The Thirst Of The Air

It is estimated that something like sixteen million tons of rain and snow falls upon the surface of the earth, on an average, every second. Day after day, year after year, and century after century this prodigious downpour goes on; yet the oceans get no fuller, the land, as a whole, gets no damper, and—strangest of all—the atmosphere, despite this tremendous drainage of its moisture, gets no drier. The explanation of the paradox is, of course, found in the process that we call evaporation. Everybody knows, in a general way, that water evaporates, but few people realize that the amount of evaporation that occurs throughout the world is, in the long run, exactly equal to the world's rainfall.

For every ton of water that falls upon the globe a ton passes off, in the form of a gas or vapor, into the air, though not, as a rule, at the same place where it fell. The oceans, lakes and rivers generally lose a good deal more water by evaporation than they receive directly as rain. The lands, for the most part, lose less by this process, than a tract of land covered with luxuriant vegetation, especially with forests, vies with a corresponding area of water in the amount of moisture it supplies to the air, because plants gather water underground with their far-spreading roots, pump it up through their stems and trunks, and breathe it out through their innumerable pores.

Evaporation is a complex process, and a whole library of books and scientific articles has been devoted to its elucidation, but from the layman's standpoint it may be said that the amount of water evaporated at any time and place depends upon two things. The first is the supply of water available. In a desert there is little evaporation, because there is rarely any moisture at the surface of the ground. Elsewhere on land different soils differ in their capacity for holding water after a rain and for lifting it from the lower levels by the action known as "capillarity"; hence some are more conducive to evaporation than others.

Both snow and ice evaporate; sometimes with astonishing rapidity. When the temperature is below the freezing point, their moisture changes directly from the solid to the gaseous form, without becoming liquid. Oceans and other bodies of water afford an unlimited supply of moisture for evaporation.

The other thing that controls evaporation is a combination of physical conditions that we choose to call, for our present purpose, the "thirst of the air. " In scientific language it is sometimes termed "evaporativity." Air that is dry (deficient in the invisible gas called "water vapor") is thirsty. Warm it and you increase its thirst. Set it in motion, and you make it thirstier still. In other words, humidity, temperature and wind are all factors of evaporativity, and there are still others that need not be considered here.

By: davidbunch

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