Why is there a rainbow halo around the sun?
Mix sunlight with some ice crystals at just the right altitude and angle, and you can get stunning displays of atmospheric optics.
Various sun halos as spotted at South Pole Station, Antarctica in January, 2012.
(Lieutenant Heather Moe / NOAA Corps)
Have you ever looked up near the sun and spotted a ring of color, as if a rainbow is surrounding the sun?
The phenomenon is technically not a rainbow, though, as its colorful counterpart, it is based on sunlight refraction.
Only in this case the precipitation is frozen.
"Some people might call it a rainbow around the sun, but it's actually caused by ice crystals," says scientist Michael Kavulich with the National Center for Atmospheric Research.
That difference is critical. Raindrops are spherical, but the ice crystals have six sides, similar to snowflakes, which refract sunlight at a different angle.
(FOX Weather)
"(Rainbows) are absolutely beautiful," Kavulich said. "But they're fairly boring compared to other kind of atmospheric optic phenomena because the raindrops are a sphere, and so the angles are always going to be the same… no matter what angle that raindrop is oriented in."
Ice crystals, on the other hand, are of different shapes and sizes.
"Just like rain, sunlight will go into the ice, and it will be refracted. And so the red and blue light will be refracted at slightly different angles," Kavulich said. "And in this case, because it's ice, it happens to be at a 22-degree angle."
Photo by Miha Rekar on Unsplash
And because those crystals are in all sorts of angles and orientations, "you get the light coming from all around the sun rather than, say, one particular spot," Kavulich said.
That's how you get a halo that completely circles the sun, provided those icy clouds surround the sun. The ring sits at a 22-degree angle, and it's how that ring gets its name: A "22-degree halo".
See the halo, find an umbrella?
Aside from making a colorful display, it could signify more traditional rainbow sightings soon to follow.
In areas that see mid-latitude storms, halos are noted as signals of rain coming within a day.
"It does have some grounding in truth," Kavulich said.
As a storm's low pressure system approaches, the leading warm front will have warm air riding up over colder air in place, creating clouds at various heights ahead of the storm, Kavulich says.
"And the furthest along (the front) you get, you get very high up in the atmosphere (where) it's always below freezing, so you're always going to have ice crystals," he said. "And so in that regard… you may have a higher chance to see a halo around the sun because there are these high, thin clouds in the atmosphere that are made of ice."
Still, don't ignore that FOX Weather app just yet.
"In truth, it's probably not more reliable than listening to your local meteorologist," Kavulich said. "But it can alert you that there's maybe a change in the weather coming because there are there are many different ways that high thin clouds can form."