Date: April 30, 2018 Author: Nwo Report
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).
Thanks to: https://nworeport.me