Q: Why does wind make you colder, but re-entry makes you hotter?

The original question was:

Why is it that, when you are outdoors and the atmosphere is moving past you at a moderate rate (wind), you get colder; but when the atmosphere is moving past a space shuttle during re-entry, it gets hotter?

Things falling from space get crazy hot. Unless they're designed specifically to survive the onslaught of air, they tend to aerosolize. That doesn't seem to be the case on the ground.

Physicist: The air right on a surface stays effectively fixed to that surface, an effect that the fluid dynamicists call “the no-slip boundary condition”.  You can imagine this like a deck of cards being spread out on a table, the bottom card barely moves, the card above that moves a little more and so on.  By the time you get to the top card there’s plenty of movement, but the first couple of cards would seem to be effectively “stuck”.

The top card moves a lot, the bottom card barely moves, and the distance each card moves with respect to its neighbors is about the same.

We experience this as a “bubble” of air around our bodies.  The air right next to our skin is almost stationary, while the air a foot or two away barely notices us at all.  The greater the air speed, the more shearing the air near our bodies is, and the thinner the layer of roughly-stationary-air.  The boundary layer (since it’s near us) is warmer than the surrounding air, and acts as insulation.  So more wind means less insulation, which makes you colder.

Unless you live somewhere terrible like Florida or Morocco.  Then the wind just blows away the air that was keeping you cool.  Terrible places to live.  If you want to know what Florida is like, get a dozen humidifiers and hairdryers, and lock yourself in a tiny, air-tight room.

When you wave a piece of cardboard, or something, through the air the resistance you feel is (mostly) a build up of pressure on one side, and a drop of pressure on the back.  Increasing the pressure on a region of air compacts it and causes it to heat up.  When you walk down the street your front should actually be a little warmer than your back, but if you really think you can feel it, then you’re probably walking with the Sun in your face.

The space shuttle does the exact same thing.  It’s just better at compressing the air in front of it, since it’s moving at about Mach 23.  The rush of air moving past the shuttle does cool it off, but the effect is completely overwhelmed by the effect of the compression.  The effect is most pronounced above the speed of sound (Mach 1).  Below Mach 1 air will just get out of the way rather than compress much, but above Mach 1 you can basically “sneak up” on the air.  It doesn’t know you’re coming until it gets hit.

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2 Responses to Q: Why does wind make you colder, but re-entry makes you hotter?

  1. Scott says:

    The real question to answer for folks, I think, is why do you blow on your hands to warm them up, and also blow on your soup to cool it off?

  2. Paul says:

    No, Scott. I don’t think so. But this would be answered in few sentences, wouldn’t it?

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