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Comment by leafmeal | Hacker News Reader

Possibly a stupid question, but why would painting your roof blue help keep it any cooler than painting it white for example, which reflects more light?

Edit: Ok it turns out there's 2 parts to a cooling roof, how reflective it is, and how good it is at emitting infrared radiation (emissivity). Reflectivity of visible and UV light prevents more energy from being absorbed, but doesn't necessarily mean the material cools down well once hot. A high emissivity means the material can cool down quickly through radiation.

I guess YInMn blue has a high emissivity.

https://en.wikipedia.org/wiki/Reflective_surfaces_(climate_e...

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I cannot think of any reason why blue would ever have a lower emissivity than actual white.

Rare earths are not going to come down in price, in my opinion. They're used in too many places and have too few sources. The US has a rare earth strategy document that makes clear just how insecure supplies of these elements is.

Thanks, I was not familiar with the concept of emissivity.

I cannot think of any reason why a material's absorption/emission through the visible spectrum would determine its absorption/emission through the infra-red spectrum.

That is why I said "actual white".

So "actual white" has little to do with the color that we see.

Is it also white to the x-ray spectrum and the radio wave spectrum? If so, I am pretty sure that this doesn't actually exist.

Right, it doesn't actually exist. It is a technical definition that by definition absorbs no light, it isn't a practically realizable thing.

Yes, you're describing a white body, the opposite of an ideal black body. It does not exist, of course. But we can try to approximate one with materials, at least for IR through UV. Now, if we do have a pigment that can reflect X-rays... I'd definitely want my roof painted with that.

> x-ray

Easy to ignore since it won't be hit by any.

> radio

I dunno, how hard is it to get materials that reflect radio waves?

It's probably reasonable to talk about materials that reflect almost all the light in frequency bands of non-negligible power as effectively being actual white.

There is a glut in supply of some rare earths as they're byproducts of refining the more valuable ones.

You are absolutely correct, but the ratios of their availability, because they are a byproduct, all go up by the same proportion. It's hard to avoid excesses of the less useful ones with shortages of the more useful ones.

More useful ones being things like neodymium and erbium and cerium.

Once one of them is not being produced in sufficient quantity due to being a byproduct, then it gets really expensive. That's for sure.

(edited for clarity)

Your edit is an excellent point, thanks for the addition. If it is white in the visible but black in the infrared that could help with cooling sure.

Personally I'd probably focus on windows and solar insolation from that first, especially since the roof is insulated, but that makes more sense.

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