1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Can anyone help with the perception of temperature difference?

  1. Feb 23, 2014 #1
    For some reason I'm having a difficult time understanding the reason why air 'feels' cold when you tighten your lips and blow air from your body, yet it 'feels' hot as you exhale with a widened mouth.

    Why is this so? The only reason why I'm having considerable difficulty is because I keep thinking of the motions of the molecules. If they are moving fast, the air should be hot right? I am a layman, and this question may seem simple, but I want to understand in detail as to why this is happening.

    Is this particular phenomena similar to wind or a breeze? And how it feels cold? Can someone provide some insight on this? Thanks.
  2. jcsd
  3. Feb 24, 2014 #2


    User Avatar

    Staff: Mentor

    The cold air you feel is the air around your face that is pushed by the air coming out of your mouth. When you open your mouth you have a much greater amount of warm air coming out than when your lips are puckered, so you can feel the warm air.

    Try this. Pucker your lips and put your hand right up to your lips and blow. You will be able to feel warm air.
  4. Feb 24, 2014 #3
    The air actually is warmer when you open your mouth wider. It's not just a feeling.

    Don't confuse the motion of the molecules with the motion of the air.

    Think of a swarm of flying insects. These swarms frequently seem to hover over one point on the ground. When that happens, the insects are constantly moving around with high velocities, but the swarm itself isn't moving at all.

    The same is true of the molecules in the air. At room temperature, the average air molecule is moving at about 450 meters per second, despite the fact that the air in the room is "at rest". The air isn't moving because all the molecules are moving in different directions, so the air as a whole doesn't go anywhere, just like the swarm of mosquitoes doesn't necessarily go anywhere even though the mosquitoes are whizzing around like crazy.

    With the air molecules moving at 450 meters per second, how much difference do you think it makes that you blow them out of your mouth at an additional 1 meter per second? That does add some energy, but it is insignificant relative to the amount of heat already in the air. It has almost no effect on the temperature that you feel.

    The explanation is actually quite simple. Air blown out of your wide-open mouth feels warm because it is at body temperature, which is warm. If you instead produce a narrow jet by pursing your lips, the air is cooler because most of the air in the jet comes from the surroundings instead of coming out of your mouth. (Note: there might also be a small adiabatic cooling effect as the air expands after leaving your mouth. This is, at best, responsible for a small part of the effect.)
  5. Feb 24, 2014 #4
    Okay, so you're saying that it has to do with the air around me? Is the lower pressure (generated by me blowing) causing the perception of cooler air then? I never considered the relationship between the air inside my body and the air around me.

    In addition I did put my finger closer to my lips as I puckered and exhaled the air by the way, and it does indeed feel warmer! Thanks for that suggestion.

    But perhaps, is there some principle behind this? Or any universal theory that explains this experience?

    Thanks for your time.
  6. Feb 24, 2014 #5
    Oh, now I think I understand. Thanks for your response. So in a general sense, it's because the air surrounding me is at a lower temperature than the air coming from my body..which has the "cooler" perception of temperature. Am I accurate in this response?

    Wow, I never thought of that! Now it kind of makes sense. I actually conducted a little experiment to corroborate your explanation.

    As I had my mouth widened, and exhaled, I had my hand held at a LARGER distance...and felt cool air! So perhaps you were right, that the molecular interactions are negligent when talking about a HUGE volume of the surrounding air. Thanks for helping me again.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook