Rare Ghostly 'Fogbow' Appears Over Florida

We've all heard of a rainbow but what about its ghostly cousin, the fogbow? The rare aerial phenomenon appeared in the sky above Florida yesterday afternoon.

The National Weather Service in Melbourne, Florida, shared the photo of a fogbow stretching over a highway in Viera on social media in a post that has been viewed nearly 20,000 times. The photo was taken by one of NWS's forecasters.

"Never heard of or seen a fogbow," said one Facebook user.

"That's pretty dadgum neat!!" said another.

Fogbows, also known as ghost bows or white rainbows, are much rarer than their colorful counterparts and only appear when conditions are just right. In both cases, light interacts with droplets of moisture in the air.

Light travels more slowly through water than it does through air, so when it enters a water droplet, it slows down and bends. This is called refraction. The light is then reflected off the inside of the droplet and goes back the way it came, bending again as it leaves the droplet.

But why the colors? The light that we see is made up of a spectrum of different colors, and some of these "bend" more than others. So when the light is refracted, these colors separate out, with violet bending the least and red bending the most.

So why are fogbows white? The difference comes down to the size of the droplets. Raindrops tend to fall between 0.5 and 2 millimeters (0.02 to 0.08 inches) in length. The water droplets in fog are generally much smaller, at around 0.05 millimeters.

Because of this smaller size, the color separation in a fogbow is weaker and, as said by the NWS, "more smeared," meaning the fogbow appears mostly white. This smearing can also make fogbows appear wider than most rainbows.

In December 2022, the same phenomenon was seen above San Francisco and captured by photographer Stuart Berman. "Yesterday morning, I went for a walk in the Marin Headlands and saw a sight unlike any I'd seen before..." he said in a post on Twitter.

Patchy dense fog is still in place across much of Melbourne and may continue to build further south throughout the day, NWS Melbourne said in a tweet.

For any keen skygazers, EarthSky.com says that the best time to catch a fogbow is during a thin fog when the sun is bright.

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