An unusually pronounced sundog produced by sunlight passing through thin cirrus clouds. The true sun is located outside of the picture to the right.

A sundog (or sun dog) is a relatively common atmospheric optical phenomenon associated with the reflection/refraction of sunlight by the numerous small ice crystals that make up cirrus or cirrostratus clouds. Sundogs typically appear as a bright and sometimes colorful spot in the sky at a position 22 degrees to the left and/or right of the sun. Sundogs are closely related to, and sometimes appear together with, a halo. The scientific name for the phenomenon is parhelion (Plural parhelia). Less common associated phenomena, collectively called ice halos, are the circumzenithal arc, upper tangental arc, parhelic circle, and lower tangental arc. There are 14 other named ice halo phenomena that could be seen given optimal conditions. Retreived from http://www. ... Retreived from http://www. ... The Sun is the star at the center of our Solar system. ... Halo around the sun at the South Pole (NOAA) Halos are optical phenomena that appear near or around the Sun or Moon, and sometimes near other strong light sources such as street lights. ...

The ice crystals responsible are tiny (less than 0.5 micrometers in diameter) and hexagonal. These ice crystals refract the sunlight at an angle of about 158º, resulting in the appearance of sundogs about 22º from the sun. (180º is line of sight; subtract the 158º of refraction and the image appears at 22º.) The amount of refraction is dependant on size of the crystals, so instead of a precise, neat rainbow, you get can stretched pale images. Sundogs are seen in short arcs in a horizontal plane with the sun because the short, hexagonal "plate" ice crystals that make up them predominantly tumble through the sky with their flat axis oriented horizontally. The faint halo seen with the sundogs is from longer, pencil-like crystals that are at the correct orientation for their location to refract light to your eye. A complete half-circle rainbow at Lake Zurich. ...

Because it occurs due to internal reflection in ice crystals, it is produced only by clouds cold enough to consist entirely of ice particles rather than liquid water droplets. Such clouds are usually found only at temperatures below about -20°C. These can be seen frequently during winter cold snaps in North America and Europe, as well as at high altitudes and in the arctic regions. Icicles A natural ice block in Iceland Ice is the solid form of water. ... Cumulus of fair weather A cloud is a visible mass of condensed droplets or ice crystals suspended in the atmosphere above the surface of the Earth or another planetary body. ...

Although often less vivid and more diffuse than the one depicted in the photograph, sundogs are actually rather common; they are often overlooked by amateurs because one must look in the general direction of the bright sun in order to spot them.

In remote stretches of Western Texas, sundog refers colloquially to a segment of a common rainbow. A complete half-circle rainbow at Lake Zurich. ...

External links

Wikimedia Commons has media related to:

Sun dog

• http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/opt/ice/sd.rxml
• http://www.astrophys-assist.com/wilobs/weathwin/sundog.htm
• http://www.sundog.clara.co.uk/atoptics/phenom.htm
• http://homepage.mac.com/stockdog/Pictures/Gallery/SundogIce.jpg

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See also

• Moon dog