Southern Arizona | Investigating 4 You

There’s a halo in the sky!

TUCSON – Every so often, southern Arizona is treated to a huge ring surrounding the sun or moon.

Halo Dec 21 2018
Courtesy: Mark Blomquist

So what causes it? Let’s explore the world of optics. (I promise to keep it simple)

Courtesy: Univ. of Illinois
Courtesy: Univ. of Illinois

Hexagonal ice crystals are typically found in high level clouds like cirrus. These ice crystals refract sunlight or moonlight twice, netting a 22° shift from its original path. The net result is the brilliant 22° halo that catches the eye.

Courtesy: Georgia State University
Courtesy: Georgia State University

When the sun is out, the 22° halo can create an added bonus: sun dogs. When the hexagonal ice crystals line up horizontally with the sun, one to two bright beams develop on each side. Known technically as parhelions, these beams point vertically due to the sunlight passing through the flat-falling ice crystals. It’s typical to see a slight rainbow within the sun dogs due to the prism effect from the ice crystals.

Told you I’d keep it simple!

Jeff Beamish

Jeff Beamish

Jeff Beamish is an award-winning meteorologist at KVOA. You can watch his weekday weather forecasts on Tucson Today & News 4 Tucson at Noon. He has also contributed his weather expertise to The Weather Channel, Washington Post, New York Times, AM 1030 KVOI and the Arizona Daily Star.
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