Saturday, August 18, 2012

All the color of the rainbow

And no, I don't mean "all the colors of the rainbow."

Some Saturday morning serendipity started with a Phil Plait tweet about a stunning image of clouds lit from beneath by a sinking sun and ended at an image of a red rainbow.

I had never seen a monochromatic rainbow, and the idea of it seems oxymoronic on its face. But it makes good enough sense if you understand the optics of the rainbow.

Normally a rainbow consists of dispersed full-spectrum sunlight. Sunlight includes all the colors of the...uhm...rainbow (***circumlocution alert***).

But what if the sunlight that makes it to the raindrops has already undergone significant Rayleigh scattering by passing through a great thickness of atmosphere typical of a sunset or sunrise? WIth the shorter wavelengths scattered, only the redder colors get through. And if red is the only color to hit the raindrops, red is the only color that will show up in the rainbow.

Still, I had never seen an image of a red rainbow until today. Once again, I need to get out more.

Earth Science Picture of the Day: Monsoon Sunset and Red Rainbow.

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