5 ms·

This is because the blue sky is very "cold". Point an infrared thermometer up there and see... Very cold indeed.

wongarsu 3y ago
Which usually objects can't fully exploit, because the heat they radiate away is absorbed by their immediate surroundings, which radiate part of it back towards them. You can "cheat" that by radiating away heat in a spectrum that isn't absorbed by anything, removing it from the area.
- jessriedel 3y ago
  Basically. To be clear, it's not that you are worried about getting "the same" heat reflected or otherwise radiated back toward you. (The radiation emitted by your roof doesn't appreciably change the radiation incident on your roof from the atmosphere.) It's that if you are passively emitting at some frequency you are necessarily absorbing at that frequency too (by the 2nd law). And you don't want to be absorbing at the frequency where the atmosphere is opaque and hence warm. In contrast, it's fine to be absorbing at the frequency where the atmosphere is transparent, because then you're just exposed to cold outer space.
  - dragontamer 3y ago
    I had no idea what you were talking about until I remembered that infrared is just a Radio wave and therefore everything is an antenna. Antennas that are good at transmit at X frequency tend to also be good at receiving X frequency necessarily. And different 'colors' are nothing more than different antenna resonance points.
    - pertymcpert 3y ago
      Wow. I've never thought of heat in that way. Mind blowing.
- eptcyka 3y ago
  You could use chimneys that are coated by the irradiative material on the inside, pointing all the IR up into the sky.
dontwearitout 3y ago
I had to look this up - apparently it's around 0C if you do this! https://mynasadata.larc.nasa.gov/sites/default/files/2018-12/Measuring%20the%20Temperature%20of%20the%20Sky%20and%20Clouds.pdf https://mynasadata.larc.nasa.gov/sites/default/files/2018-12...

Which usually objects can't fully exploit, because the heat they radiate away is absorbed by their immediate surroundings, which radiate part of it back towards them. You can "cheat" that by radiating away heat in a spectrum that isn't absorbed by anything, removing it from the area.

jessriedel 3y ago
Basically. To be clear, it's not that you are worried about getting "the same" heat reflected or otherwise radiated back toward you. (The radiation emitted by your roof doesn't appreciably change the radiation incident on your roof from the atmosphere.) It's that if you are passively emitting at some frequency you are necessarily absorbing at that frequency too (by the 2nd law). And you don't want to be absorbing at the frequency where the atmosphere is opaque and hence warm. In contrast, it's fine to be absorbing at the frequency where the atmosphere is transparent, because then you're just exposed to cold outer space.
- dragontamer 3y ago
  I had no idea what you were talking about until I remembered that infrared is just a Radio wave and therefore everything is an antenna. Antennas that are good at transmit at X frequency tend to also be good at receiving X frequency necessarily. And different 'colors' are nothing more than different antenna resonance points.
  - pertymcpert 3y ago
    Wow. I've never thought of heat in that way. Mind blowing.
eptcyka 3y ago
You could use chimneys that are coated by the irradiative material on the inside, pointing all the IR up into the sky.

dontwearitout 3y ago

I had to look this up - apparently it's around 0C if you do this! https://mynasadata.larc.nasa.gov/sites/default/files/2018-12/Measuring%20the%20Temperature%20of%20the%20Sky%20and%20Clouds.pdf https://mynasadata.larc.nasa.gov/sites/default/files/2018-12...