Does A Base Layer Keep A Rider Cooler in Hot Weather
#76
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Good lord, are you really asking me to geek out more than I have already?
Of course, the energy required to change the temperature of water is its heat capacity, Cp, also known as the specific heat, as I covered in a different context in #53
The thermal diffusivity, D, which I used in #59 , is equal to the thermal conductivity, k, divided by the product of the heat capacity, Cp, and density, rho D= k/(rho*Cp).
Therefore, because the form of the equation in #53 is calculated with the diffusivity, rather than the conductivity, the energy required to change the temperature of water is already considered in the calculation.
I know that there are some engineers in this forum. I hope they all find this amusing.
Of course, the energy required to change the temperature of water is its heat capacity, Cp, also known as the specific heat, as I covered in a different context in #53
The thermal diffusivity, D, which I used in #59 , is equal to the thermal conductivity, k, divided by the product of the heat capacity, Cp, and density, rho D= k/(rho*Cp).
Therefore, because the form of the equation in #53 is calculated with the diffusivity, rather than the conductivity, the energy required to change the temperature of water is already considered in the calculation.
I know that there are some engineers in this forum. I hope they all find this amusing.
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Please just take my word for it.
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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.
Please just take my word for it.
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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.
Again, for reasonable thicknesses of sweat layers, the insulating effect is negligible. I already calculated this above.
#80
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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.
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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.
Again, for reasonable thicknesses of sweat layers, the insulating effect is negligible. I already calculated this above.
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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.
Oh, wait.
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*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?
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So, transferring heat from the skin does not change its temperature? Do tell.
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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?
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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.
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.
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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/
Thanks!
However, seems to be more everything you'd ever want to know about test methods and sample collection of sweat
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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?
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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.
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.
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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.
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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..
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.