by Chris Peterson » Sat Mar 10, 2012 3:13 pm
Ann wrote:I agree with Chris that snow just looks blue when it is in the shade and is lit by the blue sky. On the other hand, I note that even though snow can often look very blue when it is lit by the blue sky, it rarely looks strongly yellow when it is lit by direct sunlight.
That's because we rarely see snow that is lit by the Sun that isn't also lit by the sky. Sunlight is visually pure white, but gets separated by atmospheric effects into yellow direct light and blue indirect light. Under the open sky, snow (which is approximately Lambertian in its scattering characteristics) recombines these, so we see a very pure white. In typical shade, the lighting is unbalanced in favor of the sky, so snow appears blue. But it's easy to see yellow snow as well (no jokes!). When you're under fairly deep tree cover, direct sunlight that filters through the branches shows a yellow cast, because the spots it produces on the snow are now preferentially lit by the direct light, and don't have much skylight to shift the yellow back towards white.
Of course, snow lit by the Sun when it is very low in the sky is commonly yellow, pink, or purple.
This is probably an example of ice that looks blue because light penetrates the ice, and the red, orange and yellow light (and some of the green light) is absorbed. Only the blue light (and some of the green light) is reflected back at us.
Of course, the situation with ice is quite different that that of snow. Those are separate discussions.
[quote="Ann"]I agree with Chris that snow just looks blue when it is in the shade and is lit by the blue sky. On the other hand, I note that even though snow can often look very blue when it is lit by the blue sky, it rarely looks strongly yellow when it is lit by direct sunlight.[/quote]
That's because we rarely see snow that is lit by the Sun that isn't also lit by the sky. Sunlight is visually pure white, but gets separated by atmospheric effects into yellow direct light and blue indirect light. Under the open sky, snow (which is approximately Lambertian in its scattering characteristics) recombines these, so we see a very pure white. In typical shade, the lighting is unbalanced in favor of the sky, so snow appears blue. But it's easy to see yellow snow as well (no jokes!). When you're under fairly deep tree cover, direct sunlight that filters through the branches shows a yellow cast, because the spots it produces on the snow are now preferentially lit by the direct light, and don't have much skylight to shift the yellow back towards white.
Of course, snow lit by the Sun when it is very low in the sky is commonly yellow, pink, or purple.
[quote]This is probably an example of ice that looks blue because light penetrates the ice, and the red, orange and yellow light (and some of the green light) is absorbed. Only the blue light (and some of the green light) is reflected back at us.[/quote]
Of course, the situation with ice is quite different that that of snow. Those are separate discussions.