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| | | Earth on Aug 29, 2014

What makes a red rainbow?

Red rainbows happen when the sun is on the horizon. They’re created in much the same process that causes a sunset or sunrise to look red.

Goran Strand of Sweden captured this double rainbow ... and, shortly afterwards, caught it again as it became a red rainbow.  Used with permission.

View larger. | Göran Strand of Sweden captured this double rainbow … and, shortly afterwards, caught it again as the sun sank lower, and it became a double red rainbow. Used with permission. Visit Göran’s astrophotography website or his Facebook page.

Astrophotographer Göran Strand of Sweden kindly gave us permission to publish this photo, which shows a double rainbow … and a phenomenal double red rainbow. He shot the upper image in the afternoon, with the sun about 27 degrees above the horizon. He shot the lower photo with the sun 2 degrees above the horizon. Notice two things: the height of the rainbows above the horizon, and their colors.

The upper rainbow is an ordinary double rainbow (if any rainbow, especially a double rainbow, can be called ordinary). Rainbows happen when sunlight shines through raindrops. If the sun is behind you, and if you see the sun sunlight emerging from many raindrops at once, you see a mosaic of light spread out in an arc in the sky: a rainbow. Double rainbows happen when sunlight inside a raindrop is reflected twice instead of once.

The height of a rainbow depends on the sun. The lower the sun, the higher the rainbow. You can see evidence of this fact by looking at the red rainbow photo, shot when the sun was setting. See how much higher in the sky it arcs than the other double rainbow?

Here's another beautiful shot of a red rainbow, from Jesper Kristensen.  It's from August 14, 2014.  Thank you, Jesper.

Here’s another beautiful shot of a red rainbow, from Jesper Kristensen. It’s from August 14, 2014. Thank you, Jesper.

It’s rare to see a red rainbow. I’ve seen only one in all my decades of skywatching … of course, I live in Texas, where it doesn’t rain much. The red rainbow I saw was in an even-drier place. I spotted it early one morning decades ago, around sunup, while driving on the dirt road leading from Chaco Canyon in New Mexico. That is such a magical place, and I fancied at the time that the magic of the canyon helped create the red rainbow. Not so.

Les Cowley of the great website Atmospheric Optics says that red rainbows are created when there is a low sun so that, he says, the blue and green of its rays are weakened by scattering during the long journey through the atmosphere. Les adds:

Chaco Canyon rainbow, via rontayyab.com

Regular rainbow over Chaco Canyon, via rontayyab.com

Sunset and sunrise rays travel long paths through the lower atmosphere where they are scattered by air molecules and dust. Short wavelength blues and greens are scattered most strongly leaving the remaining transmitted light proportionately richer in reds and yellows. The result, glorious sunsets and red rainbows.

The lesson here is that, if you’re watching a sunset, and there’s rain in the air, turn in the direction opposite the sun and watch for the elusive red rainbow.

P.S. By the way, when I googled the search term “Chaco Canyon rainbow” I found many rainbow images over this desert place. Maybe there’s some magic happening there, after all.

Bottom line: Red rainbows happen when the sun is on the horizon. They’re created for much the same reason that a sunset or sunrise looks red – because, when the sun is low, the blue and green of its rays are weakened by scattering during the long journey to your eyes through Earth’s atmosphere. The red light travels through more directly. Voila, you see a red rainbow.

Via Bad Astronomy’s Phil Plait.