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- June 17, 2013 at 9:20 AM #67179So I understand that the reason sunsets are red is because the light has to penetrate more of the atmosphere, so only the long wavelengths of the color red can get through. I also understand that when you white balance outdoors, the color temperature on an overcast day is much more blue than a sunny day.Why is this? Shouldn't it be more red under an overcast day since there is more atmosphere for the light to pass through?
My guess is that it has to do with the angle that the light is hitting the atmosphere. Just a guess though.
Yeah, I'm pretty sure that's true for the most part: typically the steeper the angle, more red the light is. The introduction of clouds doesn't change the angle of the sun though. That's part of what makes this issue confusing to me.
I should have paid more attention in physics class I guess.
It's the difference between atmosphere and clouds. Even on a clear day, the sun sets at a shade of red, right? That's because, as Brian noted, the angle of the sun is much more flat, relative to the sunset viewer. As a result, it has to cut through more atmosphere to get to the viewer. We're talking thousands of miles of atmosphere!
A cloudy day is very different. Clouds block reds to a certain extent, but allow more blues and UVs through. Plus, we usually aren't talking about more than a mile or so of cloud cover that has to be pierced. That's why you can get a sunburn on a cloudy day.
So, the difference between piercing thousands of miles of relatively thin atmosphere, versus a mile or so of heavier clouds, accounts for the difference. Hope that helps!
The Sun closely approximates a black body radiator. The effective temperature, defined by the total radiative power per square unit, is about 5,780 K. The color temperature of sunlight above the atmosphere is about 5,900 K.
As the Sun crosses the sky, it may appear to be red, orange, yellow or white depending on its position. The changing color of the sun over the course of the day is mainly a result of scattering of light, and is not due to changes in black body radiation. The blue color of the sky is caused by Rayleigh scattering of the sunlight from the atmosphere, which tends to scatter blue light more than red light.
Daylight has a spectrum similar to that of a black body with a correlated color temperature of 6,500 K (D65 viewing standard) or 5,500 K (daylight-balanced photographic film standard).
For colors based on black body theory, blue occurs at higher temperatures, while red occurs at lower, cooler, temperatures. This is the opposite of the cultural associations attributed to colors, in which "red" is "hot", and "blue" is "cold".
Holy moses… I'm quoting wikipedia. ooh boy.
Thanks Brian & Laguna. So it sounds like clounds just aren't enough to noticibly cut blues from the sun's spectrum of light, but it does introduce enough new water molecules to increase the effective "blueness" of the sky, based on Rayleigh scattering?
My brain hurts.