1. Air Pressure Air has mass, density, and pressure. Density Density- amount of mass in a given volume of air D= Mass Volume More molecules in a given volume = greater density. Fewer molecules in a given volume = less density.

2. Air Pressure Air pressure is the force exerted on you by the weight of tiny particles of air (air molecules). Weather forecasters measure air pressure with a barometer Barometers are used to measure the current air pressure at a particular location in "inches of mercury" or in "millibars" (mb). Earth's atmosphere is pressing against each square inch of you with a force of 14.7 pounds per square inch. You have air inside your body too, that air balances out the pressure outside so you stay nice and firm.

3. Altitude Affects Air Pressure Look at the stack of books. Which book has the most weight on it? The book at the very bottom has the weight of all other books. The book second from the top only has the weight of one book on it. Air at sea level is like the bottom book. Sea-level air has the weight of the whole atmosphere pressing on it. Air pressure is greater at sea level. Air near the top of the atmosphere is like the second book from the top. There, air has less weight pressing on it which means lower air pressure.