Most of us think of air as being “nothing,” but air is definitely “something,” if it is matter made up of certain gases. A gas does not have a definite size or shape, but it takes up space.

The great ocean of air that surrounds the earth and extends for many miles upward is attracted and held to the earth by gravity. Thus air has weight. And since air is everywhere about us, it adds weight to every object it fills. For example, there is a small amount of air in a volley ball. If you were to weigh two such balls, with the air let out of one of them, you would find it’s lighter than the other.

The weight of air exerts pressure. The air presses on your whole body from all directions, just as the water would if you were at the bottom of the sea. The great mass of air pushes down on the earth very hard with a pressure of nearly 1 kilogram on each square centimeter.

The kilogram is the weight of a column of air 1 centimeter square and as high as the air extends upward. The palm of your hand has about 77 square centimeters. Imagine 77 kilograms, all held up on one hand! The reason you don’t even know you’re doing this is that the air under your hand pushes up with the same force as the air above pushes down. There are about 270 kilograms of pressure on your head. But you’re not mashed flat because there’s air inside your body, too, which pushes out just as the air outside pushes in.

The higher up you go (to a mountain top, for example), the less air there is above you, so the pressure is less. At 6,000 meters the pressure is about 0.4 kilograms per square centimeter. At 3,000 meters it’s 0.7 kilograms per square centimeter. If you could get up to 100 kilometers over the earth, there would be almost no pressure.