A colourful splash in the sky known as a “fire rainbow” has been spotted in the United States, raising the question of whether the phenomenon could occur in our own backyard.
Circumhorizon arcs, a phenomenon that sees wispy rainbow-coloured shapes appear in the sky, have been lighting up social media after a runner spotted one over a national park near San Francisco, California.
The “fire rainbows” are formed when the ice crystals that make up wispy cirrus clouds catch sunlight or moonlight at a certain angle.
Meteorologist and ABC weather presenter Nate Byrne says they can happen anywhere if you have all the right ingredients.
“When it comes to the when/where question, it’s really a matter of chance,” Ms Byrne said.
“Latitude matters and the time of year depending on that latitude matters.
“You’re generally most likely to see it when the sun is high in the sky – so summer is the most likely time of year you’ll see it.”
A number of conditions in the atmosphere must culminate for the condition to occur:
- The sun or moon must be more than 58 degrees above the horizon
- Cirrus clouds (the thin, wispy ones) must be present
- Ice crystals that make up cirrus clouds must be faced parallel to the ground
If the stars align (so to speak) and the arc of light passes through the ice crystals at the right angle, the cirrus clouds will light up in a rainbow of colour parallel to the horizon.
“When the sun hits the ice crystals, you get pretty colours,” Roger Gass, a meteorologist with the US National Weather Service, told local news site SFGATE.
“It’s an optical illusion.”
And the closer you are to the equator the more likely it is.
Because the sun needs to be so high in the sky for the “fire rainbow” to form, places more than 58 degrees north or south from the equator are unlikely to see it occur.
“In the Northern Territory you’re pretty well positioned because you do get quite a few thunderstorms and cirrus clouds occur often after those storms,” Ms Byrne said.
“The only reason we’re not seeing more in Australia is because there are fewer eyes looking up to the sky.”