Circumhorizontal Arc and 22 Degree Halo Above Topanga, California

October 27, 2010


: David K. Lynch; Dave's Website
Summary Author: David K. Lynch

As monsoon moisture blew from the Gulf of Mexico, the gathering cirrus clouds put on a colorful light show. Here a brilliant circumhorizonal arc (CHA) underscored a 22 degree halo. Both were narrow, bright, and easily seen. Since both halos were present, there must have been two populations of ice crystals. The 22 degree halo is caused by refraction and dispersion of sunlight through randomly oriented hexagonal prisms, while the CHA is produced when the same process occurs in prisms that are aerodynamically oriented with a hexagonal side facing upwards. At the time of the sighting, the Sun’s elevation was 67 degrees, essentially ideal to form the CHA. This marvelous display only lasted about ten minutes, as the cirrus clouds changed their position and shape the solar elevation decreased. I was lucky to get this shot because at the time the Sun was transiting -- reaching its highest elevation in the sky, on this late August day. So remember, to get shots like this check the sky often, protect your eyes and always have your camera with you. Photo taken on August 21, 2009.

Photo details: Camera Maker: PENTAX; Camera Model: PENTAX K200D; Focal Length: 18.0mm (35mm equivalent: 27mm); Aperture: f/9.5; Exposure Time: 0.0029 s (1/350); ISO equiv: 100; Exposure Bias: none; Metering Mode: Matrix; Exposure: Portrait Mode; White Balance: Auto; Flash Fired: No; Color Space: sRGB.