The density of dry air at sea level is 0.0745 lb/ft2 at 70F or about 1/800th the density of water. But as altitude and temperature (or both) increases, the density drops dramatically. This is because the density of air is proportional to the pressure and inversely proportional to temperature. And the higher you go into the atmosphere, the lower the pressure gets. Pressure is approximately halved for each additional increase of 56 km in altitude.

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