4 ms·

What happens to the "heat" energy that gets reflected by white/reflective roofs? It seems like it'd stay in the relative vicinity of the house, heating up the o

What happens to the "heat" energy that gets reflected by white/reflective roofs? It seems like it'd stay in the relative vicinity of the house, heating up the outside air further (even by a small amount), right? If that's the case, are there diminishing returns to worry about with effectively just moving heat from one place to another, assuming the (hotter) outside air will probably continue to warm up the house anyway? Do we just bank on something else absorbing the energy instead? Does it leave the area/atmosphere if nothing absorbs it? (Please don't take this as a rhetorical "this is why X solution doesn't work" -- I'm no expert here and am just curious where all that reflected heat/energy goes!)

jliptzin 4y ago
I would assume those light rays reflect off the roof instead of getting absorbed…in that case the heat is absorbed into the atmosphere, I have a hard time believing it sits around the house and lingers there for any significant amount of time (doesn’t heat rise?)
- oneoff786 4y ago
  Heat rises is short for hot air rises. The light would not be completely absorbed by the atmosphere more than it did on the way down.
extra88 4y ago
With a light or reflective surface, a lot of the energy is reflected as light so it's not converted to heat. Relatively little will be absorbed by the air near ground level, heating the air only a little.
dexwiz 4y ago
“Heat” from the sun that we feel is mostly IR/Visible light. Mix of stuff happens if we reflect it with white surfaces. Some of it does go back into space. Some of it will get reflected by the atmosphere back towards the ground. IR light is also directly absorbed by the air molecules, but very little. Air is mostly transparent at visible wavelengths so it absorbs very little energy. Solids are much better at converting IR to thermal energy. Realistically air conditioning is just moving heat from one side of a wall to another. Overall painting buildings white won’t change the temperature of Earth, but it does help the building in question, which is all air conditioning cares about anyways.
myself248 4y ago
All that energy started at the sun, we call it "light" when it's visible and "radiation" when it's outside our visual range, but it's the same stuff. The fact that it reached the roof in the first place means the atmosphere is transparent to it. So with a white roof, the wavelengths being reflected are the same ones being received, it'll reflect right back through the atmosphere and back into space. Effectively it just increases the Earth's albedo a tiny bit. Any wavelengths which were gonna heat the air by absorption, were absorbed on the way in and didn't reach the roof in the first place. HOWEVER. In the case of dark/black roofs, the heat DOES stay in the vicinity of the house, at least for a while. The radiation was absorbed by the roof, and now two main things happen to it: One, the hot roof heats the nearby air by convection. This is the main contributor to the "urban heat island" effect. Eventually this warmed air mass will affect weather patterns, but even before that, it just makes the place feel hot. Two, the hot roof emits its own radiation, in a whole range of wavelengths determined by its absolute temperature, according to Planck's law. This includes infrared wavelengths that the atmosphere is not transparent to (the absorption region between 5 and 8 microns), so that radiation IS absorbed by the atmosphere and heats it as well. And just like convective transfer above, the warmed air contributes to heat-island and weather effects, etc. (The phenomenon of a hot object emitting its own radiation is why thermal cameras work, by the way. And if you get the object even hotter, the Planckian locus shifts into our visible range and you get an object that is "red hot", or even hotter still and it's "white hot", etc. This is why the "whiteness" of lightbulbs is expressed in Kelvin, that's the temperature of an object that would glow the same color as the bulb emits. The "warm white" and "cool white" are misnomers; the so-called "cool" color represents a much hotter blackbody source.)
- grog454 4y ago
  > Any wavelengths which were gonna heat the air by absorption, were absorbed on the way in and didn't reach the roof in the first place. What is the basis for this? Intuitively (I am no expert) it seems more likely that the amount of thermal energy produced would be a function of the distance light travels through a medium and the amount of energy carried by the light in the first place. So I would expect that the "inbound" light would produce more thermal absorption energy than the reflected "outbound" light, but it wouldn't arbitrarily all be absorbed on the way in. Reality seems to be more complicated: https://www.sciencedirect.com/science/article/abs/pii/0960168693901047 https://www.sciencedirect.com/science/article/abs/pii/096016...

jliptzin 4y ago

I would assume those light rays reflect off the roof instead of getting absorbed…in that case the heat is absorbed into the atmosphere, I have a hard time believing it sits around the house and lingers there for any significant amount of time (doesn’t heat rise?)

oneoff786 4y ago
Heat rises is short for hot air rises. The light would not be completely absorbed by the atmosphere more than it did on the way down.

extra88 4y ago

With a light or reflective surface, a lot of the energy is reflected as light so it's not converted to heat. Relatively little will be absorbed by the air near ground level, heating the air only a little.

dexwiz 4y ago

“Heat” from the sun that we feel is mostly IR/Visible light. Mix of stuff happens if we reflect it with white surfaces. Some of it does go back into space. Some of it will get reflected by the atmosphere back towards the ground. IR light is also directly absorbed by the air molecules, but very little. Air is mostly transparent at visible wavelengths so it absorbs very little energy. Solids are much better at converting IR to thermal energy. Realistically air conditioning is just moving heat from one side of a wall to another. Overall painting buildings white won’t change the temperature of Earth, but it does help the building in question, which is all air conditioning cares about anyways.

myself248 4y ago

All that energy started at the sun, we call it "light" when it's visible and "radiation" when it's outside our visual range, but it's the same stuff. The fact that it reached the roof in the first place means the atmosphere is transparent to it. So with a white roof, the wavelengths being reflected are the same ones being received, it'll reflect right back through the atmosphere and back into space. Effectively it just increases the Earth's albedo a tiny bit. Any wavelengths which were gonna heat the air by absorption, were absorbed on the way in and didn't reach the roof in the first place. HOWEVER. In the case of dark/black roofs, the heat DOES stay in the vicinity of the house, at least for a while. The radiation was absorbed by the roof, and now two main things happen to it: One, the hot roof heats the nearby air by convection. This is the main contributor to the "urban heat island" effect. Eventually this warmed air mass will affect weather patterns, but even before that, it just makes the place feel hot. Two, the hot roof emits its own radiation, in a whole range of wavelengths determined by its absolute temperature, according to Planck's law. This includes infrared wavelengths that the atmosphere is not transparent to (the absorption region between 5 and 8 microns), so that radiation IS absorbed by the atmosphere and heats it as well. And just like convective transfer above, the warmed air contributes to heat-island and weather effects, etc. (The phenomenon of a hot object emitting its own radiation is why thermal cameras work, by the way. And if you get the object even hotter, the Planckian locus shifts into our visible range and you get an object that is "red hot", or even hotter still and it's "white hot", etc. This is why the "whiteness" of lightbulbs is expressed in Kelvin, that's the temperature of an object that would glow the same color as the bulb emits. The "warm white" and "cool white" are misnomers; the so-called "cool" color represents a much hotter blackbody source.)

grog454 4y ago
> Any wavelengths which were gonna heat the air by absorption, were absorbed on the way in and didn't reach the roof in the first place. What is the basis for this? Intuitively (I am no expert) it seems more likely that the amount of thermal energy produced would be a function of the distance light travels through a medium and the amount of energy carried by the light in the first place. So I would expect that the "inbound" light would produce more thermal absorption energy than the reflected "outbound" light, but it wouldn't arbitrarily all be absorbed on the way in. Reality seems to be more complicated: https://www.sciencedirect.com/science/article/abs/pii/0960168693901047 https://www.sciencedirect.com/science/article/abs/pii/096016...