Commuter Amanda Curtis spun the Internet into a tizzy by posting a photograph of four rainbows shining in the sky above the Glen Cove (map) train station on New York's Long Island. Though some on social media expressed skepticism about the phenomenon, experts agree that getting four rainbows at once in close proximity is entirely possible.
"What you're really seeing is two offset double rainbows," says Raymond Lee, a meteorologist at the U.S. Naval Academy in Annapolis, Maryland, referring to the picture from Long Island.
The two brighter ones are the primary and secondary rainbows. The fainter pair that appears higher in the sky is the result of sunlight reflected off of a body of water behind the photographer, Lee says. A quick search shows two bays that could have done the trick—Oyster Bay or Hempstead Bay, the meteorologist explains.
A rainbow forms when sunlight enters raindrops in the atmosphere and is reflected back out. A double rainbow occurs when sunlight left over from that initial reflection comes back through the raindrops a second time.
The two double rainbows are different than true quaternary rainbows, which are incredibly rare, says Lee. There have only been four or five scientifically documented sightings of quaternary or tertiary rainbows since 1700, he says.
A quaternary rainbow forms when sunlight enters and reflects out of raindrops four times. With each pass through the raindrops, the amount of light is reduced, making tertiary and quaternary rainbows incredibly dim.
Conditions have to be just right for them to form—heavy rain in addition to direct sunlight. "You need a fortuitous small hole in the clouds combined with torrential rain," says Lee. (See a picture of the first-ever quaternary rainbow photographed.)
You also need to be looking towards the sun to see a quaternary rainbow, since they form opposite the primary and secondary rainbows. Says Lee: "The optical deck is heavily stacked against our ever seeing the higher-order tertiary and quaternary bows."
Follow Jane J. Lee on Twitter.