Delicate balance between the production and destruction of O3. Bad O3, pollution at the ground, smog. CFC last in the stratosphere for 100 years. O3 hole occurs over Antarctica in Sept. and Oct. Gravity keeps the gases from going out to space. Gravity weakens as you get farther away from surface, so the number of molecules decreases with altitude.
Density decreases with altitude.
Even though we can't feel the constant bombardment of air, the push of tiny molecules, we can detect rapid changes. Ears popping - air collisions outside the ear drum lessen. Ear popping, occurs as the molecules on the inside and outside equalize. Pressure is force per unit area. The pressure at any level in the atmosphere may be measured in terms of the total weight of the air above any point - atmospheric pressure.