Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Questions about air

  1. Dec 15, 2006 #1
    Why do warm air hold more moisture than cold air?

    Is it because the volume of warm air is larger than cold air because warm air expands?

    How can strong winds be cold, how do they work?

    If the movement of air is fast, then wouldn't it heat the body of air up? Are strong winds a moving body of air with slow kinetic energy? Cant visualize how a wind can be strong and cold.

    Why does hot air rise

    it'll really help if you can explain to me as detailed as possible using concepts such as kinetic energy and whatever that is necessary

    Thanks guys.
  2. jcsd
  3. Dec 15, 2006 #2
    Case(1): Warm air has more energy associated with it than colder air; therefore, it can evaporate and susatin more water molecules than colder air. This is probably due to the weak hydrogen bonds obetween the water and air.

    Case(2), as your speed increases, the thermal boundary layer decreases(This is the thickness from the vertical plate wall to the black line on the right hand side). When your boundary layer decreases, the heat transfer increases because it is a function of the Reynolds Number, and the Reynolds Number is a linear function of the velocity.

    http://www.ami.ac.uk/courses/ami4817_dti/restricted/u04/images/Incropera-9.3.gif [Broken]

    Case (3): Hot air rises ONLY if the temperature of the BOTTOM surface is larger than the temperature of the top surface AND the density of the fluid decreases as you go down. If the density does not decrease as you go down (as would be the case with a hot upper surface and cold lower surface), gravity can not induce body forces to 'push' the heavier fluid down to replace the lighter fluid (which wants to rise).

    The number used in that case is not the Reynolds number, but an analogous Grashof number. It is a ratio of the buoyancy forces to the viscous forces. SO, even though the temperature might be higher at the bottom surface than the top AND the density decreases as you go down, fluid motion will occur ONLY if there is enough buoyancy forces to overcome the viscous forces impeding the flow. In a sense, there is a minimum threshold you must overcome before you get natural circulation.
    Last edited by a moderator: May 2, 2017
  4. Dec 16, 2006 #3
    I think temperature is basically the "variance" in the velocity of the different air molecules, whilst the wind speed is the "average" velocity.
  5. Dec 16, 2006 #4
    The particles of air at the surface of the body have zero velocity because of the no slip condition. This means that the heat is transfered through conduction only at the skin surface. As you move away from the skin surface, the velocity slowly increases until you are at the freestream velocity. During this transition, the particles of air interact with the particles next to it, picking up some energy as you move away from the body.

    The velocity would be the ordered motion in the direction of the streamlines. The temperature would be the random energry in each particle of air, which could be in the form of translation, vibration, etc that is superimposed on the ordered motion of the velocity. I guess in a way, you could call it a 'variance', if you wanted to compare it to the case where the freestream temperature is exactly the same as the surface temperature. In that case the variance between the two would be the thermal component (I guess :confused:).
    Last edited: Dec 16, 2006
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook