, the more moisture it can hold.
On a hot summer day in the desert, the actual amount of water vapor may
be high, but yet, because the hot air can hold so much, the relative humidity may be
down in the teens. On a cold winter day, that same amount of water vapor may be
drizzling or snowing on you, with a relative humidity of 100%. From our point of
view—and that of desert plants and animals, the relative humidity is critical.
The lower it is, the faster water is lost—and woe to those of us who can't
conserve it or replace it.
Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.
Rosenberg, N.J., B.L. Blad, and S.B. Verma. 1990. Microclimate. 2nd ed., John Wiley & Sons, New York.