why do tanks of compressed air get cold, to the point of frosting over, when the gas is let out quickly?
In:
Heat is basically atoms and molecules bouncing off each other. (NOTE: there *is* radiant heat, but that’s a different thing and doesn’t apply here). When you get a temperature reading, what you’re actually measuring is how vigorously the molecules are moving and striking each other. In other words, it’s a measurement of the kinetic energy in the volume of the gas.
Another way to think of it is the *density* of the kinetic energy in that volume. When you release gas from the canister, the escaping gas takes its kinetic energy with it. That means less kinetic energy (heat) in the same volume and thus a lower energy density. I.E. fewer molecular collisions. Fewer energetic collisions = lower temperature.
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