Low Down Dog
December 02, 2002
Cirrus clouds helped compose the above late autumn scene, just before the Sun set. These clouds often take on a wispy appearance because there's very little water vapor at the altitudes which they form, usually over about 18,000 feet (5,800 m). At the lower right-center of the photo, notice the faint parahelia or sundog licking the horizon. Hexagonal ice crystals in these cirrus clouds happen to have a preferred orientation, allowing them to act as prisms. Sunlight entering the vertical faces of horizontally aligned plate crystals results in the patches of color. Unlike most other halo phenomena, sundogs can be seen almost any time of day. They're always at the same elevation as the Sun, faithfully following their master across the sky.