Difference Between air and Atmospheric Pressure?

What is the difference between air pressure and atmospheric pressure?
- ed (age 15)
arcadia, ca usa

Air pressure is what you measure with a tire gauge.  Atmospheric pressure is what you measure with a mercury barometer.  Let me be more specific.  Pressure is the amount of force per unit area that a gas exerts on a surface.  If the pressure on both sides of the surface are equal then the relative pressure is zero.  Absolute pressure is when the other side of the surface is at vacuum.   So the pressure you measure with a tire gauge is really the absolute pressure inside of the tire minus the absolute atmospheric  air pressure, which is measured with respect to vacuum.

LeeH

(published on 03/10/2010)