1. Air Pressure

2. Pressure Pressure is a force that acts over a certain area.
Liquids and gases are fluids. Fluids are any material that is able to flow. Fluids exert pressure because of the motion of their particles.
Pressure will always move from a high pressure to a low pressure area. The pressure will always try to equalize. You see this when you get a hole in your bicycle tire.

3. Inflating example
Air inside a ball pushes against the sides. The more air we put in a ball, the more the molecules push. So the pressure increases as we add air.

4. Air Pressure
Air pressure is the result of the weight of the column of air that is above you.
There is so much air above you that at sea level you have lbs/in2 pushing on you.
Gravity causes most of the air to be pulled down to the surface.

5. Why are we not crushed by air pressure?
Air pressure is equal in all directions.
So air pushes equally in all sides of us. The forces are balanced!

6. Atmospheric Pressure and Temperature
The atmosphere is held around the Earth by gravity. Gravity pulls gas molecules in the atmosphere toward the Earth’s surface, causing air pressure.
As altitude increases, air pressure decreases.
Air temperature decreases as altitude increases. Lower parts of the atmosphere are warmer because they contain a high percentage of gases that absorb solar energy.

8. Units of Air Pressure
TV weather stations and aviation use inches of mercury.
Meteorologists (and the NWS) use millibars, an SI unit. A unit of pressure equal to one- thousandth of a bar or 100 pascals, most commonly used to measure atmospheric pressure.

9. Altitude and Density
As the altitude increases, the air pressure decreases, and the density of the air decreases. The air particles are not squashed together as tightly the higher one goes. This is caused by gravity!
The air at sea level and at 6km has the same 21% oxygen, but at 6km there are fewer molecules, so you take in less oxygen with each breath.