On Wednesday, Pinnacles National Park had one extraordinary moment: a “fire rainbow” appeared in the sky. NBC Bay Area tweeted a photo of the fiery vision:
Fire rainbows, AKA circumhorizontal arcs, occur when the sun is over 58 degrees above the horizon. According to Atmospheric Optics, when the sun reaches that height, the arc can form. Atmosphere Optics writes, “In the USA you might see a circumhorizon arc (CHA) five or more times each summer. In middle latitude Europe you will be lucky to see a CHA once. In northern Europe they are impossible to see at any latitude north of Copenhagen. In comparison, a Parry arc is seen perhaps once a year. CHAs are common in the USA, rare in middle to north Europe.”
As Phil Plait explained in Slate:
A circumhorizon arc (or sometimes circumhorizontal arc) is different from a rainbow, which forms when water droplets in the air reflect and refract (bend) back to you. For a circumhorizon arc to form, you need flat ice crystals that are like hexagonal plates. As they fall, they tend to orient themselves horizontally, flat-side toward the ground. Light enters one of the six, short vertical sides; bends (in the same way a prism bends light—click that link for a great diagram of this); then leaves the crystal through the flat side facing the ground. This breaks the white sunlight into its colors, which are oriented parallel to the horizon (unlike a rainbow, which is curved).