Sun Halo, With Sundog Companions

When the sun interacts with six-sided ice crystals, ethereal optical effects can occur.

Before "selfies" were all the rage, Robert Greenler took this self-portrait in Point Barrow, Alaska, in 1978, using film. After setting up his camera, he positioned himself with his face to the lens and back to the sun. Can you figure out how he knew where to place his head so it blocked the light? Photo by Robert Greenler
Before “selfies” were all the rage, Robert Greenler took this self-portrait in Point Barrow, Alaska, in 1978, using film. After setting up his camera, he positioned himself with his face to the lens and back to the sun. Can you figure out how he knew where to place his head so it blocked the light? Photo by Robert Greenler

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 sunlight passes through a hexagonal ice crystal, the minimum angles of deviation (D) is 22 degrees. “Near the minimum-deviation orientation, the deviation of the light ray is insensitive to small rotations of the crystal,” writes Robert Greenler in Rainbows, Halos, and Glories (1980). “As a consequence, when light rays pass through crystals with all possible orientations, there is a concentration of rays deviated by angles near 22 degrees.” From Rainbows, Halos, and Glories (1980), by Robert Greenler

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.

To seem sunlight that has deviated by an angle D, you look at an angle D away from the sun. From Rainbows, Halos, and Glories (1980), by Robert Greenler

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.

Meet the Writer

About Chau Tu

Chau Tu is an associate editor at Slate Plus. She was formerly Science Friday’s story producer/reporter.