Air is the mixture of gases that surrounds Earth. It is often called the atmosphere. Air covers the land and sea and extends far above Earth's surface. We cannot see, smell, or taste air. Yet it is as real as land or water. When the wind blows, it is the air you feel against your face. Wind is simply moving air. You can also see the effect of wind in drifting clouds, pounding waves, and trembling leaves. Moving air can turn windmills and blow large sailboats across the ocean.

Without air, there could be no life on Earth. All living things animals and plants need air to stay alive. You are breathing air now. You must breathe air, or you will die. People have lived more than a month without food and more than a week without water. But a person can live only a few minutes without air.

Air does much more than make it possible for us to breathe. Air shields Earth from certain harmful rays from the sun and other objects in outer space. At the same time, it traps the heat that comes from the sun. In this way, air helps keep Earth warm enough to support life. Air protects us from meteors, most of which burn up in the atmosphere before they can strike Earth's surface. Clouds that form in the air bring us water in the form of rain and snow. All living things must have water to live, just as they must have air.

We also need air to hear. Sound must travel through the air or some other substance. Most of the sounds we hear travel through the air. Thus, the world would be silent if there were no air. Air has weight. This weight enables balloons filled with a light gas or heated air to rise high above Earth because they are lighter than the air around them. Air moving past the wings of airplanes, birds, and insects enables them to fly.

Earth has plentiful air. But the quality of the air depends largely on the amount of industrial wastes and other pollutants (impurities) that people add to the atmosphere. Air pollution is a serious problem in most of the world's big cities. Polluted air harms our health. It also injures plants and animals, damages building materials, and even affects the weather.