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Wind blowing in face when driving

  1. Jun 11, 2014 #1

    Maylis

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    Why does the wind blowing on you as you drive with the window down feel cool? It's temperature should be the same as stagnant air if I wasn't driving.

    My guess is that it has to do with convection because the air has a velocity relative to the driver and there is a heat transfer coefficient of convection.

    Is this the right explanation?
     
  2. jcsd
  3. Jun 11, 2014 #2
    Your body transfers heat to the environment by conduction (as long as the air temperature is below your skin temperature ) and by phase change of the sweat liquid into vapour. With a movement of air, both processes become more efficient.
     
  4. Jun 11, 2014 #3

    russ_watters

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    Note that "conduction" of heat to moving air has its own name: convection.
     
  5. Jun 11, 2014 #4

    Pythagorean

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    I have driven in 100+ temperatures on the I-5 where the breeze coming in the window was just as hot and uncomfortable as the still air.

    Humans detect the rate of cooling of their skin and often confuse it for absolute temperature, and a breeze removes heat based on the difference in temps between skin and the air molecules, so I imagine once temperatures are hot enough, you're not really cooling at a significant rate even when the air is moving, because the air is hotter than your skin.
     
  6. Jun 11, 2014 #5
    Yes. This is right. But, you also get some small cooling effect from enhanced evaporation rate, provided the absolute humidity of the bulk air is not higher than the absolute humidity of the air at the very surface of your skin.

    Chet
     
  7. Jun 11, 2014 #6

    russ_watters

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    The term you guys are looking for is "wet bulb temperature". When the outside wet bulb temperature exceeds body temperature, the air starts to heat you instead of cooling you.

    By WBT measurement, Philadelphia is hotter than Phoenix. Yes, it really is about the humidity, not the heat.
     
  8. Jun 11, 2014 #7

    CWatters

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    It's not just about humidity. Humans have a strange sense of hot and cold. There is a trick you can do involving putting both hands into a bucket of water. With the right prep one hand tells you the water is hot and the other that it's cold. The human body is typically warmer than and looses heat to it's surroundings. If you change the rate at which heat is lost you can be fooled into thinking it's hotter or colder when it isn't really.

    Clothes also trap heat and in a wind that effect can be disrupted.
     
  9. Jun 11, 2014 #8

    Pythagorean

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    I think of humidity as a modulator on the driving force (the temp differential). Obviously temperature gradient is the dominant factor; having lived across conditions ranging 140 degrees Farenheit, I can tell you that no amount of moisture difference makes -40 F feel like 100 F or vice versa. But moisture certainly can make 80 feel like 90, so in certain parts of the world, it makes a huge difference to comfort.
     
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