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As you travel with your hand sticking out of a car the sensation you have is one of cooling: even though molecules are transferring energy to your hand, you interpret it as being colder. Now, at a certain speed your hand would actually melt down, and I'm assuming you'd be feeling warmth by the time that happened.

I'm wondering how this works: how can your body sense warmth and cold from a phenomenon that, physically speaking, is just about molecular collisions?

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    $\begingroup$ You feel cool because of the evapoative cooling of the thin layer of sweat that exists. There must be a tipping point where the sweat evaporates altogether and air friction takes over. Ya, im just guessing. Sure for the first part. $\endgroup$ – Polisetty Oct 22 '16 at 4:41
  • $\begingroup$ @Polisetty Pretty solid guess in my opinion. As you said, sure for the first part, good guess for the second part. $\endgroup$ – Dart Feld Oct 24 '16 at 19:47
  • $\begingroup$ This is the same principal behind the wind chill factor. Your body makes heat. This heats up the air around your body. If that air is quickly replaced by unheated air, you have to spend energy to reheat that air, which is again quickly replaced; your body is working a lot harder to warm up the air around you, which makes you sense that it's cold. $\endgroup$ – James Nov 25 '16 at 5:36
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You have many different types of sensors in your skin; you probably feel the pressure of the air moving past your hand in addition to the temperature.

The temperature increase in your hand due to collisions isn't very significant, you have to be going at really high speeds to encounter that phenomenon (spacecraft reentry would be an extreme example, for example see this wikipedia page).

Temperature changes due to evaporation are probably most significant (this is why even warm air blown from a fan can feel cool, especially if you are perspirating), as well as the temperature of the outside air around your hand compared to the air inside the car.

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  • $\begingroup$ Does that mean I will not feel the wind as cool if I'm not sweating? Because that doesn't really look obvious to me. $\endgroup$ – QuantumBrick Nov 14 '16 at 2:05
  • $\begingroup$ If the air is colder than your body, you feel it as cold because it is actually cooling your hand; this will be enhanced by evaporation, which will be greatest if you are sweating but even if you aren't sweating your skin still contains water that can evaporate. $\endgroup$ – Bryan Krause Nov 17 '16 at 1:25
  • $\begingroup$ @QuantumBrick Sweating is a physiological mechanism to cool down anyway, why would the brain want to actively feel water evaporating as much as it wants to actively think about heartbeats. $\endgroup$ – Liu Tianyi Nov 24 '16 at 3:13
  • $\begingroup$ The few points mentioned in the threads are correct. Physical collision do not transfer that much heat when it comes to air. Multiple temperature sensors can also be activated simultaneously. For example you could be in a cold room and feel conducted cold, and have the sun shine at you through a glass panel with radiated heat. But this almost never happens with friction with air of course. $\endgroup$ – Liu Tianyi Nov 24 '16 at 3:15
  • $\begingroup$ @Liu Tianyi: I have to disagree. It's quite normal to feel a breeze as hot, if for instance you're in Las Vegas in the summertime. Or think of warm air blowing from central heating vents... Though of course it's not air friction doing the heating, but convection & conduction. $\endgroup$ – jamesqf Dec 25 '16 at 4:02
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The collisions result in heat loss because the air molecules bounce off your skin with more energy than they impact with. You have to be moving a tremendous speed before the friction heating is greater than the heat loss through transfer. By moving the air (or moving through the air) each molecule is new and at a lower energy state than your hand and there are more collisions. this cools it faster than standing still where the air around you becomes slightly heated by the collisions and have to diffuse away, so many of them get bounced back now at higher energy states when they contact again resulting in less heat loss. your hand is cooled and thermoreceptors in your skin detect the lower temperature. these sensors just detect specific temperature ranges, warm or cool past these ranges and your body can't detect the temperature.

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