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Yes, in other words: the darker a thing is, the darker it is. Is someone here actually suprised by the everyday phenomenon that dark objects get hotter than lig

Yes, in other words: the darker a thing is, the darker it is. Is someone here actually suprised by the everyday phenomenon that dark objects get hotter than light-colored ones? (Edit: I apologize for the uncalled for smugness. But "albedo" is not some special physical quantity, it's literally a measure of how light/dark colored a material is. "Alba" is Latin for "white". Although granted, albedo is usually measured across the whole spectrum, not only the visible part.)

powerhour 4y ago
No need to be an asshole about it. Some folks probably don't know the relative darkness of asphalt and tarmac.
- Sharlin 4y ago
  But according to GP's numbers, tarmac is less dark than asphalt so should get less hot than asphalt. But those numbers are suspect and probably only refer to very fresh asphalt/tarmac; the albedo aka "lightness" of worn asphalt is something like 0.3.
Swizec 4y ago
The difference between "not surprised" and "understand why" is vast. For example we (humanity) still don't understand _why_ gravity works despite not being surprised by its existence. And don't forget the 10,000 rule: https://xkcd.com/1053/ https://xkcd.com/1053/
- Sharlin 4y ago
  Yes, but GP's quote was not an explanation, it was literally saying that "light objects reflect more light". Albedo literally means "whiteness" ie. how light something is.
  - Swizec 4y ago
    From GP: > Traditional asphalt has a low albedo, which means it reflects radiation poorly and instead absorbs it Seems like a pretty good high level explanation of why that matters. Darker objects absorb energy instead of reflecting it. I'm sure there's more to it than that when you get into details.
Scoundreller 4y ago
Dark will get hotter in sunlight but can be cooler at night if the sky is unobstructed.
- capableweb 4y ago
  > 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.
ryanmcbride 4y ago
- the_only_law 4y ago
  If you're lucky there's a course you can overpay for.
onlyrealcuzzo 4y ago
Snow can have an albedo lower than 50% - and non-white sand can have an albedo higher than that. It's not as simple as color. In particular, there are dark things that have an albedo higher than asphalt's 0.05...
shadowgovt 4y ago
https://xkcd.com/1053/ https://xkcd.com/1053/
ridgeguy 4y ago
Not necessarily true. It depends on the object's optical properties. I well remember working on my parents' car in Phoenix summers. The chromed wrenches on our driveway surface got to instant-blister temperatures. Tools with black surfaces were uncomfortable to pick up, but were usable. The chromed tools had a lower absorption of sunlight than the black tools, but their IR radiation coefficient was even lower. So equilibrium temperature (at which radiated energy = incident energy) was much higher for the chromed tools.
- WalterBright 4y ago
  I was always careful to put my tools in the shade under the car :-) (Used to live in Glendale.)

powerhour 4y ago

No need to be an asshole about it. Some folks probably don't know the relative darkness of asphalt and tarmac.

Sharlin 4y ago
But according to GP's numbers, tarmac is less dark than asphalt so should get less hot than asphalt. But those numbers are suspect and probably only refer to very fresh asphalt/tarmac; the albedo aka "lightness" of worn asphalt is something like 0.3.

Swizec 4y ago

The difference between "not surprised" and "understand why" is vast. For example we (humanity) still don't understand _why_ gravity works despite not being surprised by its existence. And don't forget the 10,000 rule: https://xkcd.com/1053/ https://xkcd.com/1053/

Sharlin 4y ago
Yes, but GP's quote was not an explanation, it was literally saying that "light objects reflect more light". Albedo literally means "whiteness" ie. how light something is.
- Swizec 4y ago
  From GP: > Traditional asphalt has a low albedo, which means it reflects radiation poorly and instead absorbs it Seems like a pretty good high level explanation of why that matters. Darker objects absorb energy instead of reflecting it. I'm sure there's more to it than that when you get into details.

Scoundreller 4y ago

Dark will get hotter in sunlight but can be cooler at night if the sky is unobstructed.

capableweb 4y ago
> 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.

ryanmcbride 4y ago

the_only_law 4y ago
If you're lucky there's a course you can overpay for.

onlyrealcuzzo 4y ago

Snow can have an albedo lower than 50% - and non-white sand can have an albedo higher than that. It's not as simple as color. In particular, there are dark things that have an albedo higher than asphalt's 0.05...

shadowgovt 4y ago

https://xkcd.com/1053/ https://xkcd.com/1053/

ridgeguy 4y ago

Not necessarily true. It depends on the object's optical properties. I well remember working on my parents' car in Phoenix summers. The chromed wrenches on our driveway surface got to instant-blister temperatures. Tools with black surfaces were uncomfortable to pick up, but were usable. The chromed tools had a lower absorption of sunlight than the black tools, but their IR radiation coefficient was even lower. So equilibrium temperature (at which radiated energy = incident energy) was much higher for the chromed tools.

WalterBright 4y ago
I was always careful to put my tools in the shade under the car :-) (Used to live in Glendale.)