Feeling Pressured? Air pressure is the force exerted on you by the weight of tiny particles of air (air molecules). Although air molecules are invisible, they still have weight and take up space. Since there's a lot of "empty" space between air molecules, air can be compressed to fit in a smaller volume. When it's compressed, air is said to be "under high pressure". Air at sea level is what we're used to, in fact, we're so used to it that we forget we're actually feeling air pressure all the time! 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). A measurement of 29.92 inches of mercury is equivalent to 1013.25 millibars. How much pressure are you under? Earth's atmosphere is pressing against each square inch of you with a force of 1 kilogram per square centimeter (14.7 pounds per square inch). The force on 1,000 square centimeters (a little larger than a square foot) is about a ton! Why doesn't all that pressure squash me? Remember that you have air inside your body too, that air balances out the pressure outside so you stay nice and firm and not squishy. Run for Cover! Air pressure can tell us about what kind of weather to expect as well. If a high pressure system is on its way, often you can expect cooler temperatures and clear skies. If a low pressure system is coming, then look for warmer weather, storms and rain. Control the weather! Use a special JavaScript barometer designed to demonstrate how air pressure and weather conditions are linked. You will also find instructions on how to build your own barometer! Earth Science Spin  Find out how air affects a baseball game! What Happens if Air Pressure Changes? Why do my ears pop? If you've ever been to the top of a tall mountain, you may have noticed that your ears pop and you need to breathe more often than when you're at sea level. As the number of molecules of air around you decreases, the air pressure decreases. This causes your ears to pop in order to balance the pressure between the outside and inside of your ear. Since you are breathing fewer molecules of oxygen, you need to breathe faster to bring the few molecules there are into your lungs to make up for the deficit. As you climb higher, air temperature decreases. Typically, air temperatures decrease about 3.6° F per 1,000 feet of elevation. Do you think a decrease in temperature could be explained in terms of air pressure? How?	Air Pressure Experiments 1. While holding your hand on your ribs, take a deep breath and observe what happens to your chest. Did you feel it expand? Did you see it expand? How would you explain what happened? (Results) 2. Blow up a balloon and observe what happens. Does it expand? Why does it make a noise when it's popped? (Results) 3. Ask your parents for an empty plastic gallon milk jug with a screw top. Fill it about a quarter of the way full with very hot water. Cap it tightly and let it stand for about an hour. What did you expect to happen? What did happen? (Results) Air Pressure Word Search Find the hidden air pressure terms in this word search game. This game requires Java-enabled browsers. Take a Balloon Ride! Control a balloon as it rises to 10,000 meters (6 miles) and see what happens! Can you guess what will happen as the balloon goes higher? (JavaScript Required.) Discussion Questions: 1. If you were on a mountain, would the weight of the air above you (air pressure) be greater than or less than it is now? 2. What do you think causes wind? 3. If you lived in Kansas and observed a sudden drop in air pressure, what kind of weather would you expect to see soon? 4. Why do hot air balloons rise? 5. Air weighs less than water, would you expect the pressure exerted by water to be greater or less than the pressure exerted by the same amount of air?
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