A ghostly rainbow formed entirely out of fog descended over Lake Fairfax on Monday as a dense fog advisory blanketed most of the region.

The fogbow was spotted by a Twitter user who was surprised by its strange appearance. The Capital Weather Gang chimed in to offer some clues on its cloudy appearance.

Rainbows form when lights enters a water droplet. Each beam of sunlight travels at a different speed, slowing down at different rates when hitting a raindrop. Light is refracted and cast into an arc of color.

Fogbows, however, form from extremely small droplets, resulting in less refraction.

"Fogbow" this morning over Lake Fairfax in Reston, Va. I have never seen one before. Can someone explain the phenomenon? @capitalweather @WeatherNation @WeatherGeeks pic.twitter.com/mcKhDQTT8G

— Rob Gates (@rjgatesontheweb) November 9, 2020