Jerri Trotter, age 11, of High Point, N.C., for her question:

WHERE DOES THE WATER COME FROM THAT FORMS ON A COLD DRINK BOTTLE?

Lately a number of industrious folk have been hard at work gathering materials to be recycled, or used again. But the idea of recycling materials is nothing new. Since time began, nature has been quite busy with countless recycling projects. The oxygen so necessary for life is continually being recycled by the plant world. And then there is the stupendous water cycle, which circulates water from the seas to the sky and back again. The filmy blanket of air that surrounds the earth has no color, no taste and no smell. Of course, sometimes we see smoke or fog in the air and dust is certainly visible. But these things are not really part of the air  they are just particles of material mixed in with the air.
Gaseous water vapor is also mixed in with the air. Each tiny particle is much too small to be seen by itself. However, when countless billions accumulate together, they can be seen as billowy, cushiony clouds. Water vapor from the air can also be seen as tiny water droplets that form on the sides of a glass or bottle filled with a cold drink.
The air can hold only a certain amount of water vapor. Warm air, however, can hold more vapor than can cold air. When the warm air is cooled, some of the water vapor changes to liquid water we can see. On a warm day, tiny beads of water form on the outside of a pitcher of chilled lemonade, or on the sides of a bottle of soda, fresh from the refrigerator. What has happened is that the warm moist air around the refreshing drink has been cooled enough to cause it to give up some of its water vapor.
It is estimated that at any given moment there are about 3,100 cubic miles of water vapor in the air. This may seem like a staggering amount of water to be floating around, especially when we realize that each cubic mile of water is enough to fill a tank a mile high, a mile wide and a mile deep. Yet it is only a small fraction of the earth's total water supply.
The moisture in the air is part of a cycle that circulates the waters of the earth. Each day heat from the sun warms the surface layers of the oceans and other bodies of water. Some of the water evaporates and mixed with the air as vapor. As it is carried up into the atmosphere it becomes cooler and cooler. Eventually the vaporous moisture reaches a level where the air is too cool to hold it all. When this happens the water vapor condenses into tiny droplets, forming different types of clouds. When conditions are just right, the moisture falls back to the earth as rain or snow.


Every year about 95,000 cubic miles of water circulate between the air and earth. Most of this water rises from the ocean, but about 20 percent comes from lakes, streams, moist soil and plants. When the moisture returns to the earth as rain or snow, most of it falls back into the oceans. Enough soaks into the earth, however, where it is available for plants and animals to continue the process of life.