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7 Nov 97

Blue Skies...and not so Blue

When we flew home over the Rocky Mountains near Denver last week, we saw a surprising amount of snow on the ground. We continued over the Colorado River and had a great view across the canyon country towards the Grand Canyon way to the south. The sky was deep blue above, but definitely lighter blue close to the horizon. This effect is common, and I usually blame it on dust and smog. But I'm sure the desert air was clear and cold. So why is the sky less blue on the horizon? For that matter, why is it blue at all?

Why is the sky less blue on the horizon? On the moon there is no atmosphere, and the sun's rays pass straight down. The sun itself is bright, but the rest of the sky is black. But the Earth's atmosphere scatters light from the sun like car headlights in the fog. Blue light is scattered most because of the chemical makeup of the atmosphere, so whichever way we look, some extra blue light reaches our eyes. Close to the horizon, some of this scattered blue light is scattered again away from us. Less blue light now reaches us, so the sky looks paler or even white.

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