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?

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.