When the sun is shining, we usually dress in white because this color reflects radiation better. Yet desert nomads also wear black clothes. Why?
How to dress to protect yourself from the sun? “In white!”, We think, because we know that this color absorbs little solar radiation. Yet desert nomads also wear black clothes. Can dark clothing be as comfortable in the sun as light clothing? An examination of the various exchanges of energies that come into play sheds light on this interesting question.
The color of a surface depends on the amount of light it reflects back to us and, by complementarity, the amount of light it absorbs. The darker it is, the more light it absorbs, and therefore light energy, which turns into heat. When it is high in the sky, the Sun brings close to the ground near 1000 watts per square meter. A black-colored object will absorb up to 90 percent of this energy. We take advantage of this energy source with “solar panels”, blackened panels where water is circulated to heat it inexpensively. Conversely, if you want to reduce solar heating, you use the color white, because it reflects almost all of the incident light. This is why the walls of houses in sunny countries are painted white.
However, this analysis is far from complete! In order to establish the true energy balance of a garment, you must also take into account how it loses energy through radiation. Every body radiates the more it is hot. Clothing at the temperature of the summer sun (40 ° C) emits, for example, in the far-infrared (around 9 micrometers in wavelength). However, good absorbers are also good emitters. Black, which absorbs a lot, also emits a lot: around 500 watts per square meter for a surface at room temperature. Conversely, white, which absorbs less, also emit much less than black. This is one of the reasons why the arctic bear is white: it has more interest in keeping its internal heat by radiating little than in absorbing the little sunlight it receives …
Energy balance
In detail, the properties of a body vis-à-vis visible light and infrared radiation depend not only on its color but also on the material it is made of. Let us compare the black and white fabrics used by the Bedouins. The energy absorbed by the black robe of a person facing the Desert Sun is 840 watts per square meter, while its infrared radiation losses are 540 watts per square meter. The net gain of 300 watts per square meter is one-third of the power input. A white dress absorbs 650 watts per square meter: this high value is due to the fact that white fabric absorbs infrared radiation just as much as black fabric, which alone accounts for half of the solar light energy reaching the ground. In addition, a white fabric emits in the infrared about as much as a black fabric: 530 watts per square meter. In the end, each square meter of white fabric absorbs only 120 watts, almost three times less than black fabric. Israeli researchers measured the surface temperatures of two Bedouin robes when it was 38 ° C in the sun and found that of the light clothing was 41 ° C while that of the dark clothing was 47 ° C.
Thus, black clothing looks dangerous, since an increase in body temperature of more than 4 ° C is fatal. To understand why desert nomads still dress in black, let’s take a look at their tents, which are as black as they are comfortable. Their comfort results from two physical effects: the shade produced by the canvas and the ventilation of the tent. Since it absorbs solar radiation better, a black canvas provides much better shade than a white canvas of the same thickness. More heated by the sun, however, it risks transmitting its heat to the air it traps. To prevent this from happening, the Bedouins open their tents wide. The air in contact with the black fabric heats up, expands, and, becoming less dense than the surrounding air, rises before leaving the shelter at the top. It draws cooler outside air into the tent, taking with it the heat transmitted by the canvas. The warm surface of the tent thus initiates a convection movement that constantly renews the air in the tent, even in the absence of a breeze. The motor of the “Bedouin fan” is indeed the black canvas of the tent.
The double robe of the Bedouins
Is it because of the ventilation that the Bedouins do not suffer from the heat that accumulates on their black clothes? Yes, but before explaining it, let’s take a look at what a comfortable garment in the Sun is: like a tent that always stays at room temperature, such a garment helps the body to maintain its temperature at 37 ° C, regardless of the weather. outside temperature. For this, it protects the body from external heat and also evacuates that which is produced permanently by the metabolism (even at rest a man dissipates at least 100 watts). This regulation takes place through perspiration. To evaporate, sweat absorbs heat and takes it from the skin. Any additional warm-up must be compensated for by increased sweating. In the tropics, you can lose more than half a liter of water per hour during the hottest times. Comfortable clothing facilitates cooling evaporation.
Now we understand the choice of dark clothes and light clothes. Clothing that is worn close to the body, such as a shirt and pants, is in contact with the skin. To prevent the fabric from getting too hot, it is best if they are clear. Bedouins on the other hand wear loose clothing, large robes covering them from head to toe to protect them from the sun, wind, and sand. The men first put on a large cotton shirt directly against the skin. The latter is then itself covered by a loose garment. The outer fabric is therefore not in contact with the skin. In addition, air circulates easily between the two garments. The air heated by the outer fabric rises inside and sucks colder ambient air under the dress. In this convection movement, it is as if the two dresses were playing the role of bellows, pushing hot air out through the fabric and down the neckline. In reality, the temperature of the air circulating under the garment, as well as the temperature of the skin, do not depend on the color of the outer garment: if the surface of the dark dress absorbs more energy than the light dress, the heating of the black surface enhances the beneficial effects of convection sufficiently to compensate for this excess energy.
