Circumhorizon Arc Above Post Falls, Idaho

July 26, 2013

IdahoCHAIMG_1412 (3)

Photographers: Terry Honodel
Summary Authors: Scott Honodel; Jim Foster

My oldest daughter, Ashley, noticed this odd but beautiful "rainbow” from the backyard of our house in Post Falls, Idaho. She was facing toward the southern sky shortly after noon. The time of her observation and the direction she was looking rules out this gorgeously colored strip being a rainbow or anything related to it. Rather this is a circumhorizon arc (CHA) -- a halo phenomenon. Ice crystals in cirrus clouds and not raindrops are responsible for the vivid colors. The only time you're able to see a CHA is when the Sun is higher than about 57.8 degrees above the horizon. Thus, they're confined to the midday hours of late spring and summer but only for latitudes less than about 56 degrees. Residents in most of Alaska and Scandinavia, for instance, can never see these gems. Photo taken on June 3, 2013.

Photo details: Camera Model: Canon EOS DIGITAL REBEL XT; Focal Length: 55mm; Aperture: f/18.0; Exposure Time: 0.0020 s (1/500); ISO equiv: 320; Software: Picasa.