Monday, July 21, 2008

Why is the Sky Blue?

The Freakonomics blog had a good guest post about Greenland at It's worth reading the whole thing, but I was particularly impressed by a good explanation of the difference between blue and white ice, why the sky is blue, but most importantly, why skim milk is blue. No word yet on if it explains old-lady hair. (You'll have to read the rest of the post to find the part about eating polar bears, and the locals' skepticism about global warming):

Glaciers are rivers of ice, but there are a number of interesting aspects to this. Glacial ice has its origin in snow that falls during a winter storm. As snow continues to fall on the top of the ice cap its weight compresses the bottom layers squeezing out the air.

About 50 feet of snow depth is required to pack the snow into typical glacial ice. That ice is white because it is full of air bubbles and crystal boundaries which scatter the light — what a physicist like me would call Mie scattering.

It’s the same phenomenon that makes milk or clouds white.

Non-fat milk is much less opaque than full-fat milk or cream and has a bluish tinge. This is a different kind of scattering, called Rayleigh scattering, and is why the sky is blue, and why deep water is blue. The difference between Rayleigh and Mie scattering is the size of the particles.
Glaciers often have liquid water in them — called meltwater. This can either be inside the glacier or can appear at the surface. Once a meltwater lake starts it tends to get deeper because it absorbs sunlight more than the surrounding ice. It is intensely blue from Raleigh scattering.

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