Halos have a long and rich history in religious art, usually symbolizing the presence of someone or something divine. In the physical sciences, the beautiful displays of light are a sign of something more ordinary—the presence of hexagonal, plate-shaped ice crystals that make up cirrus clouds. As gravity pulls the ice crystals downward, their faces become horizontal with the ground and they function as dispersive prisms, breaking sunlight into separate colors and leaving rainbow-like ice crystal halos in the sky.
Sundogs are one of the most common types of ice halo. They occur when light rays enter the side of an ice crystal and leave through another side inclined about 60 degrees to the first. (See Atmospheric Optics for a good diagram that illustrates the process.) Sundogs are most easily seen when the Sun is low in the sky; the halos occurring on either side of it at about 22 degrees. The part of a sundog closest to the Sun always forms a layer of red, while greens and blues form beyond that. Sundogs are visible all over the world and at any time of year, regardless of the temperature at the surface. For more imagery of sundogs and other optical phenomena (such as sun pillars, circumhorizonatal arcs, and parhelic circles), it’s worth checking out the archives of Earth Science Picture of the Day.
In the last few weeks, we’ve had a number of readers send us their photographs of sundogs. The image above was taken by Nina Garcia Jones; the image below comes from Isa DeSil. Thanks for sending the photos our way. To the rest of our readers: keep your interesting photos of atmospheric, meteorological, or geological phenomena coming. We’ll occasionally post the best images on this blog, and we’ll do what we can to help explain the science behind them.