Peter Selander, age 13, of Pocatello, Ida., for his question:
JUST WHAT IS AIR?
Air, often called the atmosphere, is a mixture of gases that we cannot see. It surrounds the earth just above the land and sea. Without air, there could be no living plants or animals on the earth.
The most important gases in air are nitrogen and oxygen. Nitrogen makes up slightly more than 78 percent, or almost eight tenths, of dry air by volume. Oxygen accounts for nearly 21 percent, or about two tenths of the volume of air.
The remaining one percent of air consists almost entirely of the gas argon.
Air also contains very small amounts of several other gases. They include neon, helium, krypton, xenon, hydrogen, ozone, carbon dioxide, nitrous oxide and methane. In some parts of the world, the air contains small traces of ammonia, carbon monoxide and hydrogen sulfide.
Moisture in the air is in the form of a gas. Each molecule or very tiny particle of the vapor is smaller than one millionth of a millionth of an inch. That's small!
The molecules of moisture float freely among the molecules of nitrogen and oxygen, much as bees drift among flowers. The amount of water vapor in the air is not always the same. Air expands, or stretches out, when it becomes warm. Because of this, warm air can hold more moisture than cool air can.
When the relative humidity reaches more than 100 percent, the excess water usually comes out in the form of small droplets. Relative humidity is the amount of water that a certain volume of air contains, compared with the amount air could hold at a given temperature.
Clouds are formed when the droplets come close together without touching. When the droplets actually touch and flow together, they form rain, which falls from the clouds. The rainfall removes the moisture from the air and leaves the air drier than before.
We must all breathe air to live. A person may stay alive more than a month without food and more than a week without water. But a person can live for only a few minutes without air.
Air often contains many solid particles. These, like water vapor, are usually invisible except when present in great numbers.
Air normally contains about 100 000 solid particles per cubic inch. We can see puffs of smoke or clouds of dust when many particles of soot and dirt are croweded together. The dust count has ranged from 15,000 particles per cubic inch of clean air over the mid Pacific ocean to more than 5 million particles per cubic inch in the smoky air over a large city.
Other particles commonly found in the air include salt from the oceans, pollen from plants and tiny living things called microbes.
High above the earth, men have found tiny crystals of ice in the air.
Sometimes solid particles settle into the earth's air from outer space. They are the ashes of meteors that burn upon hitting the atmosphere.