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Old 07-27-2011, 10:10 AM   #19
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Joined: May 2009
Location: Spotsylvania, VA
Oddometer: 1,774
I'm going to cut and paste another member's explanation for this. The reply was to a similar question in this thread in the equipment forum by der_saeufer. This is the first time this concept has been explained to me in a way that made sense. It is making me re-examine my views on mesh.

You're right on the money.... here comes the science

The reason sweating works to cool the body is not simply because sweat is warm but because evaporating water uses a large amount of heat to change phase. If this weren't the case, people would die of heatstroke just standing around any time the temperature got much over 100.

To raise one gram of liquid water from 0C to 100C takes 100 calories (small c, the heat unit, i.e. 1cal=4.18J). To evaporate that same gram of water, turning it from 100C liquid water into 100C water vapor takes 539cal, and you haven't changed the temperature a bit.

If that water is touching your skin when it becomes water vapor, it absorbs a fair amount of that heat from the skin, helping your body stay cool. If liquid water/sweat gets blown off your body as a mist, it absorbs all the energy for the phase change from the atmosphere and does essentially nothing to cool your body.

When you wear a "rapid wicking" t-shirt on a very hot day (say 105F/40C) while riding, especially in a dry climate, your sweat gets wicked and then blown off your skin to evaporate on the shirt or in the breeze. Same goes for that t-shirt under a mesh jacket as the wind flows freely through. Put a vented, solid jacket on with a heavier but tightly-fitting cotton t-shirt (extra points for long sleeves) and you get soaked in sweat, but all that water is evaporating while touching your skin, and it cools your body far more than it would just getting blown away.

If all that sweat is blowing away without cooling your body, the body's natural reaction is to sweat more. All that extra sweat still blows away and now you're really farked: you can't keep your temperature down and you're blasting precious water into the atmosphere.

That's why riding on a really hot day in just a t-shirt feels awesome as your clothes dry out after sitting at a stoplight, but at 70mph on the slab (once your clothes are dry) it feels like you're a loaf of bread in an oven and you get thirsty in no time flat.
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