Fire rainbow over Seattle, WA
© Don Clapp
Mother Nature put on a bit of a color show over Downtown Seattle last week with a brilliant display of a rainbow arc.

This particular arc, photographed by Don Clapp, is called a "circumhorizontal arc" or sometimes colloquially as a "fire rainbow" due to its appearance (it has nothing to do with actual fire.)

Instead this is the result of sunlight refraction. The ice crystals in the cloud refract the sunlight provided the crystals are a certain shape and at a favorable angle to the incoming sunlight.

"A small shower had just passed. I was surprised the color swath was so wide," Clapp said. "As it drifted along, the colors were very vividly glowing. When it was over the Space Needle directly, the left side was a neon blue and the right side was tending toward burnt orange. As it moved northward the colors tended to be more on the red side. It was visible for several minutes. Really was pretty spectacular. Not something I remember seeing before and I watch the sky a lot."

These colorful clouds are fairly rare sights in the mid-latitudes, because they can only occur when the sun is at a greater-than-58 degree angle above the horizon. For the Pacific Northwest, that pretty much relegates any sightings to around 6 weeks either side of the summer solstice.

Fire rainbow over Seattle, WA
© Don Clapp
For Seattle, the "fire rainbow" season begins on May 2 when the sun hits 58 degrees at solar noon (about 1 p.m. PDT -- remember we turn our clocks ahead an hour in the summer but the sun doesn't care.) So on that date, you can only see them if the sun and clouds hit the exact correct angle right at 1 p.m. But the window of opportunity grows each day as we get closer to summer solstice on June 20, when the window expands to between 12:30 p.m. and 3:50 p.m.

Clapp said his photo was taken at 2:22 p.m.!

Seattle's window ends on August 9.