Elements align for unusual sight

A rainbowlike weather phenomenon seen over Spokane and parts of North Idaho on Saturday was a rare meteorological event that involves sunlight passing through precisely aligned ice crystals in high-elevation clouds.

The phenomenon is known as a “circumhorizon arc,” one of some 15 types of ice halos that can occur when sunlight passes through ice clouds.

The circumhorizon arc is different from the more commonly seen “sundog.”

“It only forms under a very rare situation,” said Jonathan Fox, meteorologist for the National Weather Service in Spokane, who spotted the colorful cloud formation on Saturday.

“I had never seen one.”

Fox said ice crystals in clouds at elevations of 20,000 feet to 25,000 feet must be aligned horizontally, which in itself is unusual. Then the sun must be at least 58 degrees above the horizon. On Saturday, the sun angle was 64 degrees. The display was visible from about noon to 1 p.m., Fox said.

During a circumhorizon arc, sunlight enters one of the facets of aligned crystals and then is refracted at a 90-degree angle out of another facet. The crystals must be aligned at a perpendicular angle to the sun. The refraction creates a prism of light that is similar to, but can be purer than, the colors of a rainbow.

Also, the appropriate high-elevation clouds are fairly low to the horizon from the vantage point of the viewer.

Fox said the colors normally are visible only across a narrow band of the ground in which the angles are just right to see the refracted light.