Should You Wear White or Black on Hot Days? Here's the Data
Should you wear white or black during the summer? Or that other burning fashion question: Is it OK to wear white after Labor Day? Oh wait, that question really isn't important. Let's get back to the summer question.
There are two answers to the black vs. white clothing question.
1. Wear White. A white object is white because it reflects white light, and white light is a combination of all the visible colors. This means that a white shirt (or pants) will reflect most of the light and not get hot. Simple, right?
2. Wear Black. But wait! What about the bedouin in the desert regions of North Africa? They often wear black clothing, and it's super hot there. It seems they wouldn't wear black unless there was an advantage. Maybe the black clothing prevents body heat from reflecting back on the human—thus keeping the body cooler than a white outfit.
OK. Let's be clear. This black vs. white clothing isn't exactly a settled issue. People actually study this stuff—here is an article from Nature published in 1980: "Why do Bedouins wear black robes in hot deserts?". There are clearly several situations to consider with the Bedouin clothing. But what about more common outfits, like a T-shirt? Should you wear a black or white T-shirt on a warm summer day?
The first thing to consider: Does a black shirt get hotter than a white one? I can explore this question with an infrared camera. You see, everything gives off light (electromagnetic radiation). Some super-hot things (like a lightbulb filament or a stove burner) are hot enough that this emitted EM radiation is in the visible spectrum, and we can see it. For most other objects, the emitted light has a wavelength that puts it outside the visible range. Most of this light falls in the infrared region.
Using a special camera, a sensor detects the infrared radiation and uses this to determine the object's temperature (for the most part).
So let's do it. Here are some shirts hanging out in the sunlight.
Now for an infrared image. Note: this is a false-color image. Since we can't actually see infrared light, different colors in this image correspond to different wavelengths in the IR region.
From this image I can get the temperature of the shirts. OK, technically there is a small problem measuring the temperature, but I will address that shortly. The black T-shirt on the right measured 131.0 Fahrenheit and the white one on the left was 111.8. Yes, it's clear the black shirt was hotter. Other than that, there were no real surprises.
But come on. You already knew this. In fact, you can even do your own experiment. Grab some paper—a white piece and a black piece. Place them outside in the same sunlight. You only have to wait a few minutes before picking them up to realize that the black paper is hotter.
Now for the second question. Does a white T-shirt reflect thermal radiation from your body back to your body to warm you up? The answer is yes. Perhaps the question should be: Does white reflect MORE thermal radiation than black clothing (I'm equating thermal radiation and infrared light—same thing). Is a white shirt "infrared white"? Does it reflect more infrared radiation than a black shirt?
How about another test. To measure the infrared reflectivity (not a real term) of different shirts, I set up the following experiment. There is a hot (but not too hot) iron that you can use to make your clothes wrinkle-free. This is my infrared source. I placed it around a corner so my infrared camera couldn't see it. Then I put different objects in front of the camera to see how they reflected this infrared light.
Let's start with something fun. Here is a tile board. It's the same stuff those whiteboards in classrooms are made of. What happens when infrared light hits it? This happens.
This is a composite image (in case you couldn't tell). The infrared camera I am using (the FLIR One) has both a visible light camera along with an IR camera. I cut out a part of the visible image and placed it on the IR image to make it more obvious what you are looking at. The important part is the bright spot in the middle of the board. That is a reflection from the iron. Oh, you want to see the iron too? Here you go.
Notice the reflection on the floor? That's because my smooth kitchen floor reflects infrared light, and you can see an image with the camera. Yes, that's awesome.
What about a white T-shirt?
No spot. It doesn't reflect much infrared. What about a black shirt? It pretty much looks the same in infrared.
So, although the two T-shirts look different to human eyes (in the visible light range), they are pretty much the same in infrared. That pretty much answers the second question about clothing. Does white reflect back more infrared radiation on your body? Nope. Just because it's white doesn't make it "infrared reflective."
Do you know what is very infrared reflective? Space blankets—those shiny mylar blankets that you can use in an emergency. You know what else makes a difference? Water. Here, check this out. This is an image of a T-shirt with some water on it next to a piece of mylar.
That darker stuff on the shirt is just a tiny bit of water. As the water makes a phase transition from a liquid to a gas, it takes energy. This energy comes from the rest of the liquid water, causing a drop in temperature. This is exactly why humans sweat—we cool off through the evaporation process. Also, check out the mylar on the right. It looks different because it's reflecting both the visible light and the infrared radiation. That makes it rather difficult to measure the temperature with an infrared camera, because you are seeing reflected light rather than emitted light.
Now is the time to discuss this emission vs. reflection problem. In the world of infrared cameras, different materials can have a different emissivity. The emissivity of an object can have a value between 0 and 1. If an object is only radiating infrared light and not reflecting it at all, that would be an emissivity of 1. Something that only reflects infrared light would have an emissivity of zero.
The T-shirts (both the black and the white) have an emissivity very close to 1—they don't really reflect much infrared radiation. But the mylar has an emissivity close to zero.
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That pretty much answers the question. In most cases white clothes look just like black clothes in the infrared spectrum. They both reflect about the same amount of thermal radiation. That means you are going to be better off with white clothes, since they don't absorb as much visible light. But wait! Could there be a special case in which black is better?
Let's get back to the bedouin black clothing. What is going on here? Well, there is more to heating and cooling than just the color of the clothes. What about evaporation? What about wind? One possible reason for the black clothes is a type of chimney effect. The idea is that the black clothes heat up the space between the cloth and the human to promote an upward air current (like a chimney). This air current adds to the cooling of the human. But maybe you see the problem. You have to have an air space between the fabric and the skin. I don't know about you, but my shirts aren't that loose. I suspect that there are only a few people that wear clothes in the bedouin fashion—but for those people, you might want to stick to black.
But wait! There's more! There are so many variables in this black vs. white clothing question that this could be a great starting point for a science-fair experiment (you know … for kids). I'll be honest, I'm not too keen on science fairs in general, but if you are going to do a project, this seems like a great thing to study. Here are some ideas to get you started.
- Data collection: If you want to get an infrared camera (they are very useful), you can collect some great data. If you don't have an IR camera, you could still collect meaningful data using some small temperature sensors.
- Do different types of clothing material reflect infrared light differently? What about those "breathable" shirts? What about other stuff, like silk?
- Get a bunch of people and measure their body temperatures with loose vs. tight clothing.
- What about the wind? Does the color of clothing matter if there is a slight breeze?
- What about the humidity in the air? What impact does it have on clothes of different colors?