Summertime brings out the colorful cloud displays over Seattle

Circumhorizontal Arc spotted over Seattle on June 7, 2021 (Photo courtesy: Long Bach Nguyen)

Summertime may be better known for barbecues, days at the beach and occasionally struggling with the question: "Why don't we have air conditioning?" But it's also the time of year when some clouds can be painted awash in color due to a fairly rare atmospheric event.

As the sun climbs higher in the sky as the calendar approaches the summer solstice, there are some times during the day that with the right cloud alignment, you can get a gorgeous display of a "circumhorizontal arc."

Sky gazers noticed these rainbow clouds over South Seattle on Monday. 

"I saw this rainbow over South Park neighborhood just west of Boeing Field and stopped my car to watch this amazing site," said photographer Long Bach Nguyen.

Circumhorizontal Arc spotted over Seattle on June 7, 2021 (Photo courtesy: Long Bach Nguyen)

The simple explanation is that the clouds are made of tiny ice crystals that refract the sunlight into the colors of the prism. But the colors only come out when the sun is both at least 58 degrees above the horizon and you have these icy clouds in the right spot for the refraction. 

"It is a very large halo and always parallel to the horizon," says the great Atmospheric Optics web site. "Often only fragments are visible where there happen to be cirrus clouds - the individual patches of cirrus are then lit with color."

Photo: David Bertch

Why just a summertime event? At least around Seattle, the sun is only high enough from roughly the start of May through about August 10th.  But as you head farther south, the arc season last longer -- Houston, for example, can see the arcs from around mid March through the end of September.  

Photo: Brian Donegan, Central New York on May 31, 2021.

Best time to spot them is near solar noon -- which remember is around 1 p.m. these days since we're on Daylight Saving Time. The closer we are to the summer solstice, the longer the sun stays above that magic 58 degree altitude and the more chances we have to get the colorful alignment. This week look between noon and about 2:30 p.m. 

By the way, you may also hear them referred to as "fire rainbows" due to their appearance, but the name is a bit of a misnomer as it has nothing to do with fires or the exact process to make a traditional rainbow.


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