From the ABC 7 Weather team

Double rainbows: The science of these majestic arcs, explained

May 5, 2011 - 11:27 PM
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What's happening in the sky when we see a rare double rainbow? ABC7's senior meteorologist Bob Ryan discusses the phenomenon.

Another shot of the London bow.8 Photos
(Photo: Ryan Wilson/WJLA / TBD users | Date: Aug. 10, 2008)

Do you remember the first time you ever saw a rainbow?

We all tend to take the wonderful daily show the sky gives us for granted, to some extent. Then, every once in a while on a special day like Wednesday, we see a rainbow, and we stop and gaze in wonder. In our universe there are billions stars and millions of planets… but there can’t be many other places with rainbows.

We don’t see rainbows every day because everything has to be just right for the sky to produce that wonderful arc of color that takes our breath away, whether it’s the first time or the hundredth time. To have a rainbow, there has to be rain in front of you, yet the sun has to be out behind you.

The raindrops are tiny spheres, or flattened spheres in the case of big raindrops, falling in the air. The white light from the sun enters and is reflected off the back of the drops. But because white light is actually made up of many colors, different colors come out of the raindrop at different angles because of the “refractive” properties of the different colors. Thus we see the colors at an angle close to 42 degrees off the sun's axis as part of a circle, or a “bow.” If the sun is higher than 42 degrees above the horizon, we can’t see rainbows. That’s why the lower the sun gets in the sky, the bigger the rainbow gets. If the sun is fairly high, the rainbow is fairly small.

Look at the great rainbow pictures that were sent to us yesterday. Some are double rainbows – two for the price of one! Why does this happen?

If the sun is fairly high and bright, yet there are plenty of raindrops and a dark sky in front of you, the white sunlight will go through two reflections in the raindrops and a double rainbow will be created. Notice how the colors of the second bow are reversed from the inner or first bow because of the two reflections. The diagram below shows how this works:

double rainbow d.c.

And here's another diagram showing where you expect to see the second rainbow (courtesy of NCAR):

double rainbow d.c. 2

The next time the rain has just ended and the sun is positioned low in the sky, look for a rainbow. Not any rainbow, but your own rainbow! Each bow is formed by different raindrops. Your spouse or friend standing right next to you is seeing his or her own rainbow! As a matter of fact, your left and right eye see the beautiful colors reflected and refracted from different raindrops and each eye has its own rainbow. Isn’t the sky special and wonderful every day?

Send me more rainbow pictures at bryan@wjla.com and we’ll share them with everyone.

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