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Comment by JohnVideogames | Hacker News Reader

A black body will emit radiation (and receive radiation) until it's in equilibrium with its surroundings. In theory, the black bit of box radiates heat into space, and receives some of the CMB until both are at ~3K. It's how we cool satellites and spacecraft.

But the effect is quite small, and I'm suspicious of it (as the nearest surroundings are the hot planet, which transfer heat more effectively!)

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If you grow crops, your crops can freeze when it's a clear sky at night with stagnant air even though the air temperature only goes close to freezing.

In some agri areas, you'll see "wind machines" to blow the warmer air around the crops to keep them from radiating their heat and frosting.

https://www.ontario.ca/page/wind-machines-minimizing-cold-in...

Wow unradiant heat, or more accurately radiant cooling I suppose. Fascinating

There are some IR bands that are transparent between the ground and space so if you optimize your emissions in those bands (because you are not an idealized black body) and insulate yourself from conduction and convection heating, you can beat your local surroundings and cool yourself lower than ambient to a useful degree.

https://en.wikipedia.org/wiki/Infrared_window

There are materials and even paints that have emission spectrums designed for this.

For example: https://www.parc.com/technologies/self-cooling-paint/

Normally you would just be in the same thermal equilibrium with everything else having similar emission spectrums you'd give and receive similar amounts of radiative heating and cooling. Tweak your spectrum and you tip the balance in your favor.

it's easy to calculate how big the effect is with units(1)

    You have: stefanboltzmann tempC(20)**4
    You want: W/m**2
     * 418.76592
     / 0.0023879689
    You have: stefanboltzmann tempC(0)**4
    You want: W/m**2
     * 315.65782
     / 0.0031679874

so in the temperature range of interest you're radiating 300–400 watts/m², derated for your surface emissivity at the relevant infrared wavelengths. as i understand it, the temperature will drop until the heat emitted through radiation is counterbalanced through heat that seeps in through the insulation and through the stagnant, stratified air above your pan of water, and, probably more importantly, through greenhouse-effect radiation from the warm atmosphere and any opaque objects you have unfortunately included in your field of view

tens or hundreds of watts of cooling is quite significant indeed when we're talking about making some ice or cooling some food

> as the nearest surroundings are the hot planet, which transfer heat more effectively!

The box is insulated. In one case it's an unused refrigerator that opens on the top with more insulation apparently surrounding it. The planet may be hot but on a cold night the ground does nothing to warm me. Soil is not a good conductor of that heat at the surface nor is the air between it and the container. The planet stays "hot" because it is "insulated" as well.

The ground does a lot to warm you, radiating terrestrial temperature BBR to your entire lower hemisphere, basically it's the same ~100W/sqm.

Sleeping under some tree cover on a clear night vs sleeping under the clear sky makes heaps of difference.

That's why he insulates his box from the hot planet and enjoys the fact that the air is largely IR-transparent.

The black body temperature of the night sky on earth is much warmer than the temperature of the CMB in the universe. You can test this by aiming an IR temperature gun on a clear night.

Relevant stack exchange: https://physics.stackexchange.com/questions/153839/what-is-t...

It's possible to make ice in the desert with this effect. Not too hard to believe.

It’s also commonly freezing in the desert at night

Snow is a near perfect black body radiator

Therefore imagine the impact of reduced snow and ice due to global warming. It’s free cooling and we’re throwing it away.

The albedo is the part that's bad to lose. I don't think snow emits especially strongly in the frequencies relevant for this. A highly imperfect blackbody emitter could cool more effectively.

This was a key plot point in the 1990s video game: Midwinter

https://en.wikipedia.org/wiki/Midwinter_(video_game)

It's an issue for amateur astronomers too as you have a mirror in a long tube pointed directly at the sky: https://britastro.org/2021/dealing-with-dew

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