WHEN IS A RAINBOW NOT A RAINBOW? At this time of year, a colorful arc often appears in the noontime sky. It looks like a rainbow, but it’s not. “I saw one on June 23rd while I was walking through forest near Flagstaff, Arizona,” reports David Blanchard, who took this picture through the treetops:
Blanchard witnessed a circumhorizon arc–a rainbow-colored band of light caused by the sun shining through plate-shaped ice crystals floating in cirrus clouds. Actual rainbows, on the other hand, are caused by sunlight reflected from raindrops.
Summer is the season for circumhorizontal arcs because they appear only when the sun is high in the sky–more than 58o above the horizon. The arc’s enormous size and pure spectral colors make it one of the most beautiful of all ice halos.
At medium latitudes, like much of the USA, the arc is not rare.Typically, it can be seen several times each summer. In contrast, further north in much of Europe the circumhorizon arc is a rarity and impossible to see north of Copenhagen. See the charts in Les Cowley’s web page ‘How rare?’ for the visibility at your location.