Would you believe that air pressure has a fairly large effect on air temperature? Well, you probably would, you’re a science teacher. But would your students? You can explain to them that the pressure, as much as the elevation, affects air temperature when traveling up a mountain, for example, but will they buy it? Probably not; heck, the little dummies probably wouldn’t even rent it. To help you drill this concept through their thick little skulls, we present yet another handy dandy Demo Science science demo.
Compressed Air, Expanded Minds
For this experiment, all you’ll need is a bottle (can?) of compressed air like one would typically use to clean the burrito crumbs and dead skin flakes out of a computer keyboard. (Personally, I use a stainless steel keyboard, because I don’t mess around, and scrub clean it every other week or so with a sturdy industrial brush—but you probably don’t want to use that for this demo, as you’ll soon see.)
Have your students sit at their desks with one hand held out in front of them, palm up. It doesn’t matter which hand, Kevin! Stroll among the rows of desks and spray their hands with the compressed air canister. How does it feel, kids?
The answer, most likely, will be, “COLD!”
The Cold Wind of Science
The canister itself is at room temperature, so why is the canned wind so chilly? It’s the pressure, homeboy. While the air was in the canister, it was under high pressure, far higher than the ambient, atmospheric pressure outside the container (and around your students’ hands). When the air is expelled, it expands in the lower pressure, and as it expands its molecules of course spread out.
Heat, as we know, is produced by random vibrations/motions of atoms and molecules. As they spread out, these infinitesimal air bits collide with and vibrate against each other much less, which reduces the heat produced and, therefore, the temperature of the air itself.
Going back to our example from the start. At the base of a mountain, the air pressure is greater simply because there’s so much more air above it (leading up into the atmosphere, infinity, and beyond); as one scales the mountain (or drives up it or follows a hiking trail or whatever), the pressure decreases because, again, there’s just less air above it. Ergo, colder air at the peak than at the foot.