Port Townsend Circumhorizontal Arc

Provided and Copyright by: Carol Steinel
Summary Author and Editor: Carol Steinel, Jim Foster

This stunning circumhorizontal arc was observed over Port Townsend, Washington on June 26, 2008. It was first noticed at about 12:44 p.m. and was visible for a full 15 minutes. Circumhorizontal arcs are only seen when the Sun is high in the sky, at least 58 degrees above the horizon, near solar noon in summer in the mid-latitudes. Because these colorful arcs are found approximately 45 degrees below the position of the Sun, they’re easier to see than many other arcs, which take shape higher in the sky and are located closer to the Sun’s disk. In order for circumhorizontal arcs 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. After this breath-taking arc faded, a 22 degree solar halo took shape.