Diamond Dust Brings out the "Dogs"

The greens of summer have disappeared from most of Alaska, as have the vibrant yellows, oranges and reds of autumn, leaving us with a black-and-white world for the remainder of the winter. But our ashen winter environment is occasionally splashed with color when the dogs come out. Sun dogs, that is.

Sun dogs are two colorful bursts of light that appear on either side of the sun. Besides spicing up the skyscape, sun dogs indicate the presence of falling ice crystals, which also produce neat effects such as halos around the sun and moon. The ice crystals falling through the Alaska air, known as "diamond dust," also create pillars of light that extend upward from outdoor lights and occasionally downward from the sun.

Sun dogs form when ice crystals act as a prism, according to the excellent text Meteorology Today. Ice crystals sometimes take on flat, hexagonal shapes, looking like microscopic stop signs.

The six-sided, platelike ice crystals sometimes fall with their flat surfaces parallel to the earth. Sundogs become visible if the crystals fall when the sun is low on the horizon (not an unusual phenomenon in the Alaska winter) and both the viewer and ice crystals are on the same horizontal plane as the sun.

Just as white light bends when it passes through a glass prism, the ice crystals bend sunlight so it separates into different colors. Sun dogs usually feature red closest to the sun and blue on the outside.

Sun dogs also have been called "mock suns." The Greeks dubbed them "parhelia," meaning "with the sun." Jerry Dennis, author of It's Raining Frogs and Fishes, said the origin of the name sun dogs may be in the tail-like ray of white light sometimes sticking out at one end.

Halos surrounding the sun or the moon are indicative of slightly different forms of ice crystals, those within high cirrus clouds about 30,000 feet above the earth. Instead of tiny stop signs, ice crystals sometimes take the form of fresh-from-the-box pencils: still six-sided, but long rather than flat.

Sunlight passing through these crystals is once again bent, forming a halo surrounding the sun or moon at an angle of approximately 22 degrees. Geophysical Institute Professor of Physics Glenn Shaw says the old wives' tale that a ring around the moon signals approaching bad weather is true: high-altitude, thin cirrus clouds usually precede a storm front by one or two days.

A third light show caused by ice crystals is an often-seen pillar of light extending upward from outdoor lights or downward from the sun. While sun dogs and halos are caused by ice crystals bending light, pillars are caused by light reflecting off ice crystals. As either pencil- or stop-sign-shaped crystals fall, they sometimes waft down like falling leaves, allowing light to reflect off their flat undersides. This collective reflection sometimes appears as a spotlight beam above or below any bright light source--the sun, the moon, street lights or even car headlights.

Pillars, halos and sun dogs all serve to liven up the often colorless Alaska winter world. That's worth barking about.