Did Marilyn mess up again (column about evaporative cooling)?

In this week’s column, Marilyn Vos Savant explains that sitting in front of fan in hot weather “may cause your skin moisture (a.k.a. sweat!) to dry up, taking away your body’s main means of cooling itself.”

Isn’t the whole point of sweating to allow the sweat to dry up? That’s what cools you off, not moisture sitting on your skin. (Note that the context is hot temperatures, 100+F.) I seem to remember a thread that I posted to (that I cannot now find) where I said that a breeze when the ambient air is hotter than body temperature would make you feel hotter, and it was refuted by reminding me of evaporative cooling.

I think the issue of becoming dehydrated or suffering from heat stroke as a result of the body’s inability to stay cool in extreme heat is a separate issue from whether a fan cooling will cause your body to cease to cool off. She seems to be concluding that the result of the former is due to the latter, a classic cause and effect fallacy.

Please see entire column (it’s short) before responding to this thread, and then tell me what you think.

I would guess that the part of the cooling comes from the sweat spreading out on the skin before evaporating. If only the area immediately around the sweat pore is evaporating while the rest of the skin remains dry, your still going to suffer some sort of hyperthermia.

The way I’m reading that, it’s because when the fan is (key phrase) turned all the way up the sweat isn’t evaporating, it’s blowing off your skin. At least that’s what I’m getting out of it. I think ‘causing your skin moisture to dry up’ is a clumsy way of wording it. But I could very well be misreading it.
But it seems to hang on having the fan all the way up and sitting right in front of it. I would imagine, in that scenario, sweat won’t evaporate it’ll just dissipate (is that the right word) in to the air.

IOW If the fan is set high enough that the sweat remains a liquid (even if it’s atomized) when it leaves your skin it’s not going to take any energy with it…there we go, that makes more sense.
It needs to be on your skin long enough for your body heat to turn it into a gas. That will cool you…a fan on low carrying that vapor away from you is will help you feel cooler by speeding up the evaporation by moving dry air in and pushing the humid air away.

No, this lady is incorrect. Barring some physiological abnormality that prevents pores from leaking sweat ( or due to old age), the sweat evaporation will cause the skin temperature to drop as per the laws of thrmodynamics. And I don’t think that it’s physically possible for sweat glands to squish sweat so far high into the high velocity air stream, completely bypassing contact with actual skin, and bypassing the narrow (yet real) laminar air flow next to the skin.

An unrelated issue is one where hyperthermia causes the body to shut down sweat production.

When the air blowing past you is hotter than your skin, then you have two competing heat transfer effects:

-convective heating, and

-evaporative cooling.

In cases of extreme heat, I think the concern is that with high slipstream velocities, convective heating starts to win over evaporative cooling. In other words, when temperatures are 100+, you may actually achieve better cooling with low (but non-zero) air velocities than you would with jet blasts. The key is that with high slipstream velocities, the cool liquid sweat on your skin ends up absorbing more heat from the air than it does from your skin. Since the whole point of sweating is to cool your skin rather than the air, this is a bad deal, a waste of sweat. Slower slipstream velocities will nicely carry away the humid microclimate next to your skin, facilitating evaporative cooling without heating you up like a pot roast in a convection oven.

This is purportedly is part of why people in smokin’ hot climates (e.g. north Africa, Saudi Arabia) tend to wear full-coverage, lightweight fabrics: it limits the slipstream velocities near their skin.

I’ve even experienced it myself, riding my motorcycle once in 110F temperatures: I found that my face was cooler if I kept my full-face helmet’s visor slightly cracked, rather than fully open and exposed to the ambient wind.

That’s some fan. Most people don’t live in a wind tunnel.

I can imagine one small effect here. If you take a droplet of sweat and evaporate a little bit of its water, the droplet becomes cooler. It then transfers heat from your skin (by conduction), and the sweat droplet evaporates some more, etc. Before it’s all the way gone, it’s had time to transfer heat energy out of your skin.

If the fan makes it evaporate so quickly that it doesn’t have time to get as much heat from your skin, then that droplet of sweat would not have provided as much cooling as it otherwise would have. I guess this would mean that the droplet gets its latent heat of evaporation (it has to come from somewhere) from the air instead of your skin.

On the other hand, my gut feel is that this would be a really really small effect, and it still doesn’t explain Marilyn’s wording in the OP.

I’m pretty sure she’s talking about high temperatures - only when air is warmer than body temperature. At that point, you’re looking at two competing processes:

  1. sweat drying and cooling you
  2. hot air moving over dry skin and warming you.

The higher the temperature and the higher the air speed, the more I see this being a problem.

Since it wouldn’t be a problem when air is cooler than body temperature (in that case, even case 2 has some cooling benefit), the original writer really should have been more specific.

(Either that or she’s just plain wrong and I’ve merely been searching for a way to give her the benefit of the doubt.)

The answer as “Marilyn” gives is at best stupidly over-simplistic, and at worst just plain stupid. And I know a little bit about the subject.

What Machine Elf (and then dracoi) said, and also CurtC may be on to something if the droplets are so standoffish that the convected air supplies most of their heat of vaporization.

But, it’s not at all clear that Marilyn was thinking this way. My impression from reading the column was that she didn’t understand well, though there isn’t enough information there to be sure.

Could be her editor; she only has so many column inches.

A quick WAG on my part. I’d say the airflow speed in which your sweat doesnt quite drip off is the optimum speed for cooling your body (when the air temp is above your body temp). Now, that may not FEEL the best however.

She made an error recently and blamed it on her editors.

Seeing as Cecil does the same thing, I’m disinclined to believe her. Especially since no editor should be adding stuff to a column.

Just because it *shouldn’t *happen doesn’t mean it won’t. I’ve worked with proofers who were so ignorant that they’d actually *introduce *errors in punctuation (for things they weren’t even supposed to be touching, like commas), so I assume there are probably editors who do the same thing.