An observant second-grade class at Westwood Elementary School spotted a rare meteorological phenomenon last week during recess.
When students of Mrs. Veronica Starr’s class went to the playground at about 12:45 p.m. on April 25, they noticed a brilliant, multicolored cloud formation, called a circumhorizon arc or more commonly a fire rainbow, drifting high across the midday sky.
“Everything I’ve read so far [describes what we saw] as a fire rainbow,” Starr said.
The teacher, a self-described weather geek, said that her students came to her saying “look a rainbow.”
She knew that because it was the bright, clear day what they were seeing couldn’t be a normal rainbow.
When she looked at the sky, Starr knew the display was something unusual, so she snapped a few pictures with her phone.
The next day she retuned to class, research in hand.
“We had witnessed something that we may never see ever again.”
The National Weather Service describes circumhorizon arcs as differing from rainbows, “which appear in the sky as an arch after it rains. [The circumhorizon arcs] appear in more of a gentle curve, during fair weather, when the sun is at a very high angle.”
How it works is sometimes, when conditions are just right, sunlight hits ice crystals in cirrus clouds in the upper atmosphere at just the right angle and projects a vivid, rainbow-like patch of color on the insides the cloud.
In a normal rainbow, sunlight hits an asymmetrical raindrop, it refracts into the “ROY G BIV” colors of the visible spectrum. But when sunlight hits the ice crystals in a thin cirrus cloud at the right angle, the light sort spatters with the individual colors going in different directions coloring the cloud with the spectrum.
If the sun is too high or the cloud is too dark and blots out the light, nothing will happen.
Circumhorizon arcs are only visible at certain latitudes and only at the right times of the year.
According to the NWS, at our latitude, the sun is only high enough from April 1 to Sept. 15 and then only at middle part of the day.
“Even on the summer solstice, the sun is above 58 degrees for only about 4 hours and 30 minutes.”
It adds that farther north the phenomenon is less common because the sun is at the necessary angle for less time.
Plus, precisely the right type of cloud formation must be present in the right part of the sky for the proper sunlight refraction to take place.
“The clouds must be composed of plate-like ice crystals that are oriented such that sunlight enters the nearly vertical edge of the crystal, refracts through it at an angle of about 46 degrees (as in a prism), and exits the relatively flat bottom face. Only certain types of cirrus clouds have the right composition of plate-like ice crystals.”
Starr said that she wished she had realized at the time how rare these phenomena are so she could have better savored the moment.