Molecules in Motion: Air Pressure at the Earth's Surface
 

	Fun Experiment to Try at Home!
		Purpose To model the atmospheric pressure at Earth's surface.   Materials - box with lid (a small shoe box will work) - 32 marbles   Procedure 1.  Place the marbles inside the box and close the lid. 2. Hold the box in both hands so that as much of your hands touch the bottom of the box as possible. 3. Shake the box up and down and make note of how the marbles feel hitting the bottom of the box.   Sum It Up! The marbles represent the gas molecules in Earth's atmosphere in the layer closest to Earth's surface. The force of the marbles hitting the bottom of the box represents the atmospheric pressure. At 3.4 miles (5.5 kilometers) above Earth's surface, the pressure is about one-half that at Earth's surface, and at 6.8 miles (11 kilometers) the pressure is about one-fourth that at Earth's surface. To represent this, the experiment can be repeated three times: First repeat the original experiment, and then remove half the marbles from the box. Make a comparison of the force felt with 32 and with 16 marbles. Then remove half the marbles in the box again, leaving 8 marbles. Again compare how the moving marbles feel.

You are bombarded by millions of particles every day.
 
The atmosphere is a blanket-like layer of gases surrounding Earth. Gravity pulls the gases down, causing more gas molecules to be in the part closest to Earth. Thus the air near Earth has a greater density than the air at a height above Earth. Density is a measurement of how much matter is packed into a certain volume.

The gas molecules in the air are in constant, random motion. The gas molecules are widely separated, and they collide with each other and with any other objects that get in their way. The force exerted by the gas molecules in the air on a particular area is called atmospheric pressure.

As the density of the air increases in a particular area, the number of gas molecules increases, resulting in a higher atmospheric pressure. Thus the density of air molecules and air pressure are directly related. Since the density of air is greatest near Earth, the atmospheric pressure is also greatest at Earth's surface. The average atmospheric pressure at Earth's surface is about 14.6 pounds per square inch, which is about 100,000 pascals.

For more infromation about air pressure, see "Janice VanCleave's Earth Science For Every Kid." (New York: Wiley, 1991)