Air Pressure

1. We live at the bottom of an ocean of air.
2. Gravity pulls down on our atmosphere just as it pulls on all nearby matter.
3. We are under constant bombardment by an uncountable number of gas molecules every second over every part of our body. The impacts of these molecules are collectively felt as a pressure. We don't normally recognize this pressure because we are used to it.
4. The higher we go in the atmosphere the fewer molecules there are on top of us. Since gravity pulls on these molecules they are less compressed by having fewer molecules on top of them, so the gas pressure drops the higher you go in the atmosphere.
5. Notice the mountain climber's body is being hit by fewer gas molecules per second. Pressure is lower on top of the mountain and the air is "thinner" making it hard to get enough oxygen molecules into your lungs.