Atmospheric Flight

9-12 Grade Reading

Atmospheric Pressure

Imperceptible to the human eye, air is in constant, frantic motion at the surface of the Earth. As in any gas, the molecules (in Earth's case nitrogen and oxygen) are moving and bumping into each other at various 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 move more slowly. 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.

a picture graphing
the different alyers of the atmosphere

Imagine a long tube standing up from the Earth's surface all the way up to the top of the atmosphere, about 50 miles straight up. The weight of all that 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 2000 pounds! The normal atmospheric 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 1000 pounds per square inch. Why is the pressure less? Because there is less air in the tube above this point there is less weight of air pressing down.

three graphes one comparing
atltitude and pressure, one comparing altitued and pressure and the
third comparing altitude and density

But if the whole atmosphere is about 50 miles thick, how can it be that half the pressure is caused by the air in the first 4 miles? What happens is that the pressure of the air above it, compresses it. The air closer to the Earth's surface has more density, since the molecules are closer together. Near the top of the atmosphere, there is very low air density because there is very little pressure compressing 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 above it, and adding much more to the pressure below. As one moves up in altitude, the pressure and density are reduced very quickly.

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