Wind and Temperature: Why Doesn’t Windy Equal Hot?

I got forwarded a physics question last night asking about the connection between wind and temperature, which I’ll paraphrase as:

Temperature is related to the motion of the atoms and molecules making a substance up, with faster motion corresponding to higher temperature. So why does it feel warmer when the air is still and why does wind make you feel cold?

This is a moderately common point of confusion, so while I responded to the question in email, I’ll also appropriate it for a post topic. So, why doesn’t “windy” equal “hot,” given that wind consists of moving air molecules?

The full answer to this involves a couple of subtle issues, but I like to start with a simple-but-cool fact, which is that the speed of wind just isn’t that big a change in the speed of an air molecule.

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When Should You Open Your Car Windows? An Experiment

Back at the start of the summer, I asked a question about automotive thermodynamics: On a hot day, is it better to open your car windows a crack when making a short stop, or leave them closed? For a long term– say, leaving your car parked outside all day– I hope everyone will agree that leaving the windows slightly open is the better call, but the answer isn’t as clear for a short stop. There might well be some time during which the open-window car heats up faster as warm air from outside gets in, while the closed-window car holds in the air-conditioned goodness longer.

It occurred to me not long after posting that, while walking through the parking lot, that it was possible to test this with SCIENCE! A colleague in computer science has a car that is very nearly identical to mine, as you can see in this picture:

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Chris’s car is similar enough to mine that I have more than once stood next to it like an idiot pressing the “unlock” button on my car remote and wondering why the door wasn’t opening. This makes for a nearly ideal test of the question of how to arrange your windows: just park both cars in the sun, one with the windows open and one with the windows closed, and monitor their temperatures over half an hour or so.

It took a while to get together, but yesterday, we did just that.

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