LUNAR FOGBOW VS. NORTHERN LIGHTS: Deep inside the Arctic Circle, aurora tour guide Tim Nordström of Abisko, Sweden, routinely sees green curtains hanging down from the sky. On Dec. 28th, he saw something different: a pale arc reaching out of the ground:
“It was amazing,” says Nordström. “We were hiking through a frost-covered forest. The air was cold (-25 C) and crisp. At first the fog was thick above us, but after a while it started to thin out so we could see the green auroras overhead. A bright shaft of moonlight lanced through the fog –and that’s when we saw the fogbow.”
Fogbows are cousins of rainbows and they are formed in essentially the same way: light bounces in and out of water droplets to produce a luminous arc. In this case, the droplets were supercooled (to remain liquid in the freezing air) and much smaller than typical raindrops. Tiny droplets cause a diffraction effect not seen in ordinary rainbows; as a result, the colors are smeared together resulting in a nearly-white arc.