Curious Sky Colors
March 25, 2004
This curious photo was captured on July 25, 2003 above Manzanita, Oregon. The lovely colors and their position in the sky hint that this is probably a portion of a circumhorizontal arc. This is also revealed by the altocumulus cloud along the top of photo, which is likely glaciated (transformation of cloud water droplets to ice crystals) and the distended cirrus cloud, actually displaying the colors. A circumhorizontal arc is a halo phenomenon -- formed by ice crystals usually in cirrus type clouds. It's doubtful if there's a true spiral or vortex in this cloud, rather the corkscrew shape may result from wind shear in the region of the cirrus. The circumhorizontal arc can be nearly as bright as the more common circumzenithal arc, but it tends to be a little more washed out because it's always seen at a greater distance (zenith is closer than the horizon), and thus there's more intervening air light. They're usually only observed during mid-day in the late spring and summer months (in the mid latitudes) when the Sun attains an elevation greater than about 58 degrees above the horizon. See also the Earth Science Picture of the Day for June 3, 2003.