By Christine Clarridge / The Seattle Times
Although Seattle sees the occasional deluge, more often what shows up could be called drizzle or mist. The drops are smaller and more diffuse. It’s part of the reason we notoriously eschew umbrellas: We may bust them out to protect against heavy rain like we’ve gotten this week, but they’re no use when what’s outside is less a downpour than an omnipresent wetness.
But why is the rain around Seattle so unique? Why does it fall where it does, in the way it does?
When we recently asked Seattle Times readers what questions you have about Pacific Northwest weather, your curiosity about rain was overwhelming.
Some wondered why some parts of the Puget Sound region get less rain than others (“Is it my imagination?”), or why the rain here is different than on the East Coast. Others got specific: “Why are raindrops in Seattle usually so tiny? If I’m outside for just a few minutes, they don’t even penetrate my hair to wet my scalp.”
The answers to all these questions start at the Olympic Mountains, which are responsible for the convergence zone, the rain shadow and even the size of our raindrops, said Logan Johnson, the meteorologist in charge at the National Weather Service in Seattle.
In places like Quileute or Forks, currents of weather sweep in from the Pacific Ocean unimpeded, battering the Western coast of Washington with wind and dumping an average of about 100 inches of rain each year.
But when those currents travel farther inland and hit the Olympics, the slope of the mountains forces the air to rise.
“That wrings out the moisture,” Johnson explained in a recent interview.
It’s why we have a rainforest on the southwest side of of the Olympics, he said, and a rain shadow to the north and east of the range in places like Sequim, where it can be sunny and dry while every other Puget Sound locale is wet.
Rain shadows are common where coastal areas and mountains coexist. We even have more than one here: The Cascade mountain range blocks the inland flow from the ocean, keeping areas west of the mountains wet and Eastern Washington dry.
More unique to this area is our convergence zone, which is created by the way wind travels around the two mountain ranges, Johnson said.
Again, think about the storms coming in from the ocean.
As the Olympic mountains guide currents of air upward, forcing moisture out, they also split streams of weather in two. One current heads north around the north end of the range and then whips east through the Strait of Juan de Fuca and curves back south toward Seattle. The other travels around the range’s south side, through the Chehalis Gap, and is then forced north by the topography of the Cascade Mountains.
When those two currents collide, or converge, it forces air to rise again, Johnson said. That creates what’s known as the Puget Sound Convergence Zone, which can occur almost anywhere between Skagit and Pierce counties but usually happens near Everett or the King-Snohomish county line.
In the convergence zone, you’re more likely to experience bands of heavy rain, strong winds, thunder, hail and snow.
“A lot of our thunderstorms do occur in that region,” Johnson said.
As for the size of the raindrops? You can blame the Olympics for that, too.
The coast gets two kinds of rain: stratiform and convective.
Stratiform precipitation is so named because, from the air, it would look like a sheet or a layer of billions of tiny raindrops that “just fall out of the clouds,” Johnson said.
Convection precipitation is the intense kind formed in more billowy and puffy-looking clouds, Johnson said. It occurs when air from the warmer surface of the Earth rises, sometimes to great heights. Because the force of the wind is pushing upward, tiny raindrops are not heavy enough to break through on their own, but they eventually clump together, getting “bigger and bigger and heavier and heavier” until finally they do fall, he explained.
Stratiform rain tends to fall evenly across a region, while convection rain can fall heavily on one block but be barely felt just blocks away, said Dustin Guy, another weather service meteorologist.
The two types can sometimes be mixed together, Johnson said, but convection rain usually doesn’t make its way inland across the mountains. That leaves Seattle with mostly stratiform precipitation — the drizzle or mist that sprinkles gently across your hair but leaves your scalp unscathed.
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