Circumhorizontal arc over Tacoma, WA
© Mark Coates
Circumhorizontal arc spotted in Tacoma, WA on June 9, 2019.
A pretty sight in the clouds this weekend is a sure sign that summer is on the way -- and you don't even need to look at the weather forecast.

Mark Coates snapped these photos of a colorful cloud outside a Tacoma grocery store Sunday afternoon. "I have never seen a more beautiful cloud, it was the colors of the rainbow," Coates said. "The weather was pretty clear, no rain around."

Officially named "circumhorizontal arcs" (or more informally sometimes known as "fire rainbows" - although they have nothing to do with fire...or actual "rain"bows) -- the colors caused by ice crystals in the thin cirrus clouds being at just the correct angle to refract the sunlight into the colors of the prism.

These colorful clouds 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, we're eligible to see these clouds starting 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.

But if it's a sunny day, and it's close to lunch time, and you spot some wispy cirrus clouds about halfway up the horizon, maybe see if you can spot a rare circumhorizontal arc!