Because we have our own pressure in us. The further away we are, the less pressure exists. Decreases with height due to the effect of gravity. Over half of the total mass of the stmosphere is below 5300 m 90% below 16000 m. At sea level the average pressure is 1013. 25 millibar (mb), or 29. 92 inches of mercury (hg) we use a barometer. The atmosphere is divided into 2 broad regions, the heterosphere and the homosphere. Exists from the surface to 80 km altitude. Only exception is ozone concentration (o3) the ozone layer is located from 19 to 50 km. Heterosphere: exists between 80 and 480 km altitude formless, blended so thoroughly that it behaves as a single gas. Air simple mixture of gases that is naturally odorless, colorless, tasteless, and. For practically: top of our atmosphere is300 miles above earth"s surface. About the 300 mile mark is the exosphere outer space less dense atmosphere is nearly a vacuum.