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Consider two cases:

  1. You sit on a chair in a room at $45 ^\circ \text{C}/113 ^\circ\text{F}$ with closed windows and no air flow.
  2. Same case, but add a fan to blow air on to you.

Which case is going to be more tolerable? I'm interested in ambient air temperature of, say, $37^\circ \text{C}- 50 ^\circ\text{C}$. I'm inclined to think that the fan will cool you down until the air temperature is a few degrees above $37 ^\circ\text{C}$ (maybe $40 ^\circ\text{C}$?), and then it will start to heat you up.

Vincent Thacker
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EML
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1 Answers1

4

It depends on the humidity of the air well as the temperature.

There are two competing mechanisms that determine whether you will "feel" cooler or hotter.

One involves the evaporation of moisture (sweat) on the skin in which heat transfers away from the skin making you feel cooler. This mechanism is determined by the combination of temperature and humidity.

The second involves heat transfer by convection (in this case forced convection from the fan) due to the difference in temperature between the air and the skin. If the temperature of the air is higher than the skin, this mechanism involves heat transfer to the skin making you feel hotter.

Wether you feel cooler or hotter depends on which mechanism dominates. On very hot and humid days the second mechanism may dominate in which your body will gain instead of lose heat, making you feel hotter rather than cooler. Check out the following site: https://www.health.ny.gov/publications/6594/

In the absence of evaporative cooling, such as when the fluid flowing over the skin is a liquid instead of a gas, you will feel hotter. Try the following experiment in which cooling by evaporation is not involved:

Sit in a bathtub of hot (but not too hot!) water. Remain very still. Eventually you will get used to the temperature of the water. Now rapidly move your arms back and forth under the water. The skin on your arms will feel hotter than the skin on the rest of your body. That's due to forced convection over your arms in the absence of any possible evaporation of the water on the surface of the skin immersed in the water. In effect, the motion of your arms increases the magnitude of the heat transfer coefficient $h$ in the equation for Newton's law of cooling, all other things being equal.

Hope this helps.

Bob D
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