Before we continue, lets take a look at atmospheric pressure and how it behaves with height in the atmosphere.

Pressure
Atmospheric pressure is simply the force (weight) exerted by the atmosphere.  The average sea level pressure is 1013.25 mb, or 14.7 lb in^-2.  This means 14.7 pounds of pressure are being exerted on every square inch of your body!  Luckily, an equal force is exerted outward, preventing us from being crushed.

The atmospheric pressure is greatest at the bottom of the atmosphere.   This is also where the air has its greatest density.  Pressure and density decrease with height in the atmosphere.

The earth's gravitational field pulls the mass of the atmosphere towards the earth's surface.   In fact, one half of the mass of the atmosphere lies within the first 5.6 kilometers; 90% of the mass lies within 16 km; and 99.99997% of the mass lies within 100 km of the earth's surface (Lutgens 17).  At 600 km, the mean free path between air molecules is about 10 km (Lutgens 18).   From here, the atmosphere continues to get thinner and thinner with altitude, until it finally merges with the vacuum of space.