I was wondering about three things:

Wind chill effect. Human body perceives surrounding temperature as colder, with increasing air flow:

enter image description here

(Image on wikipedia)

This is understandable, as thermal transfer "consists" of convection, diffusion and radiation.

I assume that in real world example, we are being "chilled" by fan due to our skin not being 100% dry, and thus wind speed increases convection. Is this assumption correct?

  1. Imagine hypothetical situation with no precondition- human is standing in vast open space somewhere on earth, naked. Air temperature is exactly 36.6℃, there is no wind and for the sake of experiment I assume that sun does not shine, so basically I assume that this is near thermal equilibrium (human body generates energy from metabolic processes). I also assume that this man's body has 36.6℃ on whole surface.
    Will perceived temperature to this man heavily dependent on air humidity? Why, most probably, will he/she feel that it is very hot?

  2. Same situation, but air humidity is 100%, basically a sauna. Yet, wind is also extremely strong, say 110 km/h or 70 mph. Man should feel cold according to wind chill, but how it is possible(to feel cold in such conditions), since fluid (air) that has contact with his/her skin has same temperature as his body? (Assume convection/ would be relatively small because there is no thermal gradient, and diffusion would be relatively small because of high air humidity).

  • $\begingroup$ In ancient Egypt, I believe, situation 1 was exploited to cool down water. You would take a sack of reasonably dense cloth, fill it with water, which it would more or less contain, but the fabric would be wet at all times. Then you would hang up the sack in the shade. The high rate of evaporation would lead to quick heat loss. In situation 2 the wind can't cool you down below 36.6C so you don't need to produce more heat to stay warm. Additionally, at 100% humidity you can't evaporate sweat to cool yourself down. $\endgroup$ – LLlAMnYP May 7 '15 at 16:45
  • $\begingroup$ @LLlAMnYP - the same mechanism for cooling water is the basis behind the (porous) ceramic jugs called botijo. In such jugs, the water stays cooler because it seeps slowly through the porous container, and evaporates from its surface. $\endgroup$ – Floris May 7 '15 at 16:51
  • $\begingroup$ Since when did body temp drop to 36.6 C? $\endgroup$ – Bill N May 7 '15 at 16:53
  • $\begingroup$ BillN Human average temperature? @LLlAMnYP great historical reference :) regarding 100% humidity and evaporation, true, so I suppose You have same opinion as Floris that in "sauna" conditions, fan would NOT provide "chilling effect" no matter what speed? $\endgroup$ – JustMe May 7 '15 at 17:00
  • $\begingroup$ @JustMe Yes, I believe so. $\endgroup$ – LLlAMnYP May 7 '15 at 20:58

Wind chill is really due to two things:

1) colder air moves across the surface of your skin, replacing the air you heated with your body: this in essence takes away the blanket of warm air you keep making for yourself.

2) As your body loses moisture through evaporation, there is a humidity gradient of stagnant vapor around your body. The higher the humidity of the air around you, the less heat you lose by evaporation.

The conclusion from these two things is that in situation 1 you describe, the extent to which the man feels hot depends a great deal on temperature. There is something called the "wet bulb temperature": if you have two thermometers - one dry, and one wrapped in moist cotton - they will read a difference in temperature, and this difference is a function of relative humidity precisely because of the above. This is frequently used as a way to determine relative humidity, and charts are published like this one:

enter image description here

This contains all the information you need to determine how hot your person in the desert feels, depending on the relative humidity. At 40 percent relative humidity, a temperature of 100 F will "feel like" 70 F. This will be affected by wind - if the wind flows faster, there will be less humidity gradient and you will get closer to the value given. Note that the "psychrometer" works when the thermometer is in motion (you swing it around at the end of a string!) because otherwise it would not "feel the wind chill".

But it follows from the above that if you have 100% humidity, there is no "wind chill" to be felt. In fact, you feel this in a sauna - while the air is dry, you can tolerate quite high temperatures (80C). The moment somebody drops water on the stones of the furnace, it can become uncomfortable for two reasons: first, the steam condenses on your skin (which is colder than the air) which releases a lot of heat; second, there is no longer any opportunity to lose heat by sweating.

| cite | improve this answer | |
  • 1
    $\begingroup$ A ride on a motorcycle when the air temp is the same as body temp will convince you that you will NOT feel cold. Do the experiment! Edit: oops! should have posted this on the OP. $\endgroup$ – Bill N May 7 '15 at 16:51
  • $\begingroup$ @BillN - I agree. The rate at which the body can lose heat against a 37 C air flow, even if that air is quite dry, is offset by the rate at which you produce heat. Add the effect of sunlight (which is a lot of additional Watts of heating) and you are unlikely to feel a chill when riding a bike on a 37C day. After all, you have to lose heat continuously - 2500 kcal/day is about 120 W. $\endgroup$ – Floris May 7 '15 at 16:56
  • $\begingroup$ I think You will, because air humidity rarely is near 100% if ever, but and I don't think (alive) human skin can ever be 100% dry. But I'd gladly try if i have a chance. @Floris, thanks, i suppose that in case 2) wind chill would not be felt, but still i'd like to try, taking fan to sauna. I like the idea of "warm air blanket" it might somewhat answer my 1st question. $\endgroup$ – JustMe May 7 '15 at 16:58
  • $\begingroup$ Floris, but riding motorbike is rather not neglible work for human body so i think You'd produce much more than 120W, but - to evaporate 1g of sweat means to loose ~2000J which is pretty much. Sunlight even when cloudy could be a factor, but AFAIK it's around 100W/m2 $\endgroup$ – JustMe May 7 '15 at 17:07
  • $\begingroup$ Sunlight is about 1 kW per square meter (no clouds) $\endgroup$ – Floris May 7 '15 at 17:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.