Atmospheric Optics: Sun Dogs & Moon Dogs
November 2018 :
As you’re walking or driving around the region – especially if you happen to be in one of our incessant traffic jams - out of the corner of your eye have you suddenly seen what looks like a mini-rainbow, but it wasn’t coming out from the ground, as you normally see them, but just off to the side of the Sun? And, there might be a chance that there’s another one on the direct opposite side of the Sun, and possibly above it as well. What you’re seeing is a sun dog.
They are commonly caused by refraction and scattering of light from hexagonal ice crystals suspended in high and cold cirrus clouds, or possibly drifting in very cold moist air at low levels. The crystals act as prisms, bending the light passing through them and deflecting them 22 degrees. As these crystals float downward, the sunlight is refracted horizontally and we see them to both the left and right of the Sun. You might notice, when you see them that they are always colored with the red closest to the Sun, followed by other colors of the rainbow; they are never perfectly striking, but seem to almost meld together. Also, don’t be surprised if you also see one of these phenomena above the Sun, also at its 22 degree position, but most people looking at the sideways light often don’t crane their necks up, above the Sun.
More rare are the Moon dogs, caused roughly the same way, but the Moon does have to be at least a quarter phase, and, of course, it has to be quite cold outside. Also, as they aren’t as bright as Sun dogs, our eyes can’t pick up any color to them.
I’m mentioning them because it just happens one day this past month, I was able to see both of these: The Sun dog while stuck in traffic on 95 in the afternoon, and the Moon dog that night in a field in West Greenwich, under conditions that were far from perfect, but it was worth being able to see this quite rare vision. Seeing both of these was a perfect reminder to always keep looking up, except when the traffic finally moves, of course.
