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Atmospheric FlightAtmospheric Pressure
Unseen by the human eye, air is in constant, frantic motion
at the surface of the Earth. As in any gas, the molecules are moving and
bumping into each other at different speeds. Near the Earth's surface they
move at an average of 1,090 miles per hour. Warm the air and the molecules
move faster. Cool the air and the molecules slow their speed. The impacts
of billions and billions of moving molecules cause pressure. At the surface
of the Earth, the air pressure is greater than at the top layer of the atmosphere
50+ miles above the Earth's surface.
Imagine a long tube standing up from the Earth's surface all the way to the top of the atmosphere, about 50 miles straight up. The weight of all the air in that tube is pressing on the Earth's surface. How much does all that air weigh? If the area in the cross section of the tube is one square foot, then the weight of all the air in the tube would be more than 2,000 pounds! The normal air pressure at the Earth's surface is 2,116 pounds per square foot. Now, suppose we climb up the tube to a place about 4 miles above the Earth's surface. If we could drill a hole in the tube at that point and measure the pressure there, we would find that the pressure is much less, only about 1,000 pounds per square inch. Why is the pressure less? Because there is less air in the tube above this point, so there is less weight of air pressing down. What happens is that the pressure of the air above it, compresses it. The air closer to the Earth has more density because the molecules are closer together. Near the top of the atmosphere, there is very low air density because there is very little pressure pressing the molecules together. But at the surface of the Earth, the density is much higher. So, the layers of air in the lower atmosphere are more compressed than those layers of air above it. As one moves up in altitude, the air pressure and density become less.
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