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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

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...
- myself248 4y ago
  It was a bit of an oversimplification on my part, since for a given wavelength, the atmosphere isn't binary, completely transparent or completely opaque (i.e. the radiation would go a very very short distance before being absorbed), but for _most_ wavelengths it's _very close_ to binary. Either it's absorbed or it isn't, to a first approximation. Yes, some wavelengths are absorbed very weakly, and some are scattered rather than absorbed. But these represent a tiny fraction of the spectrum, the nuance would've overcomplicated the post, and they don't change the result much. For wavelengths that are not absorbed, they can be reflected back out just as easily as they came in. For wavelengths that're strongly absorbed, they heat the first parts of the atmosphere they encounter, and this is why the thermosphere is called that. For those few wavelengths that're weakly absorbed, yes they produce some heating on the way in, and if you reflect them back out, they do produce some heating on the way out. But as you've said, because the intensity of the radiation is decreasing along the whole path, the "outbound" contribution is weak indeed. It turns out to be basically a rounding-error in the thermal contribution, which is overwhelmingly dominated by blackbody absorption and re-emission, which often does produce wavelengths that're strongly absorbed. On a perfectly-white planet (or snow-covered region), you might have to start caring about weakly-absorbed wavelengths, but for most types of ground cover, other effects swamp it.

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...

myself248 4y ago
It was a bit of an oversimplification on my part, since for a given wavelength, the atmosphere isn't binary, completely transparent or completely opaque (i.e. the radiation would go a very very short distance before being absorbed), but for _most_ wavelengths it's _very close_ to binary. Either it's absorbed or it isn't, to a first approximation. Yes, some wavelengths are absorbed very weakly, and some are scattered rather than absorbed. But these represent a tiny fraction of the spectrum, the nuance would've overcomplicated the post, and they don't change the result much. For wavelengths that are not absorbed, they can be reflected back out just as easily as they came in. For wavelengths that're strongly absorbed, they heat the first parts of the atmosphere they encounter, and this is why the thermosphere is called that. For those few wavelengths that're weakly absorbed, yes they produce some heating on the way in, and if you reflect them back out, they do produce some heating on the way out. But as you've said, because the intensity of the radiation is decreasing along the whole path, the "outbound" contribution is weak indeed. It turns out to be basically a rounding-error in the thermal contribution, which is overwhelmingly dominated by blackbody absorption and re-emission, which often does produce wavelengths that're strongly absorbed. On a perfectly-white planet (or snow-covered region), you might have to start caring about weakly-absorbed wavelengths, but for most types of ground cover, other effects swamp it.