SciTech

Experts from the Carolinas shimmer science on rainbows

An Air China jet flies past a rainbow over Beijing in 2014.
An Air China jet flies past a rainbow over Beijing in 2014. AP

Whether it’s a surprise appearance outside your front door after a storm or a 2010 YouTube posting that’s been viewed almost 42 million times, a rainbow shimmers with beauty, hope, promise.

Here are insights into rainbows from Carolinas experts who still chase them, or want to.

How are rainbows made?

It’s a prism effect of sunlight passing through raindrops. April Hiscox, assistant professor of geography (with expertise in climatology) at the University of South Carolina, said it’s all in the angles: “Rainbow frequency is about the combination of weather and the right optical conditions. So as long as the sun is behind you to hit the water droplets at the right angle to reflect and refract, a rainbow can appear anywhere.”

Are rainbows the same wherever you go?

In terms of the physical process forming them, yes, said Russell Philbrick, research professor in N.C. State’s Department of Marine, Earth and Atmospheric Sciences. “But they certainly look a bit different every time we see them. They are always observed while looking directly away from the sun and into a region of falling water droplets.”

For a single rainbow, the order of the colors is the same anywhere in the world: red on top (outside edge), and orange, yellow, green, blue and purple (inside edge), according to N.C. State atmospheric science professor Sandra Yuter and Ph.D. candidate Nicole Corbin.

If you see one, can someone on the other side of it also see it?

No, and you can’t stand under one, either, said Maria Gelabert, associate professor of chemistry at Winthrop University. “Rainbows only appear when the sun is low in the sky (less than 42 degrees from the horizon), which is why we see them early or late in the day. The rainbow is produced from light interaction with water droplets in the atmosphere, so a person will only see a rainbow facing away from the sun.

“If a rainbow is in the eastern sky and a person drives many miles east to try to find the other side, they’ll only see the rainbow in the east – provided the new location has the right conditions of light angle and atmospheric water content.”

What makes a double rainbow?

Any kid knows that, says UNC Charlotte assistant professor of meteorology Casey Davenport. “A double rainbow happens when there’s two pots of gold lying around.” As for his scientific answer: “Double rainbows occur when sunlight reflects twice within a raindrop. This is a lot more likely when the sunlight is more intense, and also easier to see if the backdrop is dark (like a dark storm cloud).”

Are there rainbows at night – maybe from moonbeams?

“Yes!” said Greg Gbur, professor of physics and optical science at UNCC. “Moonbows exist, and are usually seen in areas where there is a lot of humidity in the air, such as mountain towns and near waterfalls. They rely on dimmer moonlight, and so the human eye usually can’t see the colors in a moonbow. So it appears white.”

Can you create a rainbow with a sprinkler and strong lights?

Yuter and Corbin say anyone can do this, so long as the sun is behind the observer: “A combination of a sprinkler-created rain shower and the sun can make a rainbow. A rainbow made with larger drops will have more intense colors than one made with smaller drops. Light from a flashlight or fluorescent bulb does not contain the full spectrum of colors of sunlight, and hence will not make a rainbow.”

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