Susan Amlin, age 11, of Muncie, Indiana, for her question:

What actually is atmospheric pressure?

The atmosphere seems like a weightless nothing; but, for example, the small pocket of air contained in an empty shoe box weighs almost as much as an aspirin. The vast atmosphere around the globe is estimated to weigh about 5,600 million million tons. This fantastic weight presses down on the surface of the globe    and atmospheric pressure seems like a very good name for it.

A knowledge of local atmospheric pressure is very important to weather forecasters because this fickle pressure is forever changing. It forms local masses of lighter and heavier air    creating local changes in the weather. So the atmospheric pressure is measured in small units. The barometer is used to weigh the air pressing down on a certain part of the earth's surface. This is the weight of a square inch column of air, reaching from the surface of the earth to the very top of the atmosphere.