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Cold wind and Hot wind

  1. Jul 29, 2013 #1
    What's the difference between cold moving air and hot moving air?

    For instance, suppose you put two fans, of the same type, each in one room. One room is cold because the walls are covered in ice and one room is hot because due to the effect of heaters. Turn both fans on and the air flow, the pushing of the air particles in one direction, are both blown towards one direction with the same (almost?) velocity.

    What makes one moving air hot and the other cold? The moving air are both moving at the same velocity (about) aren't they? Isn't temperature based on how fast the particles are moving.
  2. jcsd
  3. Jul 29, 2013 #2
    Well how do you know that they are moving with the same velocity?

    I don't think this assumption is valid since you started with hot air in one room and cold in the other one, so basically they both have different velocity to begin with ..

    Hope I can see more replies to this topic from other members ..
  4. Jul 29, 2013 #3
    That's why I said "about" since the temperature in both room is different, I assumed some changes in the velocity of each blowing air. But generally, in the larger picture, the fans are blowing the air at the same speed. (am I wrong?)
  5. Jul 29, 2013 #4
    One thing to be considered is the convection currents of the hot/cold rooms. This would produce a buoyancy effect, as there will be fluctuations in the air density across the room and thus you will not necessarily have the same velocity far away from the fan. This is only true if the rooms are open systems (heat can transfer in and out of the room). This is also neglecting the heat contribution of the fan to the room.

    On the other hand, you also have to consider the density of the air in a closed system. In this case, the hot air fan will actually have a slightly higher velocity because the air density is lower. This is also assuming the energy output and blade design of each fan are identical.
  6. Jul 29, 2013 #5
    But close to fan, one blowing air feels hot and the other blowing air feels cold doesn't it?
  7. Jul 29, 2013 #6


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    of course, did you expect to get hot air from a fan blowing cold air ?

  8. Jul 29, 2013 #7
    I think there is a difference between the average velocity of the constituent particles, and the velocity of the gas as a whole. The particles are moving about randomly in all directions, even in the wind, but in the wind there is a bias towards the direction of the wind. This bias could be of the same magnitude for both hot and cold winds, but the average particle velocities may not be identical, so they have different temperatures.
  9. Jul 31, 2013 #8
    Are there no definite answers?
  10. Jul 31, 2013 #9


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    Answers to what? I see a lot of answers.

    I'm not entirely clear on your question, but I'm thinking what you need is in post #7. Are you thinking that since temperature is a measure of kinetic energy that if you blow air with a fan it should get hotter? Well it actually does, but the speed of the air when blown with a fan is very small compared to its random motion, so the temperature increase is very small.
  11. Aug 1, 2013 #10


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    The speed with which the fans are blowing the air is irrelevant to its temperature. The "heat", and so temperature, of a gas depends upon its random motion. That is the average difference of speed from the overall average speed of the molecules making up the gas. Increasing or decreasing the overall averge speed does not change the random motion of the molecules making up the gas.
  12. Aug 1, 2013 #11


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    To extend the good answers in posts 7, 9 and 10: at room temperature, the random motion of molecules in air is of the order of 400m/s, similar to the speed of sound. The fan can generate wind with something like 1m/s.
  13. Aug 1, 2013 #12


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    True up to a point where theory meets practice: when you blow air with a fan, all of that non-random kinetic energy is quickly converted to random kinetic energy. As pertains to this thread, it therefore must be included.
    Last edited: Aug 1, 2013
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