June 21 not only brings the start of summer but it also brings the peak of the "fire rainbow" season, as evidenced lately by three separate sightings of the brilliant and colorful displays around the Puget Sound region these past few days.
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.
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 21, when the sun reaches its highest angle in the sky and the fire rainbow sighting window expands to between 12:30 p.m. and 3:50 p.m. Seattle's window time shrinks a little each day now, eventually ending 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, while 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.)