Weather with Cliff Mass
9:23 am
Fri June 21, 2013

Why summer solstice doesn't really mean sunshine, warmth in Seattle

It may be summer solstice, but summer isn’t quite here yet, says Cliff Mass, professor of Atmospheric Sciences at the University of Washington and KPLU's weather expert.

The reason, says Mass: “Summer solstice … is astronomical summer. It is not necessarily meteorological summer,” said Mass.”

“And in the profession, we all joke—and it’s not much of a joke—that summer starts here in Seattle on July 12. And in fact, there’s a lot of truth to that,” he said.

As a result, the arrival of the solstice usually doesn’t mean the arrival of sunshine. In fact, even the Fourth of July is often cloudy.

“But then something magical happens after the July 4 weekend: we get into a rapid drying period,” said Mass.

But why isn’t summer solstice sunny and warm, since the strong sun shines the longest on this day?

“It’s true that the sun is the strongest and out there the longest, so that’s wonderful. But it takes a long time for the atmosphere to warm,” says Mass. “Like anything else, there’s a certain amount of inertia in the system. So even though the sun’s strongest now, the warmest temperatures are not until the end of July, early August.”

The same thing happens with winter solstice, says Mass. The coldest temperatures are usually seen a few weeks after Dec. 21.

Any hope of sunshine for solstice weekend?

It may not be the best weekend we’ve seen around these parts, but we’ll see some sunshine this weekend, says Mass.

Mass says we can expect the clouds and drizzle to taper off Friday, but Saturday is the best bet for sunshine.

“By afternoon, it will be sunny and it will be much warmer with temperatures in the low- to mid-70s,” he said.

Mass predicts Sunday will be cloudy and dry till about dinnertime in the Olympia to Seattle area, as a weather system moves in from the southwest. So if you're doing things on Sunday, "don't go South; go North," said Mass.

“I believe by 1 or 2 o’clock, it will be raining in Portland, and that rain will slowly move up to the north. But I don’t think it will reach Seattle until sometime later in the afternoon. So most of the time on Sunday, Western Washington, from Olympia north, will be dry,” he added.

Come Sunday, we’ll have some sun, at least in the morning, says Mass.  

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The weekly KPLU feature "Weather with Cliff Mass" airs every Friday at 9 a.m. immediately following BirdNote, and twice on Friday afternoons during All Things Considered. The feature is hosted by KPLU Science and Health reporter Keith Seinfeld. 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 a podcast of “Weather with Cliff Mass” shows.