May. 15, 2015

Picture of the Week: Sun Halo, With Sundog Companions

by Chau Tu

Click to enlarge images
This article is part of the SciFri Science Club's Explain the Sun activity. Participate using the hashtag #ExplainTheSun.
What you’re seeing above are just two of the nearly four-dozen different kinds of optical effects that result from the sun interacting with ice crystals in the sky. This photograph—taken in 1978 by its subject, Robert Greenler, in Point Barrow, Alaska—features what’s known as a 22-degree halo, as well as two bright spots on the edges called sundogs. Both phenomena occur when sunlight passes through hexagonal ice crystals in the sky, according to Greenler, a professor emeritus of physics at the University of Wisconsin, Milwaukee, who’s studied these optical effects for decades.
When a random assortment of hexagonal ice crystals tumbles through the air, a concentration of them will refract the sun's rays by about 22 degrees. But most of that light won’t reach your eye. “The only deviated rays that can get to your eye come from those crystals in the sky that are located in a particular direction,” says Greenler—that is, 22 degrees from a sightline extending from your eyes directly to the sun. The light that reaches you appears as a halo, because the crystals it passes through lie in a circle about the sun. (For more explanation, see diagrams at left and below.)
Sundogs like the ones Greenler’s pointing to in the photo occur in a similar manner to the 22-degree halos, but with a key difference: They arise from six-sided, plate-like crystals. These flat crystals typically descend with their faces approximately horizontal to the ground—“think about how a leaf falls,” says Greenler. As a result, the only crystals with an orientation that will refract light into your eye are ones on either side of the sun.
When the photograph above was taken, there must have been two groups of ice crystals in the sky that produced both the 22-degree halo and the sundogs, according to Greenler.
The 22-degree halo and sundogs are the most common ice crystal effects that people see, says Greenler. (Each can also appear without the other—i.e. a sundog can glint without a halo, and a halo might shine without companion sundogs.) And they can appear during any season, across countries. “I've chased these things all over the world,” he says. Greenler used to keep track of various ice crystal phenomena he observed, averaging sightings about 70-80 days a year—and 22-degree halos and sundogs dominated the list.
The best trick to eyeing them, advises Greenler, is just being aware. But remember to keep those polarized sunglasses on.
About Chau Tu

Chau is SciFri's web producer. She spends a lot of her time drinking coffee, seeking out street art, listening to music, and defending Los Angeles. Follow her @chaubtu

The views expressed are those of the author and are not necessarily those of Science Friday.
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