Question for the science folks–which keeps you cooler in hot, humid weather?

What cools you in hot weather is the evaporation of water.  It takes a lot of energy to evaporate water, whether it is boiling hot or at room temperature.  That’s why critters in warm climates sweat and pant – to keep their core temperature within a narrow range even when the outside temperatures plus solar input are higher than body temperature.

And that cooling effect takes place right where the water turns to vapor.  If it is your skin that is evaporating sweat, your skin gets cooler.  If tight-fitting clothes get wet from sweat or added water, your skin also gets cool.

If loose-fitting clothes are wet and get cool, the benefit is less direct.  The air between you and the clothing is cooler, but some of that evaporation is just cooling the environment.  I notice that when I wear loose-fitting clothes, I’ll twist or turn and bring a very cool portion of the clothing into contact with my skin – telling me that it had long been evaporating water and being cool but not cooling me directly.  One point for tight clothes.

Tight clothing transmits solar input right to your skin, while loose clothing keeps your skin the shade.

My preferred combo for a Grand Canyon hike in the summer is:

• go in the Spring or Fall, instead.  When sane people would.
• a loose-fitting, loose-weave* light-colored cotton shirt so my skin is shaded from the sun, and air passing through the shirt evaporates some sweat off my skin underneath.  Cotton so when I pass a stream or water faucet I can soak the shirt.  I find it less exhausting to apply a pint of water externally to my shirt than to drink and sweat out a pint of water.  Usually, I use a retired men’s dress shirt.  The collar is good sun protection and I can un-button the front for a little more breeze to my skin.*”loose-weave”: try breathing through a quadruple layer of different shirts in your closets.  You’ll find that apparently similar cotton shirts have dramatically different abilities to pass air quickly and easily.  Note the ones that pass air the easiest for use in hot climates.
• alternately, if hiking with a umbrella like the Chrome Dome, I’d have a close-fitting, wicking polypro shirt.  Since the umbrella is providing the shade, the tight-fitting shirt lets the sweat evaporate in contact with my skin where that cooling does the most good.  A downside is the lack of absorptive capacity of the shirt, but I can use a bandana around my neck and add stream water (as I cross the stream or from a water bottle later) for cooling without having to sweat so much).  An upside of the wicking shirt is it is ready to use for warmth soon after dusk because it dries so quickly.  It can be tricky to time the drying of a cotton shirt.  At 6 pm you want it wet for cooling and it is wet because you’re sweating, but only 30 minutes later you want it dry for warmth and it takes longer than that for it to dry out completely. 

From a science/research based perspective, what keeps a person cooler in hot, humid weather

That needs to be defined, when I look at your statement, I think 80-90 degrees and +80% humidity.  In that case (IMO) there isn’t anything that will really work.  Because when the relative humidity is high, the drivers to promote evaporation don’t exist (except wind),  wuth heavy activity your clothes get soaked with sweat.  Most of those times, I found that wicking t-shirts work ok, but they still get soaked. I lived in Miami for 3 years, I’ve hiked/backpacked on tropic/subtropic islands (French Polynesia, Hawaii, Australia).

This solution is to realize that you are going to get hot & sticky and just figure that is the price for being in that environment.  At the end of the day, rinse off your clothes with fresh water (if possible), jump in the water or give yourself a sponge bath.  Sit still and let the water evaporate to cool off and then put on dry clothes (if you have them).  When I lived in Miami, I have a regular job and you just got use to the fact that you would have a thin film of sweat on you all of the time.  That is one of the reasons that I only lasted there 3 years.

Hot and lower humidity (like 50-60%) is a very different story.

As I understand it, there are 3 main factors helping or hurting your ability to cool down:

1. Evaporative heat loss, which depends primarily on air humidity and air flow.  Obviously the higher the concentration of water vapor in the air immediately next to the evaporative surface, the slower the rate of evaporation (or no evaporation at 100% humidity).  Air flow simply moves the more saturated air next to you away, replacing it with less saturated ambient air.  Fabric against your skin can increase evaporative rate by increasing surface air – absorbing and spreading the sweat out across the surface and amongst the internal surfaces of the fabric, as well as decreasing the intermolecular attractive forces between water molecules.
2. Convective heat transfer – transferring heat directly away from your skin to cooler air next to skin/tight clothing.  High air flow (ventilated clothing, windy conditions) increases this transfer.  This obviously does not cool, but rather heats, your body if the air temperature is higher than body temperature.
3. Thermal radiation – the radiant energy from the sun directly heating your skin/tight clothing.

So you basically want to maximize loss from 1 and 2 and minimize gain from 3.  I think a tight-fitting base layer made of conductive fabric is ideal, and under high sun conditions, use of an additional very loose-fitting outer layer made of a fabric that will prevent penetration of sunlight will provide shade while still allowing good air flow between the 2 layers.  I don’t think color of the outer layer really matters, but dark fabrics tend to block more sunlight, which is why Bedouin often wear black robes.  Obviously, when humidity is reaching 100% and air temp is reaching 98F, no clothing is going to work to cool you off – you need to find cooler water to immerse yourself in.

Interesting Dave, so sounds like looser, more breathable clothes tends to be better for sun exposure and tighter fitting, well wicking clothing tends to be better in shade.  Makes senses and seems to fit my experience so far.  Perhaps for Sun conditions, tight fitting, more conductive, well wicking garment plus reflective umbrella would be most ideal?

But what about highly conductive materials like certain metals or materials with high thermal inertia (like jade)–wouldn’t that theoretically, be able to help the water evaporation process cool the skin–especially when it’s very humid and the evaporation process is less than ideal?
Hi Jon, yes, i completely agree with you based on experience that when it gets hot and humid enough, nothing is going to really “keep you cool” in any comfortable way, but i have noticed some definite differences in relative comfort between certain garments.

For example, when in Costa Rica a couple years ago, an OR Echo ls shirt kept me more somewhat more comfortable than other things when active and sweating A LOT.  I’ve worn all cotton is less worse conditions (typical southeast US Summer conditions), and it just felt like a soggy, uncomfortable mess (however, i’ve found it’s pretty good in desert, very low humidity and high temp type conditions).  While in Costa Rica, i also wore typical, loose fitting linen clothes while just casually walking around (still sweating a lot, but nothing like when hiking up a volcano), and felt somewhat more comfortable than if i had been wearing say an all cotton shirt, or a typical nylon supplex shirt (tightly woven and not wicking).

While none truly kept me “cool”, some things have been relatively less or more comfortable depending.  I’m just trying to figure out which is “most” comfortable for these conditions–partly because i live in the south east and it’s coming up to Summer, and partly because i have a curious, inquisitive, thirsty for knowledge and understanding kind of mind.

I’m curious about something like a say a woven wicking nylon+aluminium blend?

“I think a tight-fitting base layer made of conductive fabric is ideal, and under high sun conditions, use of an additional very loose-fitting outer layer made of a fabric that will prevent penetration of sunlight will provide shade while still allowing good air flow between the 2 layers.  I don’t think color of the outer layer really matters, but dark fabrics tend to block more sunlight, which is why Bedouin often wear black robes.”

Interesting.  Reminds me of my desert solution/idea. I’ve had the idea for a while to wear a tight fitting, more tightly knit or woven cotton spandex blend black shirt against skin, and a very loosely woven, loosely fitting, white colored linen shirt over that.  Maximizes and prolongs evaporation cooling, while minimizing Solar heat and UV gain.

But when it comes to thin and single layer clothing, i’ve noticed a big difference between wearing white and black in the Sun.  BIG difference.  I think with the Bedouins, the fact that the robes are so thick, somewhat insulating, and yet pretty breathable counter balances the dark color/Solar gain somewhat.  Many modern desert dwellers, if not hung up in fashion or culture, wear white and light colored cotton and linen clothes.  I’ve looked at Muslim specific clothing stores.

With heat transfer, we mainly think about conduction, convection and radiation, but these are all related to material properties.  With respect to feeling cooler, the dominant effect will really be none of those but evaporation.  The latent heat of vaporization (phase change) of water is  2264.76 kJ/kg.  Even with a highly thermal conductive material, the phase change effect will be the dominate factor.

Given this, I like the idea of a snug shirt to wick and a loose shirt to block the sun (or better yet an umbrella).

If I recall correctly, if you jump into the water and hop out again you will feel cooler wearing a wet wicking t-shirt than is you had no shirt on at all (evaporative cooling:2264.76 kJ/kg).   I have a trip coming up to Mo’orea, I’ll test the theory.

You guys talking about this just as we are getting hammered here in Pennsylvania.  Near 90’s and very high humidity.  This following last week with highs in the 60’s and cloudy.  I still don’t have a good answer.  I did find last year that this “spandex” long sleeve shirt did a surprising job at keeping me cooler when I was working hard.  Yeah I was still a sweaty mess but I didn’t feel as unbearable as when I had my shirt off.  The sun was just cooking me.  I picked up an underarmor golf shirt, short sleeve, for summer hiking but I’m finding it’s not really that cool, I suppose it might be because it’s too loose but it feels a bit hot.

It’s hard to imagine wearing two shirts but maybe a very thin long sleeve shirt with a white loose fitting shirt would work well.

I can’t afford all this new science stuff, maybe there is something that works.

An interesting observation for all you west coast folks.  My quick dry trail runners and shorts got soaked during a water balloon and hose battle.  After several hours outside the shoes still squished water.  After hanging to dry over night the shorts were still wet.  Things just don’t dry here.  It might be the reason people like water proof shoes and boots.  Yes you sweat but it’s nothing like being soaked from rain or shallow puddles.

But companies are starting to come out with more actively cooling type tech, everything from flatter/wider fibers/threads and yarns, to embedding more highly conductive materials (stones, metals, etc) in the threads/yarns, to embedding heat and moisture

Correction: their marketing spin is making all sorts of psuedo-scientific claims about how wonderful their products are. But once again, 95% of all that is nothing more than pure spin. Nothing has really changed, except that they have now learnt some science words.

Shade works wonderfully: it blocks incoming radiation.
Air circulation gets rid of trapped hot air.
Wicking may help slightly by spreading the sweat out to enhance evaporation, but it is not a panacea.
Not walking in the middle of the day is probably 10x more effective than any of the latest fabrics – or even more.

Cheers

Of some relevance, we had an interesting discussion on clothing color last year.

https://backpackinglight.com/forums/topic/99727/

Loose clothing always for me.

nothing to add to the science part of it–looks like the other comments covered most of the relevant information–but some specific recommendations:

light-colored, form-fit clothing has always worked best for me. wool and synthetics have their pros and cons, but for humid weather i almost always wear a blend (Rab Meco, Smartwool Tencel, Icebreaker Tech Lite, etc…). i like shirts that are thick enough to protect against solar irradiation (so no mesh or loosely knit fabrics), and if the sun is really beating down, more coverage (long sleeves, really breathable pants, hat, sometimes a neck gaiter). i will wet my shirt cold water to enhance evaporation cooling if i do not plan to sweat much.

and i must agree with Roger…avoid being out in the middle of the day if the temperature/humidity is extreme.

“It’s hard to imagine wearing two shirts but maybe a very thin long sleeve shirt with a white loose fitting shirt would work well.”

Hi Todd, i would do the above only during very, very dry and intense solar conditions like desert type and with that specific combo of materials (or ones very similar).  I agree, i would want the inner shirt to be fairly thin, and the outer shirt pretty loose and otherwise very breathable.

“Correction: their marketing spin is making all sorts of psuedo-scientific claims about how wonderful their products are. But once again, 95% of all that is nothing more than pure spin. Nothing has really changed, except that they have now learnt some science words.”

Ouch, glad i’m not an engineer or materials expert that works in R&D for one of these companies.

But yeah, i do mostly agree with you, with some exceptions. Due to expense/budget factors, and being wary of marketing spin–i haven’t tried a lot of the new, supposedly extra cooling garments.

I have tried a few though.  One is Under Armour’s previously called “Iso-Chill” (i think it’s called something else now).  All it is, is flatter fibers and thread/yarns that contact more of your skin and still/trap a bit less air.   Interestingly, it does feel noticeably cooler to the touch than many other fabrics.  Whether or not it really affects your overall body temp, especially core temp, is probably unlikely or to such a small degree it doesn’t matter.

But perceived comfort does go up a little. I find in those truly horrid, very hot and humid conditions, every little bit helps.

On the other hand, Cool-Core actually does work pretty well, especially when the humidity is not super high.  Cool-Core is a combination of synthetic, hollow fibers with little holes, intimately knitted or woven with very well wicking synthetic fibers.

The idea is that the hollow fibers with holes, pulls in and circulates moisture in a more prolonged sense, while the very well wicking synthetic fibers, spread some of the moisture over a larger surface area and speed up the evaporation process that’s directly on or next to the skin (keeping it more dry and comfortable).

It’s a pretty solid concept, and easily tested. Wet out the fabric, snap it or expose it some movement and air circulation (wind, walking fast, etc), and the material’s temp drops quite a bit from ambient temp.

Problem is, with super high humidity (80% and higher), the evaporation process in general slows way down, and it doesn’t provide as much benefit.   The other problem, for me at least, is that it’s made out of all polyester, not treated, and can get pretty dang funky.

If they switched to using all Polygiene treated nylon, i would buy a bunch of the shirts, because like i said, i’ve found the basic concept to work pretty well in all but the most humid conditions.

Thank you for the link Ralph.

Thank you Ryan, Jon, John, and Jared for the feedback.