John Heraty, age 14, of Dorchester, Mess., or s question:
What is air pressure?
Can you hold up a load of ten tons? Oh, yes you can. The weight of air pressure around the body of an average 14‑yeur old young man is more than ten tons. And, so long as you live on this air‑blanketed planet, your body must continue to support its enormous air pressure.
We use the expression light as air and tend to think that air has no weight at all. But this is not so. True, it does not weigh much, but there is a great deal of it. The atmosphere covers the earth in a filmy shell hundreds of miles deep. All this air weighs about five and one half trillion tons.
Air as we know it is a mixture of invisible gases. The gases are composed of particles too small to imagine. In a so‑called empty thimble there are billions of gaseous air particles. What's more, there is plenty of space between them, rind these little molecules do not just sit there. They dash about at speeds of more than 500 feet a second, The traffic situation in the seemingly calm air of a room is fantastic. Every air particle averages several billion collisions a second.
And all this busy air is hugged to the ground by the pull of earth’s gravity. It presses on the ground and out in all directions, A diver feels the pressure of water around end above him. On the ground, we live at the bottom of an ocean of air. Its pressure, the atmospheric pressure is all around us.
The weight of air pressing down on a square inch of ground at sea level is 14.7 pounds. This is the standard unit of atmospheric pressure. It gives the weight of a column of air on a square inch base reaching way up to the topmost level of our atmosphere. This column of air is hundreds of miles tall.
However, most of its weight is near the surface of the earth. Three and a half miles above the ground. the air pressure is only 7.25 pounds. This means that half the earths blanket of air is contained in a layer below three and one half miles.
The air thins out as we go higher. Seven miles above the ground atmospheric pressure is halved again. About three quarters of the entire weight of air is crushed in a layer seven miles thick. Above this the air guts thinner and thinner until, some 500 or 600 miles over our hoods, it finally peters out,
Air, as we know, is forever restless. Even its pressure does not remain constant for long. It is heated and cooled, dried and moistened with the changing weather. It is forced to rise and become lights fall and become dense. And all these factors effect the pressure of the air.
The slightest change in the air pressure may be a clue for the weatherman. It may indicate a change of weather ahead. And the instruments he uses for this job are called barometers: