genejockey 

Okay, so if you reduce the amount of water on the skin surface, you don't have to cool as much water to cool the skin, right?

MinnMan

I'm not sure you are thinking about it correctly. apart from evaporation, the heat loss through this layer of water (let's call it a film) is conductive (or diffusive) loss. In other words, the film is an insulating layer. But water is an excellent therrmal conductor compared to air (or to air-filled cloth), and it has negligible effect unless the layer of water is unreasonably thick. If you were to posit a layer of water that was half a centimeter thick, its insulating properties could matter.

Please just take my word for it.

genejockey 

You're approaching understanding. You just agreed to the concept that a layer of sweat serves as insulation. The question is whether its absence is noticeable for the person sweating, and we just had someone observe that wiping off the sweat leaves him feeling cooler, AND we have an explanation for that phenomenon that you just agreed is valid. You simply don't believe anyone could notice the effect. You're not arguing science at this point. You're arguing belief - YOUR belief about what OTHERS could perceive.

MinnMan

Yes, I know everybody with an internet connection is an expert, but this is not in any way about "belief" or "perception" It's the physics of heat transfer..

Again, for reasonable thicknesses of sweat layers, the insulating effect is negligible. I already calculated this above.

asgelle

Without getting into the role of a surface film, I'd just point out that one can't sense heat transfer from the skin and we're not very good at detecting changes in core temperature. All of which is to say, you can't judge how effective something is or isn't based on feelings. For example, pouring cold water over yourself feels as if it is cooling you because it lowers the skin temperature, but due to surface vasoconstriction, you end up losing less heat afterwards and the overall effect is worse than before.

genejockey 

Indeed.

genejockey 

This must explain why nobody feels any cooler on hot days simply by standing in front of a fan.

Oh, wait.

asgelle

So you didn't notice the whistling as the point sailed over your head.

genejockey 

*sigh* We have nerve endings that perceive temperature, yes? And the result of heat transfer from the skin is change in skin temperature, yes? So, we perceive, if somewhat indirectly, heat transfer from the skin, yes?

asgelle

Yes, No, No.

genejockey 

So, transferring heat from the skin does not change its temperature? Do tell.

asgelle

https://www.amazon.com/Transport-Phe...6227342&sr=8-1

genejockey 

So, you're saying that if I read this book, I'll understand how heat can be transferred from Thing 1 to Thing 2 without Thing 1 losing any heat?

asgelle

No, but you might learn how to frame a consistent, logical argument.

genejockey 

Well, if you've read it, that's not a glowing testimonial.

dmanthree

Not for me. More layers equals more heat.

woodcraft

Kudos to you guys for keeping cool in a hot discussion.

Does anyone have explanation for the radical variation in the amount that folks sweat?


Personally, I sweat relatively little, live in a lower humidity area, and wear a base layer on the theory that sweat that drips off is not available to cool you as it evaporates.

Less cooling leads to more sweating in attempt to cool, and results in more dehydration. So the in-the-moment feeling of coolness is not the main point.

MoAlpha

Just about everything you'd ever want to know on the subject of sweat rate differences between and within individuals is in this comprehensive review: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5371639/

woodcraft

Thanks!

However, seems to be more everything you'd ever want to know about test methods and sample collection of sweat

Seattle Forrest

And if so, is it a Rapha?

colnago62

Where I live, for most here, 85 degrees is considered very hot. In other places, it doesn’t get as low as that all summer. Our bodies adjust to the perception of cold and hot at some level. I wonder if the same thing happens with those of us who wear base layers in the summer; we get used to the added feel of heat?

genejockey 

If a cyclist wears Rapha but nobody can see the stripe, is is still elitist?

Carbonfiberboy 

I don't think you are correct in this first assertion. We cannot feel temperature. We only feel heat transfer. For instance in a hot oven everything is the same temperature. One puts one's hand in the oven and the hand feels a bit warm. We touch the hot pot, and we immediately sense burning. This is because convection (the air) is much slower to transfer heat than is conduction (the pot). Our nerves certainly tell that conduction is more effective at heat transfer than is convection.

Your second assertion is only partially true. Whether or not cold water lowers core temperature depends entirely on the time and volume of the cold water. Standing under a waterfall definitely lowers core temperature as does a cold plunge. In both these cases our skin will definitely feel heat transfer. Pouring a little cold water on one's head will likely produce the effect as you say. Pouring 4 cold water bottles on one's head is more likely to increase heat transfer, which we will feel. A sock full of ice around the neck definitely cools one - there are two phase changes involved, and our neck skin definitely detects heat transfer until it becomes numb.

asgelle

Not quite. Air being a poor conductor of heat, your skin remains close to body temp in the oven. The pot being a good conductor raises the skin temperature almost immediately. The difference in how the two situations feel is a result of the difference in skin temperature. We know the skin temps are different because for the pot there is cell damage from the high skin temperature. In air, there is no immediate damage because skin temperature remains low at least for a while.

MinnMan

This isn't wholly true. I profess no expertise regarding thermoreceptors, so unlike others here, I won't comment where I don't have expertise. But regarding your oven analogy, whereas it is true that the temperature of the air and the temperature of the hot pot are the same, the nerves are not feeling the temperature of the external world. They are feeling the temperature at the nerve ending, inside your skin. The reason that you feel the greater signal from the pot is this case results in a larger increase in the temperature of the nerve ending - by the more rapid process of conduction from the steel handle.

Additionally, the process of heating from the hot oven air is still conduction, not convection. Convection requires that the air be in motion, but even if the air is stationary, you will feel the heating effect of the air. the reason you feel less of an effect is the vast difference in thermal conductivities of the two materials - air versus steel.

Additionally, for materials with the same thermal diffusivity, convection is MORE efficient at heat transfer than conduction. Here, its the difference in thermal properties between the materials that matters, not the heat transfer mechanism

Note that if you juxtapose two objects at a different temperature with one another (i.e, your cool hand and hot oven air, or your cool hand and hot pot handle), the instantaneous temperature at the interface at time = 0 is always half the difference (T1+T2)/2. So your hand does not feel the the exterior temperature instantaneously. In the case of contact with an excellent conductor, this changes - FAST.