People always say you should wear white when it's hot
to avoid overheating.
Is that true?
And to what extent is that true?
Is there a real difference between wearing a green t-shirt,
or a blue or white one - do those differences matter?
We're about to test this today and we're at the perfect place to do so:
we're in the Grand Canyon and it's very, very hot.
Come on, let's do it!
I don't have t-shirts of all colors with me so I'll work with paper instead.
Each bit of paper starts off at 27.2°C and the floor is at 57°C.
Quickly enough, the temperature rises for each paper.
After 45 minutes, the white paper is the coolest.
The next coolest is the yellow one; the other colors are all comprised in a tight range of temperatures.
The black piece of paper is the warmest of all.
My thermometer's precision is of about +/- 1°C
so there's no real difference between the red, blue and green colors.
After doing this experiment, I asked myself many questions:
what had I really expected to find?
How do you explain the fact that all the colors are about the same warmth
apart from yellow?
And how do you explain all the exceptions like that of the white polar bear?
Okay, so polar bears aren't actually white - they're transparent.
Their hairs are hollow tubes that let the light through.
Right then.
Bye!
Thanks Leo!
But let's go back to our bits of paper.
Ranking our colors experimentally is easy; we just did it.
But could we have reached the same conclusions purely from reasoning?
Let's go from what we already know about colors and heat.
When you make a camp fire, the hottest point is right above the embers
and the coldest, around the outside.
It's similar to a Bunsen burner like the ones you used in high school:
the blue flames are the hottest and the orange flames, the coolest.
We can visualize temperature with color with the following diagram.
The blue side is the hottest while the red side is the coolest.
There's a big difference between the light emitted from a fire and the light we perceive from a t-shirt.
The fire emits light but the t-shirt doesn't.
To be precise, the t-shirt emits some light, but only in the infrared spectrum,
which is invisible to us.
In the visible spectrum, a t-shirt only absorbs or reflects light.
When a t-shirt is being lit by the sun, it absorbs a few colors that contribute to its heating up;
the light(s) it doesn't absorb are reflected and determine the color of the t-shirt.
If a t-shirt is red, it means that it has absorbed all other colors of the visible spectrum
which warmed it up.
But let's go back to our diagram.
Red is the coolest color when it is emitted.
But when it's being reflected, it means the fabric has been warmed up
by all the other colors which are warmer.
So the red color that we see is the color that'll be the warmest.
Our diagram for the temperature of emitted colors is therefore the reverse to our diagram for reflected colors.
This is the right diagram:
We need one extra piece of information:
we want to know about t-shirts left outside in the sunlight.
And the sun doesn't emit red-to-blue light in constant quantities.
On this image, you can see the amount of photons received from the sun
as a function of their wavelength i.e. their color.
Our sun emits much less blue light than any other color.
What we want to know is how much energy we receive.
We need to take into account the fact that red is cooler than blue.
So if we look at how much energy the sun sends us,
the diagram looks like this.
All in all, the red color doesn't matter as much; green and yellow are the major ones.
That information complicates our ranking quite a lot.
In order to deepen the research I got help from Bunker D
who created a computer script
that can simulate what each t-shirt absorbs and reflects.
But in order to use it, we'll need one extra piece of information:
the spectrum of each t-shirt.
The spectrum will tell us what wavelengths the t-shirt really reflects.
Just because we perceive a t-shirt to be yellow doesn't mean that all it reflects is yellow light.
It could just as well reflect red and green light that our brain interprets as yellow.
In order to make that distinction, we need a spectrum.
How do we get it? By using...
a spectrometer.
A spectrometer is a tool that decomposes light into separate wavelengths.
That way we can find out if we really see yellow or a combination of colors instead.
I created one with a cardboard box, a webcam and leftovers of a CD.
I put the link in the description that explains how to create this stuff.
There's a whole community working on it.
I kind of expected these results.
The yellow paper actually emits red and green light; it's a combination of colors.
Now that we have the spectrum of each t-shirt, we can import them into Bunker D's script.
If we take the script of the white piece of paper, we can see it reflects almost all colors.
By analyzing this image, we can see that the t-shirt reflects this:
If it reflects all of this part of the spectrum, then it absorbs all of that part.
What you can see here is the energy absorbed by the t-shirt aka what warms it up.
If we calculate the surface under the curve (via an integral, just like in high school),
we'll get the amount of energy absorbed by each t-shirt.
This is the t-shirt ranking, from coolest to warmest.
Right, I have a question!
I'm working on a video about polar bears and I was wondering:
are the yellowish ones warmer than the white ones?
Um...
Well, yes.
Technically yes: white means that all colors are reflected.
A yellowish bear's fur would absorb all colors but yellow so yes, it should be warmer.
Is it the same thing for greenish or purpleish animals?
Yeah, same thing.
'K great thanks!
Find the video about polar bears at the end of the video
Back to our ranking.
All of this is well and good but we can't generalize the findings to a certainty
for two reasons:
(1) in order to create this ranking, we had to measure light spectra
and we can't know whether another blue t-shirt would have the same specter as our blue t-shirt.
(2) our cardboard spectrometer is (surprisingly) limited.
It can only measure wavelengths in the visible spectrum.
But 50% of the light we receive from the sun is invisible infrared light
but that still contribute to the warming up of the fabric.
Depending on the t-shirts' pigments, they don't all absorb infrared light in the same way.
But that's something we cannot know.
If our eyes were as efficient as a good spectrometer, it would be simple to figure out
what color to wear in the summer.
But in the meantime, with our limited eyesight,
we have to stick to the notion of pale color vs dark color.
A white t-shirt (not a color btw) will reflect all colors and thus be the coolest (mmh hmm) t-shirt to wear.
A black t-shirt (not a color either) will absorb all the colors and thus be the warmest t-shirt to wear.
That's why solar showers are black and houses in hot regions are painted white.
And this is when I expect you to come up with tons of counter-examples.
I expect you to mention Bedouins, for instance.
They wear long, black robes and live in black tents in the middle of the desert -
the exact opposite of what they should be doing to protect themselves from the heat.
In the 80's, scientists studied this paradox and went to measure the difference in temperature between
a bedouin wearing a long black robe and one who wore a long white one.
They found that on the surface of the robes, there was up to 6°C of difference.
47°C for the black robe vs 41°C for the white robe.
But the temperature of the skin was at 33°C for both.
It's all because of the thick layer of air between the robe
and the second, tighter layer of clothes that they wear underneath it.
The black robe heats up the air that's inside, which travels upwards to come out through the collar,
thus creating an in-draft that calls for fresher air from the bottom of the robe.
It's a convection movement.
So there is extra heat because of the color of the robe,
but it's compensated by the convection effect.
And there's of course the case of polar bears,
which appear white but aren't really so.
But you'll learn more about that in Leo's video.
Let me now encourage you to go and watch Leo's video about polar bears
and that adds to this video.
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