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Always Cold Wind

  1. May 17, 2007 #1
    Why is it that air going at a high rate of speed always cold? For example, fans throws air at high speed and it's cold and when we, human blow air fast is always cold.
    WHY?
     
  2. jcsd
  3. May 17, 2007 #2

    chroot

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    The air is not actually at a different temperature just because it's moving fast. In other words, it's not really colder.

    It just happens that a fast-moving stream of air is capable of carrying heat away from something (i.e. your body) faster than a slow-moving stream of air.

    - Warren
     
  4. May 17, 2007 #3
    You can prove this for yourself, too. Simply get a fan, put a thermometer in front of it, and notice that the temperature does not change regardless of whether or not the fan is blowing (as long as it is not in the sun).
     
  5. May 17, 2007 #4
    That's really phenomenal!
    I guess that's the way windchill works.
     
  6. May 17, 2007 #5

    chroot

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    Yep, that's wind chill.

    If you walk out of a warm building into a cold evening, you'll get chilled faster if the wind is blowing than if it's not. This is because moving air can remove heat from your body faster than can still air.

    If you live in a hot climate, you can experience the same thing in reverse, too. If you walk out of a cold air-conditioned building into a very hot summer day, you will warm up faster if the wind is blowing than if the air is still. You might even feel like the wind is hot, like someone's aiming a hair dryer at you. This is the opposite of "wind chill," but it is the same effect.

    - Warren
     
  7. May 17, 2007 #6

    Danger

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    You will notice also that the effect is far more pronounced if you're wet. Moving air amplifies evaporative cooling.
     
  8. May 18, 2007 #7
    More importantly I think that others have fail to state is its to do with Newtons laws of cooling, forced convection moves away 'preheated' air allowing for a faster expenditure of heat from an object ( say the body) than a sample of air with a temperature excess closer to zero (the temp of the object w.r.t. the temp of environment), it is exponentially proportional
     
  9. May 18, 2007 #8

    rcgldr

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    As Funker impled, heat up the thermoter a bit first, and it will cool off faster with the fan on than with the fan off.

    As a motorcycle rider, it has to get pretty hot before the wind fails to cool you off significantly. I don't recall it ever being so hot that moving was hotter than not moving (must be sweating / wind cooling effect).
     
  10. May 18, 2007 #9

    AlephZero

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    I agree the main "wind chill" effect is because of increased convenction cooling, but you need to be careful interpreting experiments like "putting a thermometer in the flow to measure the temperature".

    There is a similar temperature effect to the Bernoulli effect on pressure:

    T = t + v^2/2Cp

    T = the stagnation (total) temperature of the gas
    t = the static temperature
    v = velocity
    Cp = specific heat.

    In that sense, increasing velocity (by a fan) can reduce the static temperature. What you measure with "a thermometer" depends on the local flow pattern round the thermometer. The velocity at the stagnation point on the upstream side of the thermometer is zero. And the velocity on the surface of the thermometer is zero everywhere, if you believe that classical boundary layer theory is exact.
     
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