We’re celebrating 20 years of Earth Science Picture of the Day during the month of September! Today’s photo features a popular EPOD from the past. Thanks to all of our followers (on the blog, Facebook, Instagram and Twitter) for supporting us. Thanks also to all of you who’ve submitted your photos. We’re most appreciative. This EPOD was originally published January 16, 2008.

Provided and copyright by: Lisa Gonnelli

Summary authors & editors: Jim Foster, Lisa Gonnelli

On Sunday afternoon, July 22, 2007, a cold front was crossing our area after several very humid and hot days. I could tell by the upper atmosphere that it would be a great day to observe halo phenomena. Still, I was surprised when the splendid circumhorizontal arc, shown above, appeared and remained in place for approximately 35 minutes. It was spectacular! The arc was first noticeable about 12:45 p.m. (Eastern Daylight Time). It was very hard to take my eyes off this beautifully colored strip of sky. I followed it with my camera, taking a shot about every three minutes.

In order for a circumhorizontal arc to form, the Sun has to be at least 58 degrees above the horizon. In mid-latitude locations, this means that these lovely arcs are most often observed near noon during the summer months. Sunlight enters the vertical side faces of atmospheric ice crystals and exits through their lower horizontal basal face. At times the circumhorizontal arc can be nearly as brilliant as the more common circumzenithal arc, but it tends to be a bit more washed out because it's always seen at a greater distance (the horizon is further than is the zenith), and thus there's more intervening air light.

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