Summer solstice, the longest day of the year, is upon us — marked in Seattle by the colorful Fremont Parade (famous, among other things, for body-painted cyclists). Though the sun sets after 9 p.m., it’s not the sunniest part of summer. June in the Pacific Northwest typically means lots of low clouds, our annual "June Gloom."
That’s been tempered lately by lots of iridescent clouds — another feature of the season that is fed by the sun’s position, high up in the sky. KNKX weather expert Cliff Mass expects a little of both this weekend.
‘BEST DAY’ SATURDAY, THEN GLOOM
“A little bit of gloom before it’s over,” Mass said of the weekend. He says expect the low morning clouds to burn off on Friday, ahead of temperatures in the 70s after isolated morning showers.
“The best day of the weekend is Saturday," Mass said. “I think it’ll be mostly dry, there could be a few sprinkles toward the mountains, but pretty much dry — temperatures getting up again around 70, maybe some clouds in the morning.”
He says Sunday will be the worst day, as an upper-level trough of low pressure comes into the region, bringing more persistent low clouds that Mass expects will stick around most of the day.
“There could be a few sprinkles in the lowlands, but there will be almost guaranteed some light rain the mountains on Sunday,” Mass said. “So if you’re going to go on a hike on the western side, you’d better be prepared for a few light showers. If you get to the eastern side, you’ll be OK.”
Mass says Monday will be fairly dry with temperatures in the upper 60s. Then, there will be a transition with another upper-level disturbance bringing in more damp weather, with clouds and showers aloft. He says expect thicker clouds and slightly cooler temperatures Wednesday and Thursday.
“So we’re going to be a little bit up and down, but temperatures are going to stay seasonal and we’re not going to have any heat waves for the next week — that’s pretty much guaranteed,” Mass said.
IRIDESCENCE BRIGHTENS JUNE GLOOM
Although this is all quite typical gloomy weather for June, Mass added, there has been another weather feature many people have been asking him about: colorful iridescent clouds floating in the sky.
“The number one question I have been getting the last several weeks — and the number one picture that has been sent to me during the last month — are these absolutely marvelous, colorful clouds in the sky,” Mass said. “These are called ‘iridescent clouds.'" A popular term for them is "rainbow clouds."
Mass says you probably know just what he is talking about: you see them when the sun travels through very thin clouds and you see a rainbow of colors, generally more in the lighter pastel realm. “They are absolutely extraordinary,” he said.
They’re caused by extremely thin clouds that have either water droplets or ice crystals in them with uniform-sized particles that bend the light, producing the colors.
“This is a phenomenon called ‘defraction,’” Mass said. It’s what happens when light bends around objects. “If you took physics or an earth science class, you may have heard about this.”
He says although the display looks like a rainbow sometimes nestled in the clouds, it’s produced by a completely different scenario. Rainbows are generally seen near the horizon, when the suns shines through precipitation.
“That occurs not with clouds, but with raindrops,” Mass said. “The light enters the raindrop and gets reflected back… and refracted, and bent inside the raindrop. That’s where the colors of the rainbow come from.”
He says these are very different mechanisms: “Refraction that breaks up the colors — versus defraction, that occurs in these iridescent clouds.”
Mass says the long days in the month around the solstice create preferential conditions for the iridescence, because the sun is high in the sky, closer to the clouds where it occurs.
Rainbows are seen less frequently this time of year, because the sun has to be on the opposite side of the sky from the sun.
So, don't look for a rainbow if you're celebrating the solstice this weekend or for that matter, for Pride weekend coming up; look for iridescence, in clouds that are near the sun.
To hear the full conversation, click on the "play" icon at the top of this post.
Weather with Cliff Mass airs at 9:02 a.m. Friday, right after BirdNote, and twice on Friday afternoons during All Things Considered. The feature is hosted by KNKX environment reporter Bellamy Pailthorp. Cliff Mass is a University of Washington professor of atmospheric sciences, a renowned Seattle weather prognosticator, and a popular weather blogger. You can also subscribe to podcasts of Weather with Cliff Mass shows, via iTunes or Google Play.