'Tis the season for brilliant 'fire rainbows'

Circumhorizontal arc taken over Spokane, Wash. on June 3, 2006. (Photo: Ron Glowan)

The first week of May is probably better known around here as the Opening Day of Boating Season but did you also know it's when we kick off the fire rainbow season?

Fire rainbows, or more officially (and more boringly) known as "circumhorizonal arcs" are caused by ice crystals in the thin, distant clouds being at just the correct angle to refract the sunlight into the colors of the prism.

Ron Glowen, now of Arlington, Wash., just sent me these photos that were taken in June of 2006 while visiting his hometown of Spokane.

"We were returning to our motel around mid-day when my wife said she was seeing a gold glint in the sky through the car window. (We thought maybe it was the tinting,)" Glowen said. "We got out of the car--and saw an absolutely stunning, brilliantly multi-hued band of wispy color amid the cumulus clouds. I grabbed my then-new camera and took some shots. Came outside again--it was still there. This must have lasted for up to an hour before it faded away."

He said he looked around the Internet and discovered it was the circumhorizontal arc. "The web site Atmospheric Optics calls it the "Great Spokane Arc" that was probably seen by more people than any other. I felt lucky to have been there to see it (and, it was my home town!)"

These "fire rainbows" (so nicknamed by appearance, not by having anything to do with fire or rainbows) are fairly rare sights in the mid-latitudes, because they can only occur when the sun is 58 degrees or higher above the horizon. For the Pacific Northwest, that pretty much relegates any sightings to around 6 weeks either side of the summer solstice.

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. Seattle's window ends on August 9.

Note that we're lucky because if you go too far north, like north of 55 degrees latitude, you'll never see them as the sun never reaches that required height above the horizon. So those of you in Copenhagen have never seen this before, whose those in Houston can see them from March through October.

So next time it's a sunny day and those wispy cirrus clouds are floating around, keep an eye for these brilliant displays. (And if you see one, I'd love to see a photo! I keep a running photo gallery in my weather FAQ.)