# A Lot of Pressure

Gravity pulls the atmosphere toward the earth. The air near the earth’s surface compresses, creating greater air pressure near the earth and lower air pressure at higher altitudes. Because of the pressure exerted on the lower atmosphere, it is packed much more densely than the thin air at higher altitudes. In fact, of the roughly 180 miles-thick layer of atmosphere, fully 80 percent of it is packed into the lowest 3% miles.

At sea level, atmospheric pressure amounts to about 14.7 pounds per square inch. The higher we go in the atmosphere, the lower the pressure gets, until at the very top of the atmosphere, the air pressure diminishes to about zero.

 Plane Talk The air, like any gas, is composed of tiny atoms that speed through space bouncing into other atoms. Not only do atoms bounce into each other, they also bounce against every other surface, whether it’s our skin, the walls of a building, or the body of an airplane. The combined force of these tiny collisions is called atmospheric pressure, or simply air pressure. We measure pressure by comparing the total push of all the little collisions over a certain surface area, usually in units of pounds per square inch.

Meteorologists measure air pressure using instruments called barometers, which measure pressure with long mercury-filled tubes. The higher the air pressure, the higher the mercury rises in the tube. Mercury barometers measure pressure in units called inches of mercury, the units pilots most commonly use. The equivalent of 14.7 pounds per square inch is 29.92 inches of mercury.

 On Course You’ll find an aneroid barometer on every airplane instrument panel. Pilots use it to measure altitude. The altimeter registers the decrease in air pressure that takes place as the airplane climbs, then displays the lower pressure not as a measure of inches of mercury but as higher altitude.

A cleaner and more compact barometer, the aneroid barometer, uses a small, flexible box with the air sucked out of it. High air pressure compresses the box, and low air pressure allows the box to expand. The changes in the shape of the box move a needle on the scale on the barometer’s face.