Q: What is a sundog and what
makes it happen?
(Trsitan, 10 years old, Renton, Washington)
A sundog on the right side of a halo around
the sun. The globe blocking the sun sits atop a pedestal marking the South Pole.
Photo courtesy of U.S. National Science Foundation, 11 Jan. 1999
A: A sundog is a rainbow-like spot in a
cirrus cloud. Light shining through ice
crystals in the cloud makes a sundog,
much like light shining through
raindrops makes a rainbow. "They are
reddish on the side facing the sun and
often have bluish-white tails stretching
horizontally away from them," say
David Lynch and William Livingston
in Color and Light in Nature.
Cirrus clouds--those high fleecy white
bands or patches in the sky--are mostly tiny particles of ice. Ice can take on many forms and
shapes. The cloud ice, however, is shaped like hex bathroom tiles or stubby pencils each no
bigger than the tiniest grains of sand. These ice crystals bend light like a prism, disperse its
colors, and cause sundogs.
When the crystals line up like tiles on a table, the light shining through makes sundogs. The
horizontal crystals bend the light 22 degrees, say Lynch and Livingston, as the light enters and
exits the crystal. Light colors fan out from the bending and display as a sundog.
Sundogs are among the most commonly seen sky phenomena, appearing most prominently when
the sun is low.
"They usually appear in pairs two handbreadths on either side of the sun when it rises or sets
behind a very thin veil of high cirrus clouds", says astronomer Richard Teske of the University of
Michigan. Hold your arms straight out to estimate the two handbreadths.
Halo Display lights Pole's sky, USATODAY.com
Watch for halos, pillars and sundogs in Michigan's winter skies, The University Record, 12 Dec
Sundogs and Light Shafts, Article #36, Alaska Science Forum, 16 April 1976