1. Air Density

Ears "pop" due to air pressure differences. Photo courtesy of CK-12. 

If your ears have ever "popped", you have experienced a change in air pressure. Ears "pop" because the air pressure is different on the inside and the outside of your ears

The air desnity (the number of molecules in a given volume) decreases with increasing altitude. This is why people who climb tall mountains, such as Mt Everest, have to set up camp at different elevations to let their bodies get used to the decreased air density. 

Why does air density decrease with altitude? Gravity pulls the gas molecules towards the Earth's center. The pull of gravity is stronger closer to the center, at sea level. Air is denser at sea level, where the gravitational pull is greater.