Black and White

[BeBMan]

So there's this Nasa mission to the sun: https://www.bbc.com/news/science-environment-45160722

Yesterday I watched some coverage, and the heatshield was specifically mentioned, as the thing has to withstand temperatures around 1300-1400?

One expert explained the shield, (among other stuff) said it's white, which helps as white reflects light way more than black which heats up more instead as light is absorbed.

That is well-known to most ppl, and the expert wanted to give a popular example, saying it's also hotter in a black car in the sun compared to a white one etc.

So I was wondering - how much of a difference does this make when you're confronted with temperatures of 1300 and above? Is it really significant (short answer: if not they wouldn't have dunnit, I know, but does anybody know how much of a difference it makes?)


I was also thinking on ppl's clothes - many seem to prefer white t-shirts in summer, otoh I have that image in my mind of desert guys (Tuareg I think) who often use also dark clothes for some reason. How much of an impact would it make there?

[Proteus_MST]

Tuaregs wearing black actually was something that puzzled scientists as well, therefore in the 1980s they made a practical experiment,
putting 4 test subjects into the Israels Negev desert, one with black and one with white Bedouin robes, one with a tan army uniform and one with shorts.

It turned out that oit doesn't matter whether a person wore black or white bedouin robes, the heat that reached the skin was always the same (despite the tissue of the black robes, of course, absorbing more heat from the sun).
Obviously the secret behind Bedouin robes is, that they are worn loose fitting and that there is a significant cooling effect due to the convection effect of the air below the robes.
So, all additional heat that is taken up by the colour of the robes dissipates due to the built in air cooling of the robes

https://www.theguardian.com/science/2012/aug/19/most-improbable-scientific-research-abrahams

[BeBMan]

Good finding Thanks.

[rah]

Yes, quite interesting. THX

[Lorizael]

The sun's energy is mostly a mix of UV (10%), visible (40%), and IR (50%) light. So if you're considering the difference between a perfectly white and perfectly black surface in space, the former is absorbing 60% as much energy as the latter. But of course we're dealing with real materials, so in truth the figure will be higher.

[BeBMan]

That seems to be quite someting. My (rather amateurish) thinking was previously kinda like, hey, if you're at 1300-1400, what does it matter.

So this is kinda....cool (hehe).

[ZEE]

Why do you idiots think so much about these things? Stop thinking.

Drink some booze, get lit, and do something relaxing. Let the Gods handle the universe.

שֶׁקֶט רָגוּעַ

[Bereta_Eder]

My dermatologist (who is very good) says that to protect you from the sun a black t-shirt is more efficient than a white t- shirt

[Lorizael]

That seems to be quite someting. My (rather amateurish) thinking was previously kinda like, hey, if you're at 1300-1400, what does it matter.

So this is kinda....cool (hehe).

Well, like I said, that's the best case. In reality creating a material that blocks 100% of light between 400 and 700 nanometers (perfectly white) is not something we can do. It won't have total coverage across that span, and it won't block 100% of light at any particular wavelength.

[Lorizael]

My dermatologist (who is very good) says that to protect you from the sun a black t-shirt is more efficient than a white t- shirt

Well, when you're talking about "protection" from the sun, you're mostly worrying about UV rays. So you want a material that's going to prevent UV rays from hitting your skin. It's totally plausible that black dyes in clothing have that property, whether through reflection or absorption, independent of the fact that they're worse at reflecting visible light.

[Bereta_Eder]

If you say so. I apparently have a "predispotition to skin cancer" and I'm supposed to wear a black t-shirt on the beach. fat chance. I don't want to look like a dork plus it's uncomfortable. TRhere are 50+ blocking agents and almirikia (trees that feed off salt water and provide SHADE). So I don't need black t-shirts.

[Lorizael]

My (rather amateurish) thinking was previously kinda like, hey, if you're at 1300-1400, what does it matter.

Oh, another thing about this. Our personal experience of temperature is in large part not based on the light a hot object is emitting, but conduction of heat through particle collisions. If you put your hand over boiling water, it's the water molecules slamming into you that feel hot. And in that case, whether your hand is white or black (maybe you're wearing a glove?) doesn't matter; what matters is the thermal conductivity of the material.

So your intuition is right that color couldn't be that big a deal on earth. But in the near vacuum of space, conduction isn't the primary thing; it's radiation from the sun. There is the solar wind to contend with, which would heat up materials via conduction, but I'm not sure how much that contributes to the temperature quoted for the probe. /me shrugs.

[pchang]

I would probably switch conduction to convection.

[pchang]

Oh, and google radiometer.

[Lorizael]

Sure, convection is occurring when hot air moves toward or away from you, but it's the thermal conductivity of your skin/clothes that determines how much heat you absorb and consequently feel.