Greetings from the blogosphere! My name is John Ristvey. I am the new director of the UCAR Center for Science Education. I have been a science educator for over 25 years. I began teaching Earth and life science at Seabrook Intermediate School (Seabrook, Texas) in 1990. I joined McREL in 1999 and then moved to UCAR a few months ago.
For my first blog entry for UCAR, I would like to share a bit about sundogs. I got a good look at this atmospheric phenomena out the window of a bus on my way to Boulder, CO on an April morning.
A sundog (also called a mock sun or parhelia) is a brightly colored spot that appears to the right and/or left of the Sun. Sundogs most often form when there are uniform cirrostratus clouds covering the sky and the Sun is near the horizon. The cirrostratus clouds are made of hexagonal ice crystals which bend light, creating sundogs and other atmospheric optics such as the 22-degree halo.
I posted these pictures on Facebook and asked my friends what they wondered about sundogs. “What is the leash law for sun dogs in your community?” one friend responded. Another asked, “What do you feed them? Do they play well with other dogs. Are they easy to train?”
These were funny, but there was one great question from Christopher, a friend of mine from high school. He asked why sundogs are prominent at the end of April.
Sundogs are brightest in winter because ice crystals are more common, but can be seen at any time of year. I asked Peggy LeMone, an NCAR scientist who studies clouds, Christopher’s question, whether sundogs are most prominent in April. Here is her response:
“I think they tend to occur more in the winter because the atmosphere tends to be more stable, which favors the formation of horizontal clouds like cirrostratus, the cloud associated with haloes and sundogs. In the summer, convective clouds penetrate high into the troposphere. The fact that we had them in April was, well, because the weather was right."
So, the answer is that sundogs form when the weather is just right.
What do you wonder about sundogs? Tweet your sundog questions and photos to us (@UCARSciEd). Let us know what other atmospheric optics interest you too!
- Check out Peggy LeMone's blog post about atmospheric optics: Incredible Optics on a Winter Afternoon
- Learn more about optics on the Roping Rainbows page I helped develop for NASA’s Genesis mission. Try the matching quiz and see how many optical phenomena you can recognize.
- Watch The Science Behind a Sundog for more about sundogs.