Air, the mixture of gases that surrounds the earth and forms the earth's atmosphere. Air is invisible and tasteless, and normally has no odor. Air has weight, can be expanded or compressed, and, at extremely low temperatures, can be changed into a liquid or solid. Air in motion, called wind, has enough force to turn windmills and to move ships; such strong winds as hurricanes and tornadoes uproot trees and destroy buildings.

Composition of Air

Air is a somewhat variable mixture, consisting chiefly of nitrogen (about 78 per cent by volume) and oxygen (about 21 per cent by volume). Air also contains argon, carbon dioxide, water vapor, neon, helium, methane, krypton, and ozone.

The oxygen in air is essential to all land animals and many plants. Through a process called respiration, animals and plants take in oxygen, use it to obtain energy from food, and release carbon dioxide. Carbon dioxide in the air is needed by plants for photosynthesis, the process by which they use sunlight to make food. During photosynthesis, plants release oxygen into the atmosphere.

Carbon dioxide forms only about 0.03 per cent of the air by volume. Carbon dioxide is produced not only by the respiration of animals and plants but also by the burning and decay of organic matter.

The water-vapor content of the air is referred to as humidity. The humidity varies with conditions on the earth's surface.

The air usually contains a variety of tiny solid particles, such as volcanic dust, pollen, spores of molds and algae, bacteria, soot, and finely pulverized earth. Dust particles in the air can be seen in a beam of sunlight that penetrates into a darkened room. The scattering of sunlight by dust and other particles in the air causes the coloring of the sky at sunrise and sunset. (The blueness of the clear sky during the day is caused by the scattering of sunlight by the molecules of the gases making up the air.)

The contamination of the air by impurities emitted from furnaces and motor vehicles has created serious problems over some cities and industrial areas.

Air Density and Pressure

Air is densest (heaviest) at or below sea level. At sea level, the density of the air is about 0.08 pounds per cubic foot (1.3 kg/m3). The weight of the atmosphere causes air at sea level to exert an average pressure of 14.7 pounds per square inch (101.3 kPa) in all directions. This pressure is called one atmosphere. (On weather maps, air pressure is usually given in millibars, abbreviated mb; one atmosphere is equal to 1,013 millibars.) Air pressure decreases with altitude. At an altitude of 20,000 feet (6,100 m), for example, the average air pressure is 6.8 pounds per square inch (47 kPa). Air pressure is measured by an instrument called a barometer.

A balloon will rise when it is filled with helium or any other gas that is lighter (less dense) than the surrounding air. Birds and airplanes, which are heavier than air, fly through the air by means of wings. As a wing moves through the air, it experiences an upward force (called lift) because the pressure exerted by air passing over the wing is less than the pressure exerted by the air passing under the wing.

Special Forms of Air
Compressed Air

When air is compressed that is, forced into a smaller space than it normally occupiesit has a tremendous expansive pressure. This pressure is used to drive such piston-operated machines as rivet guns and rock drills. It also is used to operate hoists, jacks, hammers, and molding machines. In sandblasting, compressed air is used to hurl jets of sand against a hard material, such as stone or glass. The air cushion vehicle, a device developed in the late 1950's, rides on a layer of compressed air.

Liquid Air

Liquid air is an intensely cold, clear, pale-blue substance. Its density is about nine-tenths that of water. To liquefy air, it is first compressed, thereby raising its temperature. Next, heat is removed from the hot air by passing it through chilled pipes. The cooled air is then allowed to expand. As the air expands, its temperature drops, causing some of the air to liquefy.

Nitrogen, oxygen, argon, neon, krypton, and xenon are produced commercially by the fractional distillation of liquid air. This process, which was developed by the German physicist Carl von Linde in 1895, depends on the fact that the gases that make up the air have different boiling points. Liquid air is also used as a refrigerant and is used in low-temperature scientific investigations.