math prof asked us a bio question today....

[Balain]

He was wondering if the temperature in a room is slightly above body temperature and a fan is blowing on you does it cool you off? He figured it would because it would help evaporate your persperation. he was also wonder if the same room was very warm like 50c would the fan still cool you?

[Seerow]

Having lived through Florida summers where AC was broken, I can say definitely yes, the fan will help cool you even if the air is hot. I can't say for certain that it's true above 100-110ish (farenheight, so around 35-40 celsius) but I see no reason why it would not hold true up until the point where the air is so hot you are going to die if you don't get somewhere cooler.

[Kato]

While not having lived in Florida or such but still having experienced hot days... yes. Although I can't entirely explain the phenomenon of why wind feels cooler than still air, at least not at high temperatures. (At lower ones it likely is because the blanket of warm air forming around your body is always blown off and replaced with cold air) Likely it is something similar, your body alwas extra-heats the air around you and then it gets replaces with cooler air when it's blown away. (?)

[factotum]

This is more of a physics question than a biology one. Basically, the moving air the fan blows onto you will help to encourage evaporation of sweat, and that takes energy, thus cooling you down. The only time this wouldn't be effective is if the air was already saturated with water vapour, thus preventing evaporation taking place; the actual temperature wouldn't make much of a difference.

[NichG]

Even at zero humidity, there's a temperature at which the airflow heats you up faster than the evaporating sweat cools you down. A bit of internet research reveals that in North India, these winds are called Loo, and are around the 40-50C range - the wind is hot enough that it actually increases the chance of heatstroke compared to still air.

[factotum]

Even at zero humidity, there's a temperature at which the airflow heats you up faster than the evaporating sweat cools you down.

Wouldn't that also be the case if you were in still air at the same temperature, though? In fact, it might even be worse because the sweat isn't evaporating as fast and thus you heat up even quicker.

[NichG]

Wouldn't that also be the case if you were in still air at the same temperature, though? In fact, it might even be worse because the sweat isn't evaporating as fast and thus you heat up even quicker.

When the air heats you up, it cools down. That means that once the air near your skin has dumped out its energy, the air surrounding that air takes even longer to transfer heat to you, and so on. When the wind is blowing, the air that heats you (and thus cools down) is being constantly replaced.

[Jay R]

But in general, in reasonable warm temperatures, a fan does cool you off. That's a common use of a hand fan - outdoors in the summer.

[Cespenar]

It's a thermodynamics problem, basically.

On one hand, higher wind speed = faster evaporation = lower body temperature.

On the other hand, if the ambient temperature is higher than your body temperature, higher wind speed = heats you faster = higher body temperature.

The balance point would also depend on relative humidity and your sweating rate besides the more obvious data.

[noparlpf]

It's a matter of conduction vs convection, isn't it? We did a lecture on that in physiology. And for humans, sweat evaporation factors in too. Let's see...

Hot body/cool, still air: The body heats up the air just around it quickly, but then it takes longer for the heat to diffuse away from the air just around the body, and that warmer air also sucks less heat from the body. (Peripheral blood vessels also constrict, slightly decreasing the loss of core heat.)
Hot body/cool, moving air: The body heats up the air just around it quickly, but that warm air is constantly replaced by cool air, drawing more heat from the body. (It's even colder, so those peripheral blood vessels stay constricted.)
Hot body/warm, still air: The body heats up the air just around it, then sits in a bubble of hot air. Adding in human sweating, there's not much chance for the sweat to evaporate because the bubble just around the body is saturated and it takes time for the wet air to diffuse away. (Peripheral blood vessels open up to help you lose heat, because it's already plenty warm and left alone your body is producing enough heat.)
Hot body/warm, moving air: The body heats up the air just around it, but that hot air is constantly replaced, so it feels a little bit cooler. Adding in sweating, the sweat evaporates into the new drier air more quickly, sucking out more heat.
Hot body/hot, still air: You're not really losing any heat here, and your sweat doesn't evaporate quickly, so your body heat builds up. (Peripheral blood vessels stay wide open, but it doesn't do much here.)
Hot body/hot, moving air: You're not really losing any heat to the air, but with human sweating, some heat is evacuated by evaporation.
Hot body/very hot, still air: You're actually drawing heat from the air now, but not much because it takes time for the slightly-cooler air around you to diffuse away/for heat to diffuse into the slightly-cooler air around you. (Peripheral blood vessels stay wide open, but now that's actually a problem because it facilitates heat transfer to the core.)
Hot body/very hot, moving air: This is the worst; now the slightly-cooler bubble around you is constantly replaced with hot air. Your sweat does help a little bit, depending on exactly how hot it is, but now you're being dehydrated quickly as well as heated up.

So basically, when the temperature is well above body temperature (40-50° makes sense, because the body is normally 36-38°) you're screwed eventually whether or not it's breezy, so try to stay in the shade and find plenty of water (or juice, because you lose electrolytes to sweat). Just above body temperature around 38-40° the breeze probably helps; somebody would have to find the exact numbers for heat lost by sweating vs. heat gained by convection, and there's probably calculus involved, and lots of variables for body mass and surface area and shade and humidity and junk. Around body temperature on a hot summer day up here in New England, a breeze does help because your sweat evaporates faster as the wet air is replaced. Below body temperature, you're losing heat due to diffusion (but if it's only a little below body temperature and it's still, the bubble around you hits body temperature and then you stop losing heat and start to heat up thanks to your metabolism), and a breeze would make you lose heat faster due to convection.

[Cikomyr]

It's a thermodynamics problem, basically.

On one hand, higher wind speed = faster evaporation = lower body temperature.

On the other hand, if the ambient temperature is higher than your body temperature, higher wind speed = heats you faster = higher body temperature.

The balance point would also depend on relative humidity and your sweating rate besides the more obvious data.

The amount of energy required to provoke a state change is pretty higher than a simple temperature change. This is why i suspect with a reasonable rate of vaporation, you will do fine

[NichG]

The amount of energy required to provoke a state change is pretty higher than a simple temperature change. This is why i suspect with a reasonable rate of vaporation, you will do fine

The real question is 'how fast do you sweat?', I suspect.

For water, the energy required to transition from liquid to gas is 550 times the energy required to raise its temperature by 1 degree C. That seems like a huge number, but at the same time its the evaporation of a very small amount of sweat that has to cool your entire body against the heating provided by the wind. If convection/conduction/etc are involved, the calculation is unobvious, but we can think of this very crudely as 'there is an amount of heat which is dumped into your body all at once; can evaporating all of your sweat account for all of that heat?'. In that case, the amount of sweat needed to keep you cool is a fraction of your body mass: 0.2% per degree C that the heat would increase your body temperature. Then the conduction/convection problem just becomes, how quickly does that much heat get delivered?

[factotum]

Well, there's also "How long can you sweat at that rate before keeling over from dehydration?"...

[ace rooster]

I live somewhere pretty cool (not that sort of cool), so I don't have experience of this, but doesn't a fan still cool you at 100% humidity? Someone from Washington DC would know I think. If it still works well at 100% then evaporation is not the only major mechanism.

Incidently evaporation is crazy effective. 100 grams of water evaporating will cool 1 kg of water by 50 degrees. Another way of thinking about it is that 2kg of water evaporating will cool a 50kg person by 10 degrees, which is why wet clothes are so dangerous in the cold.

[noparlpf]

I live somewhere pretty cool (not that sort of cool), so I don't have experience of this, but doesn't a fan still cool you at 100% humidity? Someone from Washington DC would know I think. If it still works well at 100% then evaporation is not the only major mechanism.

Incidently evaporation is crazy effective. 100 grams of water evaporating will cool 1 kg of water by 50 degrees. Another way of thinking about it is that 2kg of water evaporating will cool a 50kg person by 10 degrees, which is why wet clothes are so dangerous in the cold.

If the air is at all cooler than your body, then yes, a fan will cool you at least a bit. Once the air is higher than 37°, 100% humid air will heat you instead.

[factotum]

If it still works well at 100% then evaporation is not the only major mechanism.


The only mechanism other than evaporation that takes place is that the air which is being warmed by your body is being constantly replaced with air at ambient temperature. If that ambient temperature is significantly cooler than your body (which I think would be the case in Washington DC) then yes, the fan will cool you more than sitting in a still room would even if you're somehow not sweating.

[noparlpf]

The only mechanism other than evaporation that takes place is that the air which is being warmed by your body is being constantly replaced with air at ambient temperature. If that ambient temperature is significantly cooler than your body (which I think would be the case in Washington DC) then yes, the fan will cool you more than sitting in a still room would even if you're somehow not sweating.

I dunno, it's breaking into the 30s some days up here in NY already. If you're sitting outside in DC it might be up to body temperature some days by this time of year.

[ace rooster]

Looks like DC doesn't get those sort of conditions. I knew it got 100% sometimes, and temperatures up that high, but they seem to be mutually exclusive. Even in Dhahran (http://en.wikipedia.org/wiki/Dhahran) the dew point (http://en.wikipedia.org/wiki/Dew_point) is never quite up to 37.

Interesting that. Sometimes you get 100% humidity, and sometimes air temperatures greater than body temp, but never both at the same time.

[halfeye]

Looks like DC doesn't get those sort of conditions. I knew it got 100% sometimes, and temperatures up that high, but they seem to be mutually exclusive. Even in Dhahran (http://en.wikipedia.org/wiki/Dhahran) the dew point (http://en.wikipedia.org/wiki/Dew_point) is never quite up to 37.

Interesting that. Sometimes you get 100% humidity, and sometimes air temperatures greater than body temp, but never both at the same time.
That's probably related to the fact that the moisture carrying capacity of a gas increases with temperature, so if the temperature falls and the air is 100% humid some water precipitates out, then if the temperature rises again, the humidity must go down.

[noparlpf]

Looks like DC doesn't get those sort of conditions. I knew it got 100% sometimes, and temperatures up that high, but they seem to be mutually exclusive. Even in Dhahran (http://en.wikipedia.org/wiki/Dhahran) the dew point (http://en.wikipedia.org/wiki/Dew_point) is never quite up to 37.

Interesting that. Sometimes you get 100% humidity, and sometimes air temperatures greater than body temp, but never both at the same time.

I wonder if there are cases where it's over 37° and it's also pouring rain (like in a tropical rainforest or something). That probably counts as 100% humidity even if it's not a dew point per se.

[ace rooster]

That's probably related to the fact that the moisture carrying capacity of a gas increases with temperature, so if the temperature falls and the air is 100% humid some water precipitates out, then if the temperature rises again, the humidity must go down.

Ah yes, lol. I'm a moron sometimes. 100% Humidity will probably only be reached by hot air evaporating lots of moisture then cooling. To get high humidity at high temperature the air would have to stay hot for a significant length of time, which it does not.


Rainforests don't seem to get above 34 degrees. Convienient, isn't it. :smallsmile:

[Cespenar]

The amount of energy required to provoke a state change is pretty higher than a simple temperature change. This is why i suspect with a reasonable rate of vaporation, you will do fine

Your body doesn't sweat that much in grams, though. Also, only a fraction of your sweat can be evaporated at all. The rest trickles off before that, becoming useless for the reaction in question. Also, if it's hot and humid, evaporation slows even more, etc.

[noparlpf]

Rainforests don't seem to get above 34 degrees. Convienient, isn't it. :smallsmile:

Well, something like a sauna then. I don't see why you couldn't artificially produce close to 100% humidity at 40°. (How do you make a degree sign on the English keyboard on an iphone? I had to switch to one of the Japanese keyboards to find it.)

[Brother Oni]

I wonder if there are cases where it's over 37° and it's also pouring rain (like in a tropical rainforest or something). That probably counts as 100% humidity even if it's not a dew point per se.

From personal experience, with urban heat phenomenon Hong Kong can hit high 30s and still have 100% humidity, but when the temperature goes above 40C, the humidity tends to drop to a comparatively drier 60-80%RH.

[Killer Angel]

He was wondering if the temperature in a room is slightly above body temperature and a fan is blowing on you does it cool you off? He figured it would because it would help evaporate your persperation. he was also wonder if the same room was very warm like 50c would the fan still cool you?

Now, the real question could be... what were the prof's thoughts on the matter?