Elizabeth Kalatyir, age 12, of New Brunswick, N.J., for her question:
How much water is in the air?
Weather experts refer to this water as moisture. It is, of course, suspended in the air above the earth, and does not resemble the heavy supplies of liquid water that rest on and in the ;round. True, some of it is liquid water, buff it can be held, aloft only as misty droplets like those that form the foggy clouds. However, most of the moisture in the air is in the form of separate molecules of gaseous water vapor.
The atmosphere reaches up hundreds of miles above the surface of the globe and all of it contains a percentage of moisture. The air around you and above your head contains tons of moisture. On a cloudy day, much of this atmospheric moisture is liquid droplets, small and light enough to hang up there in the air. There are tons of invisi¬ble Saseous moisture in the air, even when the sky is clear blue.
It is not feasible to figure the precise amount of moisture in an isolated patch of air because the amount is changing, often from moment to moment. After all, the airy atmosphere is a global item. It exists in a world wide state of breezy turbulence and its local conditions are as restless as the changeable weather.
Weather experts measure the local humidity, which is the moisture content of our air. But since local humidity is changing, it is easier to grasp the moisture content of the global atmosphere.
Let's start with the total weight of the atmosphere, which is estimated to be about 5,600 trillion tons. This includes the fairly uniform blend of atmospheric gases, quantities of dusty particles plus about 3,100 cubic miles of moisture. The total at¬mospheric moisture equals about three quintillion gallons of liquid water, which is 0.001 per cent of all the water in the world.
This airy moisture blends with the other gases of the global atmosphere. In the form of saseous vapor, some permeates to altitudes of 45,000 feet. Some takes the form of liquid cloud droplets. All of it changes from moment to moment, because all of it is involved in the never ending water cycle.
This enormous weather engine runs a ceaseless exchange of moisture between the earth and the air. It is powered by the sun, which evaporates moisture up from the surface, drives the winds, and changes the temperature to shed moisture back down again to the earth. During the averse year, about 95 million cubic miles of water are involved in this global exchange between the earth and the air. The amount of moisture in the air varies from place to place and changes with the weather. But the global content stays more or less the same.
Above arid deserts the vapor content of the air may be as low as 1 per cent. It is highest where the warm sun causes fast evaporation above tropical seas. There the moisture content may be 4 per cent, or higher. On a global scale, the annual evaporation equals the annual downpour. Locally we get droughts and deluges. But every minute, in various places on land and sea, the total precipitation of moisture from the air equals a billion tons of water.