Under what conditions are light coloured long sleeve shirt and long pants cooler than bare skin?

Question

In the latest heat wave I'm well aware of the power of the sun. Step out of the shade, and it hits with sledgehammer force.

Arabs are portrayed in loose head to ankle white. Cowboys in the old west always have long sleeves and long pants.

A friend claims that longs are cooler than shorts.

When do longs work better than skin for keeping cool?

My suspicions:

A single loose garment that is white or close to white, may be a better reflector of light that my skin is. Plus it moves and flaps as I move, creating different levels of ventilation. But so far haven't confirmed this.

Answer 1

British people (perhaps among others) are known for removing as many clothes as practical or possible when it is hot. But in many hot countries, they don't do that: they wear more clothes, perhaps several layers.

The problem is when you get sunlight directly on the skin. As we know, most things in direct sunlight get hot, sometimes very hot, and skin can burn.

If the skin is covered in fabric, that will get hot, but a loose fitting garment allows air to circulate. So the skin underneath the clothing is not only protected from the direct radiation from the sun, but cooled by moving air.

If you add another layer of clothing, the inner layer of fabric next to the skin does not get directly heated by the sun's radiation. But it seems counter-intuitive to wear more clothes – in my country we put on more clothes when we are cold.

What about the colour? Light coloured clothing reflects heat more effectively, but is a poor radiator. Conversely dark clothing is a poor reflector but a good radiator. But we also need to lose body heat: that is from the cooling effect of moving air, as well as by radiation. There have been studies comparing the effect of dark and light clothing. Some cultures seem to prefer light, others dark.

Following a comment from henning a previous answer states

The amount of heat gained by a Bedouin exposed to the hot desert is the same whether he wears a black or a white robe. The additional heat absorbed by the black robe was lost before it reached the skin.

I have satisfied myself that it is better to wear clothing to stay cool, by my own experience. I was waiting for a shared taxi to get its full complement of passengers before it would leave. That took about 2 hours. The morning was getting hotter and the sun higher. Inside the taxi was furiously hot, so it wasn't realistic to wait in it, and there was no other shade.

I rolled down my sleeves, put on a brimmed hat, plus a sleeveless fleece with the zip open, and stood with my back to the sun. The fleece was hanging away from my back so that air could circulate. I was quite comfortable, but another person present was suffering. They caught the sun, and drank all their water before the journey had even begun.

Answer 2

I would assume any time strong direct sunlight is involved. Doubt there would be much thermal advantage to wearing long clothing during an ultra-hot night or in a jungle for example.

It's not just white/light colored clothing however. Google bedouin traditional desert clothing and you will see a fair bit of darker-colored clothes.

I suspect part of it is air acting as an insulator between the layer which gets hit by the sun and your body. Absent clothing, your skin will absorb all the solar light heat that it doesn't reflect. With it, the heat ends up on the clothing. Some of it will be radiated back out - which is were dark clothing shines - and to affect you it has to make its way to your skin, through the air gap.

That's the reason loose garments are better. There is also a notion of loose clothes forming bellows to push air over the sweat on your skin.

On the other hand, vigorous exercise that results in a lot of motion of your whole body - like jogging - probably doesn't do well with long clothes - wind resistance, chafing, sticking and extra weight will all nullify those gains.

And even jogging promotes longer, looser clothing, though not full-length:

If you're running in hot weather, chose lightweight, loose-fitting clothing for your outing. Flowing, long-sleeve shirts can keep you cooler than short-sleeve tops or tanks because they shield more skin from the sun. When it comes to running shorts, choose longer, loose-fitting styles, which protect more of your legs from the sun and also help keep you cooler by offering more ventilation. Tops and bottoms should be light in color because light colors reflect the sun. Dark colors absorb the sun's heat and can cause you to overheat much faster than whites or pastels.

Personal experience? Plenty of times doing full day cycling/kayaking in high heat, always with white long pants and long sleeve shirts. I'd hate to have done that in shorts and t-shirts.

And that's leaving aside the question of sunburns. Those you would avoid (somewhat) with sunscreen and that messes up your sweating.

black or white?

From a 2014 study (PDF), "WHY DARK CLOTHES CAN PROVIDE BETTER THERMAL COMFORT IN HOT CLIMATE THEN CLEAR CLOTHES" by Monika Baczek, University of Technology, Katowice, Poland/Lubos Hes, Technical University of Liberec/Kausik Bal, Technical University of Liberec

From the experiments follows, that black clothing absorbs more solar radiation, both in the visible and infrared part of the spectrum, thus causing the temperature increase of the outer cloth. This effect brings the increased speed of free convection flow in clothing gap, due the chimney principle. Thus, the level of convection evaporation of moisture (sweat) from the underwear or the skin also increases and cause the required cooling effect. Contrary to expectations, the results indicate that black free cloth can also provide intense body cooling, thus offering satisfactory thermal comfort to the wearer. Measurement results confirm that those, who are dressed in black cloth are smarter than these who wear the white cloth, as their dark clothing may cool the body more the white clothing, provided that the gap between the outer and inner cloth (or a skin) is large enough to support the free convection in the gap.

Answer 3

I think a lot of it is about protection from the sun’s UV radiation since people don’t/didn’t have sunscreen and spent considerable time outside.

As a former runner, hard training in full summer noon heat in central Europe works quite well when you wear as little clothing as possible. Movement (or wind) helps the sweat evaporate on the skin. Clothing only becomes okay-ish once it’s completely soaked in sweat. However this is purely from a heat perspective, for short (<1.5h) training sessions damage due to UV radiation is of little concern and there is no need to protect against it with clothing.

Looking at photos of long-distance runners or cyclists in hot climates they usually cover their head and shoulders but otherwise wear as little as possible. You can see some with tight, white arm “warmers” probably also to protect against the sun’s UV radiation.

Answer 4

Anecdotal, from living in the Dallas Texas area for 20 years... In Dallas, the high temperature hits about 95 F (35 C) about May 15th and stays above that until the end of September, often getting a little above 100 F (38-39 C). It very rarely gets above 104 (40 C). The hottest it ever gets is about 110-112 (43-44 C). The weather forecast for this holiday weekend is literally "unseasonably cool, the highs won't get much above 90 F (32 C)".

Wear cotton, loose if possible (like sports-shirt cotton, not T-shirt cotton). Short sleeve is fine. I wear shorts from March through November when I'm not at work (all the time for the last year). Don't be afraid to sweat, a lot. Let the sweat evaporate. Linen works, but some linen has a heavier weave than cotton often has and doesn't breath well.

I used to have a motorcycle. I always rode with leather footwear, long pants and a Jacket. The preferred Texas jacket is a very ventilated weave, synthetic jacket (often called a "mesh jacket", example: http://www.joerocket.com/mesh-jackets). I found riding with that jacket and a short sleeve shirt was cooler than with a long sleeve shirt.

One thing to note is that there's a singularity in how things feel right around 98.5 F (37 C). When the temperature outside is noticeably warmer than body temperature, it feels weird. The faster the air, the hotter it feels. If you put your hand outside the car window, it will feel like it's being air fried (try this with the air conditioning off in the car).

Answer 5

No garment can be loose when you are wearing a backpack with waist and chest straps.

Use an unbuttoned shirt.

Advantages:

• protects from sunburn and sun heat directly on the skin
• if cycling, and especially if using panniers instead of backpack, is cooler than naked because it whirls the wind around your body!
• if walking, being able to unbutton or button it gives some freedom for cooling by sweat evaporation instead of it being wicked into the fabric
• concerning sweat, use synthetic instead of cotton - synthetic does not wick
• in a city, an unbuttoned shirt is far more socially acceptable than bare-chested.

Answer 6

We need to also factor in the effects of humidity. In desert climates, sweat evaporates immediately, and dehydration is a huge killer; a layer of loose clothing (of whatever colour) will prevent too-rapid loss of moisture. In humid climates the balance swings to maximizing fresh airflow over the skin.