San Francisco: Rainbow Spotted Over City, Wows Twitter

April 7, 2015 Updated: July 18, 2015

A double rainbow formed over San Francisco after a storm rolled over Northern California on Tuesday.

Users on Twitter were stunned by the occurrence.

Some spotted an even rarer double rainbow.

A rainbow is formed when sunlight goes into a raindrop in the air, and splits the light into colors. “This light then reflects off the back of the raindrop and re-emerges in the direction in which the light first entered. The light emerging from many raindrops creates a rainbow,” reads an explainer from Dartmouth.

“When a ray of sunlight enters a raindrop it bends (refracts). The light then strikes the back of the raindrop, where some of the light passes through and some is reflected. As the light exits the raindrop, it is refracted again. The angle at which the light emerges depends on the wavelength of light,” it says.

Mike Pigott, an AccuWeather meteorologist, said that in order to form a rainbow, there are a few requirements. “The sun needs to be at a low angle. Usually, the magic number’s actually 42 degrees above the horizon or less,” Pigott said in 2010, per ABC News. “The sun would also need to be at your back and the rain at the front.”