What is air pressure?  Have your ears ever popped when going up or down a big mountain?  If you have experienced this, then you have experienced air pressure.  As a matter of fact you are experiencing air pressure all the time. 


Air pressure is due to the weight of all the air molecules that are piled on top of you from the edge of outer space to the ground.  Earth’s gravity is strong enough to hold molecules of nitrogen and oxygen around it, and not let them float into outer space.  In contrast, the moon’s gravity is too weak to hold air molecules such as nitrogen and oxygen around it.  Therefore the moon has no atmosphere. 


When you are on top of a very big mountain, you have less air on top of you than when you are standing on the shore of the ocean.  We call the air pressure at the ocean, 1 atmosphere of air.  See some exciting experiments on air pressure by clicking on the Air Pressure Video Link.