4 ms·

> Dark will get hotter in sunlight but can be cooler at night if the sky is unobstructed. What would the sky be obstructed by? Clouds? How could that affect h

> Dark will get hotter in sunlight but can be cooler at night if the sky is unobstructed. What would the sky be obstructed by? Clouds? How could that affect how cool tarmac on the ground gets at night?

onlyrealcuzzo 4y ago
Physics is strange: > At night, when cloud coverage increases, downwelling infrared radiation also increases. So, clouds act like space heaters, emitting energy towards the ground. Blankets only limit the transfer of heat energy away from your skin(convection).
- capableweb 4y ago
  Huh, that's very interesting and non-intuitive for a non-physicists like me. Thanks for the explanation!
Sharlin 4y ago
This is what the greenhouse effect is, literally. At "thermal IR" wavelengths the sky is a blanket. Cloudy sky is even more of a blanket.
- Scoundreller 4y ago
  I live near a grape growing are, and they have wind turbines to blow air on cool nights slightly above freezing because the radiation from the ground to to sky can be enough to frost-damage their crops. So they turn them on the blow the stagnant warmer ambient air around and mitigate that.
myself248 4y ago
Yes, clouds. With a clear sky, you're radiating (based on your own temperature) out to the blackness of space, which is radiating back at 4 Kelvin. Which is to say, it's contributing basically nothing. You're just giving up heat to the cosmos. With a cloudy sky, you're radiating (based on your own temperature) out to the bottoms of the clouds, which are reflecting your radiation back to someone else on the ground, and their own heat is reflecting back to you. And such. The cloud looks white in the daytime because it's very reflective, so it acts like a mylar blanket to hold the heat in.
ChrisMarshallNY 4y ago
I was told that light reflects heat, and heats more quickly, but also radiates badly (cools more slowly). This is why heatsinks are often painted black. It is also the reason for all the strange white/black colors on rockets.
- ChrisMarshallNY 4y ago
  > heats more quickly Errata: “heats less quickly”.

onlyrealcuzzo 4y ago

Physics is strange: > At night, when cloud coverage increases, downwelling infrared radiation also increases. So, clouds act like space heaters, emitting energy towards the ground. Blankets only limit the transfer of heat energy away from your skin(convection).

capableweb 4y ago
Huh, that's very interesting and non-intuitive for a non-physicists like me. Thanks for the explanation!

Sharlin 4y ago

This is what the greenhouse effect is, literally. At "thermal IR" wavelengths the sky is a blanket. Cloudy sky is even more of a blanket.

Scoundreller 4y ago
I live near a grape growing are, and they have wind turbines to blow air on cool nights slightly above freezing because the radiation from the ground to to sky can be enough to frost-damage their crops. So they turn them on the blow the stagnant warmer ambient air around and mitigate that.

myself248 4y ago

Yes, clouds. With a clear sky, you're radiating (based on your own temperature) out to the blackness of space, which is radiating back at 4 Kelvin. Which is to say, it's contributing basically nothing. You're just giving up heat to the cosmos. With a cloudy sky, you're radiating (based on your own temperature) out to the bottoms of the clouds, which are reflecting your radiation back to someone else on the ground, and their own heat is reflecting back to you. And such. The cloud looks white in the daytime because it's very reflective, so it acts like a mylar blanket to hold the heat in.

ChrisMarshallNY 4y ago

I was told that light reflects heat, and heats more quickly, but also radiates badly (cools more slowly). This is why heatsinks are often painted black. It is also the reason for all the strange white/black colors on rockets.

ChrisMarshallNY 4y ago
> heats more quickly Errata: “heats less quickly”.