In hot summer, is it better to wear white or black clothes? Or to switch to another hot fashion question: Is it okay to wear white after Labor Day? Oh, okay. Maybe this question is really not that important, but let us return to this question that our souls struggle with every summer.
There are usually two different answers to the question of black or white clothing.
Wearing white seems to be a good choice. After all, white is a complex light. It is a combination of all visible light and can reflect all colors of light. . This means wearing a white shirt (or pants) will reflect most of the light so people don’t feel too hot. Obvious, isn’t it?
Hey, wait! Have you considered the Bedouin people in the desert areas of North Africa? Even though the temperature in the place where they live remains high, they Still often wears black attire. In theory, they shouldn’t be wearing black, unless black really has some hidden virtues. Perhaps black light-absorbing clothing prevents body heat from being reflected back into the body and is therefore cooler than wearing white clothing.
Okay, let’s be clear. Wearing black or white is not a question that can be perfectly explained. In fact, people have been studying these things – there was an article published in “Nature” magazine in 1980: “Why do Bedouin people still wear black robes in the hot desert?”. Obviously, the Bedouin people’s Dressing habits need to consider a variety of factors. But what about more common garments, like T-shirts – should you wear a black or white T-shirt on a hot summer day?
The first thing to consider is: does wearing a black T-shirt really make you feel hotter than wearing a white T-shirt? I can explain this to you using an infrared camera. Everything emits light (electromagnetic waves), and some super-hot objects (like a glowing lightbulb filament or a burning stove) are hot enough that the emitted electromagnetic waves stay within the visible light range, so we can see it. For most other objects, the wavelengths of light they emit are outside the visible range, mostly falling in the infrared range.
The sensor of this special camera can detect infrared radiation, which we can use to determine the temperature of an object (in most cases).
Here are a few shirts hanging in the sun that we can try.
The following picture is the resulting infrared image. NOTE: The colors in the picture are not realistic. Since we can’t see infrared light in reality, we use different colors to correspond to different wavelengths in the infrared region in this image.
▲ Use infrared camera to measure temperature
From this picture we can see the temperature of the shirt. Okay, technically there’s a little problem with measuring the temperature, but I’ll get to that as soon as possible. The black T-shirt on the right measured 131.0 degrees Fahrenheit (about 55 degrees Celsius), and the white T-shirt on the left measured 111.8 degrees Fahrenheit (about 44 degrees Celsius). It’s obvious that black shirts are warmer. Other than that, there seemed to be no other surprises.
Well, you already knew the result. In fact, you can also experiment on your own. Take two pieces of paper – one white and one black – and place them together outside under the same lighting conditions. Just wait a few minutes before picking them up, and you’ll notice that the black paper is noticeably hotter.
Let’s look back at the second question. Will a white T-shirt reflect heat radiation from your body back to your body, making you feel hotter? The answer is yes. Maybe we should ask: Will white clothes reflect more thermal radiation than black clothes (here I equate thermal radiation and infrared light)? Will a white shirt reflect more infrared radiation than a black shirt?
How about testing it again? To measure the infrared reflectance (not a real term) of different shirts, I set up the following experiment. Use an iron that is heated (but not extremely hot) enough to iron out wrinkled clothing as the infrared source. I placed it in the corner so my infrared camera couldn’t pick it up. I will then hold different objects in front of the camera to see how they reflect infrared light.
The experiment will start somewhere fun, with a board made of the same material as the whiteboard in the classroom. What happens when the camera captures it? Lets come look.
▲ Whiteboard under the infrared camera lens
▲ The iron under the infrared camera lens
Did you notice the reflection on the floor? That’s because the kitchen floor is too smooth and reflects infrared light, which can be seen from the camera. How cool!
Then change to a white T-shirtWhat kind of scene would it be?
▲ There are no spots visible on the white T-shirt under the lens. It does not reflect much infrared rays. What about the black shirt? There is basically no difference from the camera.
▲ Black T-shirt under the lens
So, although the two T-shirts appear different to the human eye (in the visible light range), under the infrared camera lens Almost identical. This roughly answers the second question about clothing. Does a white t-shirt reflect more infrared radiation back to your body? No, the mere fact that it is white does not make it “infrared reflective.”
What things are more reflective in infrared? Space Blanket – That shiny Mylar blanket that can be used in an emergency. Is there anything else that can do it? It is undoubtedly “water”. Come on, let’s take a look at this. It’s a t-shirt with some water sprinkled on it and a piece of mylar next to it.
Those dark things on the shirt are just a little bit of water. When water changes from a liquid to a gas, it removes heat, causing the temperature to drop, which is why humans sweat – we cool down by evaporating heat from the surface of the body as sweat evaporates. Also, check out the mylar on the right. It looks different because it reflects both visible and infrared light. This makes measuring temperature with an infrared camera quite difficult because you see reflected light rather than emitted light.
Now it is time to discuss emission and reflection. In the world of infrared cameras, different materials can have different emission coefficients (the ratio of the radiative power of an object to the radiative power of a blackbody at the same temperature). The object emission coefficient can have values between 0 and 1. If an object only radiates infrared light and does not reflect it at all, then its emission coefficient is 1. The emission coefficient of an object that only reflects infrared light is 0.
The emissivity coefficient of T-shirts (both black and white) is very close to 1 – they don’t actually reflect much infrared radiation. But the emissivity of polyester film is close to 0.
This roughly answers the original question. In most cases, white clothing looks the same as black clothing in the infrared spectrum, and they reflect the same amount of thermal radiation. This means you’ll be cooler wearing white clothes because they don’t absorb as much visible light as black clothes. Wait, someone may want to ask, is there a special situation in which it is more advantageous to wear black clothes on hot days?
Looking back at the black clothes of the Bedouin people, what is going on? ?In fact, whether it is cold or hot depends not only on the color of the clothes, but also on evaporation, wind and other factors that must be comprehensively considered. One possible reason why wearing black clothes makes you feel cooler is the chimney effect (a phenomenon in which indoor air rises or falls along a vertically sloping space, causing air to strengthen convection). The idea is that black clothing heats up the space between the fabric and the body, thus promoting upward airflow (like a chimney), which makes people feel cooler. But you can probably see the problem, which requires a gap between the fabric and the skin. Don’t know about your clothes, but my shirt isn’t that baggy. I suspect that only a few people would be able to do this in something similar to a Bedouin robe – for whom black might be more appropriate.