What Makes a Double Rainbow?

Dec. 23, 2010— -- The Internet just can't get enough of those double rainbows.

After a double rainbow formed over Los Angeles last night, Twitter erupted with posts about the phenomenon, breathlessly referencing the wildly popular "double rainbow" viral video that swept the Web this summer.

Actor Jason Alexander posted, "And L.A. peeps, did you see the amazing double rainbow? I don't wanna be 'double-rainbow' guy, but that was spectacular. Almost worth the rain."

"Reading Rainbow" host LeVar Burton Tweeted a photo and wrote: "The very elusive Double Reading Rainbow…"

But what actually makes a double rainbow?

To See Rainbow, Sun Needs to Be at Your Back, Rain at Your Front

Mike Pigott, an AccuWeather meteorologist, said that to make any kind of rainbow you need a couple of key ingredients.

"The sun needs to be at a low angle. Usually, the magic number's actually 42 degrees above the horizon or less," Pigott said. "The sun would also need to be at your back and the rain at the front."

When a ray of sunlight hits a raindrop, it refracts (or bends) and then strikes the inside wall of the circular drop. As the light reflects back out of the raindrop, it bends again.

That internal reflection produces the arc of sunlight spread out across its spectrum of colors -- a rainbow.

Pigott said a double rainbow is formed when there are two reflections inside a raindrop.

"In the case of a secondary rainbow, you'd still have the primary rainbow -- you'd have that first reflection at 42 degrees. But the sun's rays can also have a second reflection off the back [of the raindrop]. So instead of reflecting off the back once, it will actually reflect twice. And that's typically at a different angle," he said.

The secondary rainbow tends to be fainter than the first and it also displays the colors in the reverse order from the primary rainbow.

Double Rainbows Not the Rarest Kind of Rainbows

Los Angeles' weather conditions Wednesday night were ripe for rainbow-making, Pigott said.

"In the case of L.A., this would certainly be perfect because the clouds were clearing out from that heavy rain," he said. "The sun would be setting in the West and the rain would be moving off to the East, and L.A. would be dead in the middle."

And as "all the way" amazing as double rainbows can be, scientists say that, on rare occasions, people have even spotted tertiary rainbows.

Kevin Garrett, a physical scientist with the National Oceanic and Atmospheric Association, said that sixth order rainbows are even possible.

He said that whether you see the higher order rainbows (double rainbows and above) depends on the intensity of the light because as the light reflects internally, some of its intensity is lost. That's also why the primary rainbow is always brighter than the secondary one.

In some cases, light may have reflected more than once inside a raindrop, but if the background is too light an observer wouldn't be able to see the second bow.

"It also helps if you have a darker background in the sky for larger contrast," he said. "If you've got darker storm clouds as you're viewing the rainbow, you're better able to see the second order rainbow if the light is less intense."