The residents of Singapore were treated to an iridescent spectacle on Monday as a rainbow-like wisp hovered in the sky. The phenomenon lasted for about 15 minutes, leaving plenty of time for people to take to social media to compare the sight to an airborne Paddle Pop. Experts believe the display was very likely a “fire rainbow” — but don’t be fooled by this misnomer. Known among scientists as a circumhorizontal arc, the multi-colored glow isn’t a rainbow and has absolutely nothing to do with fires.
A circumhorizontal arc emerges when sunlight passes through a cloud full of ice crystals. As the sunlight hits the crystals, it bends, separating out of the cloud into a prism of color. The clouds that make this sight possible must be cirrus clouds, which are thin and wispy puffs of vapor living at high altitudes.
“It started as a small orange circle and then grew bigger and bigger till all the colors came out,” Singaporean resident Fazidah Mokhtar told the BBC. “It lasted for about 15 minutes and it slowly went off.”
Other residents took to social media to share the sight:
Fire rainbows don’t appear often because circumhorizontal arcs don’t show up every time light passes through an icy cirrus cloud. The conditions they require are very specific, and very rare. The most important factor is latitude: For the optical phenomenon to happen, the sun must be higher than 58 degrees above the horizon. Equally important is the nature of the ice crystals themselves: if the ice isn’t shaped like a thick plate lying parallel to the ground, then the light won’t refract and separate horizontally to create the rainbow-like prism. In situations like the one in Singapore, the patchiness of the clouds means that people will see an oblong shape instead of a full arc.
If you’re looking to catch a fire rainbow in the United States, you’ll need to time it by month, time of day, and location. While people in Houston, Texas, for example, have a chance at seeing one as early as March, the best shot for most is looking up at the sky in mid-afternoon sometime between June and July.