The Physics that Explain Why You Should Wear Black This Summer

Illustration for article titled The Physics that Explain Why You Should Wear Black This Summer

We're all encouraged to wear white in summer, since white clothing is supposed to keep us cool — but it doesn't. In fact, black clothing is the best way to keep cool in the heat. It's basic physics. And biology. Find out why cool people will wear black this summer.


Generally in summer, we're treated to lines of loose summery white clothing. Not only is the white supposed to look nice floating around the edges of a picnic — until a few seconds into the event, when it has its first grass stain — people claim that white is the ideal way to keep cool in the summer. When we see white, we're seeing the combination of all possible visible light. This means that white clothing reflects a great deal of wavelengths of energy coming in. This means it should reflect the sun's rays back, instead of letting them cook us. And that's perfectly correct.

Except that this explanation is also incomplete. Heat is not just coming in off of the sun. It's also coming off a person's own, sweating, warm-blooded, mammalian body, which is a lot closer than the sun is. When all that body heat hits the white clothing covering it, it gets reflected right back towards the body. When we wear white, we cook ourselves.

The best color to keep cool in the heat, it turns out, is to wear black. Black absorbs everything coming in from the sun, sure. But black also absorbs energy from the body instead of reflecting it back. Now, the helpfulness of black clothes depends on finding black clothes that are the same thickness and looseness as those summery white clothes. Black clothing also needs a little help from atmospheric conditions. Once it has absorbed heat, it has to have some way to radiate it away. If there's even a little wind, black clothing is the better choice for those who want to keep cool, like goths who understandably don't like sweating through their make-up. So find something black to wear this summer.

And if people ask you what you're in mourning for, tell them you mourn their limited grasp of physics.

Via Straight Dope.


Stephan Zielinski

The conclusions of the cited study (Walsberg, Campbell, & King, 1978. J. Comp. Physiol. 126B: 211-222, abstract here) can't usefully be applied to humans. They studied pigeons, which mass somewhere around half a kilogram. Humans mass around 70 kilograms. Hence, pigeons have a much, much higher surface area to volume ratio than do humans. Radiative heating and cooling thus plays a far greater role in their thermoregulation than it does in animals our size.

Even were this not the case, the numbers don't add up. The metabolic heat a human produces on a 2000 nutritional calorie per day diet is about a hundred watts. Human surface area is about 1.73 m^2. Simplifying to a rectangular human in a 1:1:5.5 ratio with one long face exposed to the sun, about 5.5/24 or 23% of the total surface area's going to be exposed at any time. (It's more complicated than that— humans aren't rectangular, and actually half a person will be in sunlight impacting at various angles from dead-on to skimming— but just trust me that 23% turns out to be in the right ballpark.) This is around .4 square meters. Direct sunlight at noon on the equator on a clear day is 1 kW/m^2, but let's be generous and assume haze and a somewhat rakish angle, halving it to 500 watts/m^2. 500 W/m^2 * .4 m^2 = 200 watts.

Again being generous, let's assume white clothing has an albedo of .9 and black clothing one of .1. White clothing will thus absorb 20 watts of the incoming 200, and black clothing 180. So even if black clothing magically wicked away all the 100 watts of metabolic heat a human produces and white clothing magically trapped all of it, black clothing would mean a human has 180 watts of unwanted heat to deal with, while white clothing leaves 120 watts— a hundred from metabolism plus twenty from the sunlight.

Various desert peoples wear black clothing because it looks good and it wears well, and they have enough sense not to run around like mad dogs and Englishmen in the midday sun. Plenty of folks who can afford to maintain white clothing wear that instead.