A Neighborly CHA Above Norfolk, Virginia

June 16, 2013

CHA Modified

: John Adam; John’s Web site
Summary Author
: John Adam

On Saturday May 25, just before 2 p.m., I walked across the street and startled a neighbor by asking, “Have you noticed that?” Clearly, she hadn’t, but as she wearily put down the book she was reading, it became obvious that she had never seen ‘one of those’ before! I was referring to a circumhorizon arc (CHA). CHAs are only seen when the Sun is high up in the sky, at least 58 degrees above the horizon. This limits their visibility to near solar noon in late spring and summer in the mid-latitudes. Unlike circumzenithal arcs (or CZAs), that are low sun, high arc displays, these are high sun, low arc displays, and thus are more readily noticed by casual observers. In order for CHAs to form, sunlight must enter the vertical side faces of ice crystals, such as those typically composing cirrus clouds, and exit through their lower horizontal basal faces. Several hours before spotting this colorful arc, there was a bright 22-degree halo around the sun that lasted nearly two hours.

Photo details: Camera Maker: OLYMPUS IMAGING CORP.; Camera Model: SP570UZ; Focal Length: 18.2mm; Aperture: f/5.6; Exposure Time: 0.0013 s (1/800); ISO equiv: 64; Software: OLYMPUS Master 2.04W.