NoWhammies

I am in the wearing a base layer camp. Specifically the Under Armour Heat Gear. I like the compression aspect of the layer. That said, I am not riding in 30+ degree heat. But for the riding around here, and the riding that I do, I find the base layer does a good job of keeping me cool.

Iride01 

I know you didn't take the opposite position. You came out swinging at me by telling me I've got the physics wrong. However I'll stand by my initial post. And since you are attributing implications to it that I did not say, then you simply are misunderstanding what I said.

In my case sweat accumulates faster than it will ever evaporate. So an ever growing pool of sweat against my body will be overwhelming any cooling from evaporation. Cooling effect from evaporation occurs on the surface. If I have too much sweat between me and the surface, I won't get enough benefit.

Believe me, at no time, other than a short time after starting my ride in the current temperatures here are me or my clothes dry. I simply sweat a lot. There have been times when I stop that there is steady stream of sweat dripping from me. I can't imagine how keeping that sweat on me will benefit my body.

Sweat dripping off of me does carry away heat with it. It has a measurable temperature and whether or not it is the same temperature or cooler does not matter. It is taking away heat that once was belonged to me. So the total heat contained on and in my body is reduced. Don't confuse heat and temperature. I know I do that too often.

Have you ever taken a towel when you come in from the heat and wipe the sweat off of you? Then you should realize that by removing the sweat that is not immediately being used for evaporation, you get a much better cooling effect on your skin.

Why? Because you've made a more ideal situation for your body to benefit from the cooling effect of evaporation. You've removed the excess sweat that buffers the effect and you've brought the surface being cooled in much closer contact to your skin.

You body benefits from bringing the cooling effect closer to it. A base layer and outer layer will only serve to keep it further away.

MinnMan

Sorry if you are offended, but your physics are still wrong. Removal of sweat at the same temperature as your body has no cooling effect on your body.

I'm sorry if you, in your own words, often confuse heat and temperature. I don't The key relationship is, for a reversible process, dQ= Cp dT where Q is heat, T is temperature, and Cp is the heat capacity. Heat capacity has units of energy/mass (as in Joules/gram, usually). The sweat that leaves at a particular temperature carries with it a certain amount of heat, but also commensurate mass, leaving the mass-averaged heat and therefore the temperature of the remaining body the same.

Iride01 

Then please enlighten me with your knowledge.

I didn't say it in itself cools me, I said it carries away heat. The less heat I have on my body, the more efficiently the evaporative cooling will be for the heat in my body.

colnago62

Dude, he knows more than you about this. Let it go.

Iride01 

I know, I feel like those villagers long ago that said rocks sometimes fall from the sky.

I'm going to ride, I'll be gone for two or three hours. Thankfully it's cooler. Though the humidity will still have the sweat dripping from me in streams.


Temperature 79.9 F
Dew Point 76.1 F
Humidity 88 %
Wind Speed SSW 0.4 mph
Gust 0.9 mph
Pressure 29.90 in

asgelle

Can you explain why there's a different physics for swamp cooler areas? I've seen people claim magical physics just for bikes, but this is a first.

By the way, low dew point conditions are exactly where you don't want a base layer. You want the heat of vaporization to come directly from the body; the last thing you need is an insulating layer between the skin and the surface where evaporation takes place.

Nachoman

Don't forget to wear a baselayer.

MinnMan

If you are talking about the insulating effect of a layer of water inhibiting the cooling to your skin ,then it would have to be a rather thick layer. The thermal diffusivity of water (D) is around 0.2 mm/sec^2, which means that for a layer (x) 1-2 mm thick (that's a thick layer, think about it), the thermal equilibration time (x^2/D) is just a few seconds.

BTW, a uniform layer 1 mm thick on your entire body would be almost 2 liters of water (average male body has a surface area of 1.9 m^2, etc.).

walnutz

My head hurts.

MoAlpha

Here's another factor to consider: Skin wetness inhibits sweating through a mechanism involving higher-order sensory perception (since there are no specialized wetness detectors in the skin). Therefore, the wetter you feel, e.g., in a clammy shirt(?), the less you sweat. That could help conserve water and Na+.

genejockey 

Well, the thing is, you have a point, but not the one you seem to be making. Dividing, let's say, 70 kg at 98 degrees F into 69.95 kg and 0.05 kg, both at 98 degrees won't cool the 69.95 kg at all.

BUT!!! What is happening when sweat drips off or is wiped off is that you don't have to also cool down the sweat that's on your skin before you get to the skin. If your body is covered in sweat, the evaporation at the surface will cool the sweat down first. A thin sheen of sweat, like you leave when you wipe your face, cools your skin directly.

Is this what you were getting at?

genejockey 

I must be missing that mechanism, because I swear to god, the more I sweat the more I sweat!

MinnMan

I covered that in #59 . For a realistic layer, the effect is negligible. The "before" is seconds, after which, steady state heat transfer occurs and the effect is null.

genejockey 

You may wish to look up the energy required to change the temperature of water.

Seattle Forrest

Seattle Forrest

It takes 1 calorie to heat 1 gram of water by 1* c.

1 calorie = 1 Joule

1 Joule (per second) = 1 watt

Therefore, doing hot yoga in a heated vest gets you an extra 75 watts.

This is how everyone else is so fast on group rides.

You should look into heated saddles, like in your car.

asgelle

More magical physics. 1 cal=4.184 J.

genejockey 

Ah, but are they wearing a base layer under that?

Seattle Forrest

​​​​​​That's what provides the energy required to get the rabbit out of the hat.

noisebeam

For me a thin base layer makes things worse in the desert. Best is tight fitting single layer. Every drop of water facing wind evaporates immediately and the main issue is body sweat rate can not keep up. I've left home in 117F 2% RH with a dripping wet kit and it is bone dry 5min later except my back which takes another 5min. Then the nearly dry layer just feels like it traps more heat.

dr_max

Hotter means no layers

asgelle

It may feel that way, but unless you're suffering from heat stroke, you're still sweating and losing heat from evaporation.

noisebeam

Correct, but the double layer doesn't help with evaporation. It just feels hotter. I've done this 110F+ ride in full sun hundreds of times and experimented with varied strategies.

That is why I said 'nearly dry' - I'd rather have a single thin nearly dry layer on my body than two which traps more heat.

It seems you agreed with me in this post:

MinnMan

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.