Sundogs

(This 729th Buffalo Sunday News column was first published on March 20, 2005.)

One morning a few weeks ago I stood at one of the picture windows of the Beaver Meadow Nature Center waiting for a meeting to begin. I was idly watching the birds at the feeders when I felt a firm hand grip my arm. It was my friend Elmer Marien, past president of the Buffalo Audubon Society.

"I want to show you something," he said, and he pointed out an atmospheric phenomenon that I had never noticed before.

Elmer had me look out toward the sun that was barely showing through the icy clouds. But which sun? It looked as though there were two separate suns out there.

"That 'second sun'", he told me, "is a sundog."

Others began to gather and most of them said things like, "Oh, look, there's a sundog." It seemed that everyone knew what a sundog was but me and I had never even heard of such a phenomenon. No meteorologist I.

Elmer gave me a much-appreciated short course in sundogs that morning and I have since supplemented his information by studying some weather texts.

It seems that a sundog, also called a mock sun or parhelion, is formed when sunlight is refracted by a certain kind of hexagonal plate-like ice crystals. When the sunlight passes through those ice crystals it is refracted and the result is an extra apparent light source 22 degrees away from the sun at the same elevation in the sky.

A Sundog with the true sun covered. Note the dim halo.

On that midwinter day the sun was quite low and in the best location for observing sundogs. They occasionally do occur when the sun is higher but in those cases their angular distance from the sun may be greater due to additional refraction.

Sundogs often appear in pairs, one on either side of the true sun. Earlier on that same morning, Elmer told me, he could see both, but there was only one when I was observing.

This strange result occurs only when those ice crystals are similarly oriented. The same kind of crystal randomly oriented can cause a full halo around the sun. That halo, a somewhat less common atmospheric effect, is also 22 degrees from the sun. Apparently the reason sundogs are more common than halos is the ice crystals usually drift slowly down through clouds like falling leaves, maintaining their horizontal orientation. Halos also occur at night around the moon.

The sundog I saw that morning was simply a whitish area in the clouds, but they can be brightly colored. This is like the effect Isaac Newton produced with a prism. White light is broken down into the colors of the spectrum because of their differing refraction angles.

Sundogs and halos are only two of a number of atmospheric effects that are worth looking for. Crownpoint Institute of Technology astronomer Colleen Gino maintains an excellent website that includes attractive photos of many of them on her "Weather Window". Her site is: www.astrophys-assist.com/wilobs/weathwin/weathwin.htm. Much of the following is drawn from that source.

Two atmospherics with which we are all probably familiar are rainbows and mirages. Here are more that Professor Gino and others have cited.

Silver linings occur when sunlight is diffracted by large water droplets along the edges of thick clouds giving those edges a brighter appearance.

Often seen from airplanes, glories are colored rings that appear around shadows. They are caused by miniscule water drops.

Crepuscular rays occur when sunlight is partially obstructed by clouds. Crepuscular is a synonym for twilight, the time they often occur. Ms. Gino adds: "Although the rays appear to converge at the sun, they are actually parallel. The convergence is a matter of perspective, in the same way that parallel train tracks appear to converge at the horizon."

Coronas, colored circular light bands that form around the sun or moon, are caused by the diffraction of light by small water droplets of uniform size in thin clouds.

Cloud iridescence, sometimes with rainbow-like colors, also occurs when sunlight is diffracted by uniformly-sized water droplets in mid-level, thin clouds.

A sun pillar is a vertical column of light, usually appearing above the rising or setting sun. This occurs when light is reflected by horizontally oriented ice crystals in high-level wispy clouds or ice fog near the earth's surface.

Science columnist Edward Willett says that we don't often see phenomena like sundogs because in cold weather "we tend to keep our faces turned firmly to the ground, with occasional glances up to make sure we're not about to walk into traffic." That may well be why I hadn't seen them before.

If I injure myself falling in the weeks ahead, you'll know it was because I was looking for more atmospherics.-- Gerry Rising