Brainstorm Answer: Snowbows
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Dave Thurlow, Host
Hi, I'm Dave Thurlow from the Mount Washington Observatory and this is The Weather Notebook. For one of our recent Brainstorm questions, we went to listener Ellen Nordstrom Bear from Norwich Vermont:

"I know that there's always a rainbow when the sun shines and there's some precipitation. I'm just curious why when we have snow on sunny days why we don't have a snow bow?"

Well for part of the explanation, we hear from another listener, Jerry Jones in Golden, CO:

"The reason, as I understand it, is first of all you see the rainbow when there's water droplets in the air and the sun is behind you. Each one of the droplets of water is what reflects the sunlight back and it's reflected from the backside of the droplet, as I understand it, and refracted as it passes through the front side of the droplet back toward you. Depending upon the angle of refraction, you get a different color."

Ya that's it... Sunlight shines from the sun behind you into the raindrops in front of you. The back of the raindrop acts like a mirror and the front of the raindrop bends the light like a prism. But what about snowbows? Well they don't exist. Jerry will tell you why.

"Since snowflakes don't have such a rounded surface and you're not able to pass light through them and reflect off the back curved surface, they don't refract the light the same way."

And the result is no such thing as snowbows. But there are such things as snow halos that look like rainbows only you see them when facing the sun.

Listener questions and answers are always welcome at 1-888-RAIN-001. Thanks go to Subaru and the National Science Foundation.

Related Links

Refraction of Light - University of Minnesota

What causes a rainbow? - How Stuff Works