Double rainbows spotted over Middle Tennessee — what causes them

NASHVILLE, Tenn. (WSMV) – Isolated showers and storms over the next few days will make for more brilliant color displays across the sky.

Rainbows have been very common across Middle Tennessee for several evenings now. With all the recent rain, conditions have been ideal for fabulous displays of brilliant colors. Some of you have even reported seeing double rainbows. WSMV4 viewer, Leslie Whited, captured the one above, early Tuesday evening, July 14th.

To find out how double rainbows form, let’s first examine how a single rainbow occurs.

Single rainbows form when the sun, positioned behind you, has its light refracted through raindrops ahead of you.

Those raindrops bend sunlight as it passes into the drops. Then, some of that light reflects off the back of the drop and is bent one more time as it exits the drop. That entire process is called single reflection. Single reflection produces the primary or brightest rainbow.

Sometimes, some light reflects twice while in a raindrop before exiting. This is called double reflection. Double reflection produces a secondary rainbow. The order of colors within a secondary rainbow is a mirror image of the primary rainbow (i.e. the reverse). Secondary rainbows are not as bright as primary rainbows because less light is double reflected than is single reflected (i.e. some light is lost or attenuated every time light is reflected). Notice the fainter secondary rainbow in Leslie Whited’s double rainbow/storm picture at the top of this article.

The ideal time to see a rainbow is when the sun is relatively low in the sky (and has the best chance of being at your back). That translates to early morning or evening. Since in our current weather pattern, showers and storms are most numerous during the late afternoon and evening, that’s when you’ll have the best chance of seeing a rainbow through the rest of this week. If you’re very lucky, you might even see a double rainbow.

Happy sky watching!

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