Montshire Minute: Air Pressure

Originally aired during the week of January 4, 1999

Monday

Watch a thundering waterfall and we can really appreciate the force of liquid matter. It's much harder to get a handle on the nature of gases because they are invisible. But while air may be invisible, it's around us all the time. It's like we're swimming in a big ocean of air, which is moving in streams and current. Since gases and liquids flow in much the same way, they are both called fluids. When you get right down to it, the big difference between a gas and a liquid is the distance between molecules. Gas molecules are far apart so they move about more freely. Air is actually made up of many different gases - about 78 percent of it is nitrogen, and 21 percent is oxygen. Oh there's a bunch of other stuff in there - small quantities of carbon dioxide, argon, helium, and yes, Superman, krypton!

Tuesday

Kernels of popcorn are full of potential. But they're not very tasty. To get popcorn, you have to activate the kernels by applying heat. This also applies to air molecules. What I mean is that air molecules are energized by heat from the sun, which makes them pop up above ground level. Actually, the sun's rays don't heat the air very much in a direct way. The air is mostly warmed by heat radiated back from the land and sea. Anyway, the force of gravity keeps the lid on the little buggers as they zip about. The area in which these gases move is what we call our atmosphere. Air molecules are packed tighter together near the bottom of the atmosphere (the air we breathe). Air surrounding the earth extends upwards hundreds of kilometers. So you could say we live in the bottom of a huge ocean of air! Hey, it's like we're drowning in oxygen and hydrogen down here!

Wednesday

When we want to describe something as really insubstantial we say "it's lighter than air!" But air is heavy stuff, dude. At sea level, one cubic meter of air has a mass of about 1.25 kilograms. So the air in a small child's bedroom is approximately equal to his own weight! Like the water in a pond, air in the atmosphere exerts a pressure. We're usually not aware of air pressure because our bodies exert an opposite pressure that counterbalances it. A simple mercury barometer can measure this mysterious force. It consists of a glass tube filled with mercury and set, open end down, in a dish of mercury. The liquid runs out of the submerged open end of the tube and into the dish until the level in the tube falls to 76 centimeters. Like a see-saw in balance, the barometer balances the when the weight of the liquid in the tube exerts the same pressure as the air outside.

Thursday

Hold a plastic bag of water and you can feel its weight. But hold the bag while you're submerged in the swimming pool and you won't notice the weight anymore. It's sort of the same with air- air has weight, but we don't notice it because we've become acclimated to the pressure it exerts. Air presses in all directions. To demonstrate this, fill a glass to the brim with water. Place a square of cardboard on top of the glass, making sure it makes a strong seal. Now, holding the cardboard in place with one hand, carefully turn the glass upside down. (I forgot to mention, you should do this experiment over the sink.) Anyway, if you carefully release the cardboard . . . "voila!"... or "eureka!" if you prefer... the cardboard stays in place! Upside down! Why? Air is pressing up on the cardboard with enough force to hold the water in the glass.

Friday

Wind is created when air moves from areas of high air pressure to areas of low pressure. These differences in pressure are caused by uneven heating and cooling of the air. Warm air is lighter than cold air so it rises, causing cooler air to flow in beneath it. Sometimes you can feel this taking place in your house on a cold winter day. Cold air may get into the room through windows or under doors. It is warmed by the radiator, lightens up, and rises to the ceiling--making room for more cold air to come in underneath it. This phenomenon is called a convection current. You may just call it "drafty!" Lest you think I'm full of hot air, come to see Montshire's new exhibit AirPlay, which opens on Saturday. You can fire off our "Air Cannon," or watch how air movement propels objects through what might be the world's longest vacuum cleaner hose. It'll blow you away!