What Causes a Red Rainbow?

Bad Astronomy
The entire universe in blog form
Aug. 4 2014 11:43 AM

Red Rainbow

Knowing my love of optical phenomena in the sky, photographer (and frequent BA blog appearer) Göran Strand sent me a really interesting pair of pictures he took recently of a rainbow over Östersund, Sweden.* He shot the first one in the afternoon when the Sun was about 27° above the horizon, and the second one right as the Sun was kissing the horizon. Can you spot the difference?

red rainbow
That's like four pots of gold. Click to leprechaunenate.

Photo by Göran Strand, used by permission

Beautiful, aren’t they? The top one has a number of familiar features. For example, because the center of the arc of the rainbow is opposite the Sun in the sky, when the Sun is high, the rainbow is low. The primary rainbow has purple and teal supernumerary arcs inside, the origin of which is a bit complicated. I see them fairly often in bright rainbows after storms where I live in Boulder, Colorado.

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Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

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You can also clearly see the secondary rainbow outside the primary one, its color order reversed. Those occur when light is reflected twice inside a raindrop instead of just once, and are common when the primary is very bright.

There is also the interesting effect that the sky appears brighter inside the primary bow (light reflected by water droplets in the air inside the arc of the rainbow contribute to that), and darker between the two (called Alexander’s dark band, due to light from that area being reflected away by the droplets in that direction).

All very cool, interesting, and pretty, but what the heck is going on with the red rainbow Strand photographed later?

A hint: Remember, it was taken when the Sun was right on the horizon. See how the rainbow is higher, because the Sun is low?

The red color is because when the Sun is on the horizon, it looks red. Yes, really. When the Sun is on the horizon, its light is passing through more air to reach you because the atmosphere is curved. Air scatters away colors with shorter wavelengths (like blue), and the more air there is, the longer the wavelength that gets scattered. When the Sun is high, the blue is scattered most, and the sky looks blue. When the Sun is lower, green and even yellow and orange get scattered away, leaving the Sun looking red.

Since predominantly red light gets through the air, that red light makes up the rainbow. Of course, not only red light gets through, usually, so you see some hint of colors as you do in Strand’s photo. There’s a secondary bow, too, faintly glowing off to the right.

Amazing. Have you ever seen these effects? If you look up a lot, you might. If there’s a better reason to pay attention to the sky than the magnificence and wonder that go with a rainbow, I’m not sure what it is.

*I always wind üp using a löt öf umlauts when I write aböüt Göran’s wörk. Make sure you check out his gallery of incredible photographs!

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