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Published: 2011-01-31 07:33:39
Updated: 2011-01-31 07:33:39
Posted January 31, 2011
By Mike Moss
A viewer named Vivian sent us the photo you see with this post (click it to enlarge). She shot the image just before sunset on January 9th, and wondered what it was she had captured. Many of you may recognize the feature (or combination of features) as a "sundog," also known as a parhelia, and given the length of the colored bands in the image, I think we may be looking at both the sundog and a coincident segment of a larger 22-degree halo. When the sun is very low in the sky those two optical phenomena are just about co-located at a 22-degree angle to the left and right of the sun, while for solar elevations higher above the horizon, the parhelia move outside the halo a bit and also are displaced above the elevation of the sun.
These colored spots or bands are the result of sunlight being refracted as it passes through cirrus clouds made up of millions of plate-like, six-sided ice crystals with their large faces mostly parallel to the ground. If there are also column-shaped six-sided crystals present in the clouds, and those columns have a semi-random orientation, there may also be the 22-degree halo that makes a large circle around the sun. In both cases, sunlight refracted through the crystals is bent more sharply at short (blue) wavelengths and less sharply at longer (red) wavelengths, so that the colored bands are oriented with the red closest to the sun and vice versa. This property tells us that in Vivian's photo, the sun is off to the right and outside the frame.
Another subtle clue tells us the same thing about the sun's position. While it's not real dramatic, a glance at the picture shows the clouds are uniformly darker to the right of the sundog/halo position and vice versa, a result of the fact that while the ice crystals involved may refract light at angles greater than the roughly 22 degrees of the halo and parhelia, they do not refract any light at lesser angles. So, inside the halo you are only seeing light scattered by the air and reflecting off the outside of the cloud crystals, but outside the halo you are also seeing some extra light being refracted at angles greater than 22 degrees and on out to about 50 degrees or so, adding a little to the brightness of that region relative to the inside of the halo.
I've included a few links to pages at the "Atmospheric Optics" web site that provide diagrams, explanations and many more photos that illustrate the means of formation and variety of appearances of sundogs and 22-degree halos. Thanks to Vivian for sending us this picture, and, keep your eyes on the sky!