Dress in black to protect yourself from the sun

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? The examination of the various exchanges of energies which come into play sheds light on this interesting question.

The color of a surface determines the amount of light that it sends back to us and, by complementarity, the amount of light that it absorbs. The darker it is, the more light it absorbs, and thus light energy, which is transformed into heat. When it is high in the sky, the Sun brings to the vicinity of the ground nearly 1,000 watts per square meter. A black colored object will absorb up to 90 percent of this energy. We also take advantage of this source of energy with “solar panels”, blackened panels where water is circulated to heat it at little cost. Conversely, if we want to reduce solar heating, we use the color white, because it reflects almost all the incident light. That is why the walls of houses in sunny countries are painted white.

However, this analysis is far from complete! To establish the true energy balance of a garment, it is also necessary to take into account the way in which it loses energy by radiation. Any body radiates all the more as it is hot. A garment 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: about 500 watts per square meter for a surface at room temperature. Conversely, white, which absorbs less, also emits 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 that reaches it...

Energy balance

In detail, the properties of a body with respect to visible light and infrared radiation depend not only on its color, but also on the material of which it is made. Let us compare the black and white fabrics used by the Bedouins. The energy absorbed by a person's black robe 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 the white fabric absorbs infrared radiation as much as the black fabric, which alone represents half of the solar light energy that reaches 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, which is 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 the light garment was 41°C while the dark garment was 47°C.

Thus, black clothing seems 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 derives 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 produces much better shading than a white canvas of the same thickness. More heated by the Sun, it is however likely to transmit its heat to the air which 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 ambient air, rises before leaving the shelter at the top. It draws less warm outside air into the tent, taking with it the heat transmitted to it by the canvas. The hot surface of the tent thus initiates a movement of convection which constantly renews the air in the tent, even in the absence of breeze. The engine of the “Bedouin fan” is indeed the black canvas of the tent.

The double robe of the Bedouins

Is it thanks to the ventilation that the Bedouins do not suffer from the heat which accumulates on their black clothes? Yes, but before explaining it, let's examine what a comfortable garment is in the Sun: 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 outside temperature. For this, it protects the body from external heat and also evacuates that which is permanently produced by the metabolism (even at rest a man dissipates at least 100 watts). This regulation is carried out by perspiration. To evaporate, sweat absorbs heat which it takes from the skin. Any additional heating should be compensated by increased perspiration. In the tropics, it happens to lose more than half a liter of water per hour at the hottest times. Comfortable clothing facilitates cooling evaporation.

We now 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 better that they are clear. The 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 on their skin. The latter is then itself covered by a loose garment. The outer fabric is therefore not in contact with the skin. In addition, the air circulates easily between the two garments. The air heated by the outer fabric rises inside and draws colder surrounding air from underneath the dress. In this convection movement, everything happens as if the two dresses played the role of bellows, rejecting the hot air through the fabric and through 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 robe absorbs more energy than the light robe, the heating of the black surface sufficiently reinforces the beneficial effects of convection to compensate for this excess energy.

Even under the Sun, the laws of physics allow us to choose, according to our taste, the color of our clothing.