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Urban Deer Populations
Sun Dogs (parahelia) flank the sun as it rises near UWGB Friday, February 7, 2003
If you were up early last Friday you probably noticed the rainbow like halo that encircled the sun at about 22 degrees that seemed to explode into light as it reached the ground. This weather related phenomena are parahelia (mock suns) or sun dogs. Under certain conditions in the winter, flat hexagonal crystals of ice collect in the atmosphere. Any light from the sun or moon will pass through the crystals and refract light similar to the way a prism does creating a rainbow-like halo of light. Because of the hexagon shape of the crystal they can only deflect light at angles between 22 and 50 degrees. The largest amount of light is deflected at angles near 22 degrees so the halo appears around the sun or moon at that angle.
If the crystals mostly fall with their flat surfaces parallel to the earth the results are a special halo called a parahlion or a sun dog. The sun has to be low on the horizon and both the viewer and ice crystals must be on the same horizontal plane as the sun. That is the crystals themselves must be close to the earth (sometimes called diamond dust). This usually only occurs at very cold temperatures. As can be seen in the photo parahelia are usually red closest to the sun and blue farthest away.
Sun dogs are visible during cold still weather a couple times each year, but only as the sun is rising or setting.
© 2001-2004 The Cofrin Center
for Biodiversity and the University of Wisconsin Green Bay,
All Rights Reserved
Last
updated on
May 9, 2014