What Is The Olympic Rain Shadow?
Sequim's Best Friend
Ever wonder why Sequim residents refer to their area as the "banana belt"? They can thank the Olympic Rain Shadow.
The Olympic Mountains act as a wall that protects the northeastern Olympic Peninsula and San Juan Islands from the bulk of the rain that moves into the Pacific Northwest.
The dominant airflow during rainy days around here is from the southwest. As that air runs into the southwestern face of the Olympics, the mountains push the air upward.
As the air lifts, it condenses and squeezes out its moisture -- think of it as the mountains acting like a sponge soaking up and then squeezing out the rain. That's the reason there are vast rain forests on the southwestern side of the Olympics. They receive over 200 inches of rain a year.
On the flip side, once the air reaches the Olympic Summit, now it's pretty much lost its moisture. As it goes over the top of the mountains and comes down the northeastern slopes, it sinks. And just like rising air condenses, sinking air dries out as it encounters warmer air near the surface. So you already have semi-dry air becoming even drier.
And, of course, Sequim sits on the northeast side of the Olympics, so they are almost always in this dry slot -- although the shadow affects Port Townsend and the San Juan Islands as well.
Here is a visible satellite image of such an event:
Sequim only gets about 18 inches of rain a year. Meanwhile, just 90 miles to the west, Forks receives over 120 inches of rain a year.
Check out this map of state annual rainfall totals. You can see how dry it is around the Sequim area, and how wet it is on the southwest side of the Olympics.
One interesting statistic: Port Angeles receives about 27 inches of rain a year. However, for each mile you go west of that city, you pick up an extra inch of annual rainfall.
See The Rain Shadow in Action
Here are two YouTube videos that actually demonstrate the rain shadow in action from the ground. This first one was one I shot while in Victoria, B.C.. Winds were gusting about 35-45 mph so it's difficult to hear what I am saying, but you can see the clear "blue hole" over Sequim. The second one was taken by KOMO photographer Peter Mongillo from the Keystone (Whidbey Island) ferry dock during another storm:
How about a drive through the rain shadow? My parents took video as they drove to Seattle from Port Angeles during a rain shadow day. Watch as they drive through blazing sunshine in Sequim and eastern Clallam County, only to have the skies darken and then open up as merge onto SR-104 just past Discovery Bay -- a distance of only about 25 miles.
Here is a map of where each location is in the video:
Not Just For Sequim
But the Olympics just don't cast their rain shadow over Sequim. In cases where the wind pattern is more westerly, the shadow will then be over the Seattle Metro area.
That's why Seattle only receives about 37 inches of rain a year -- there are plenty of rainy days where Seattle gets less than others as we get the benefit of the rain shadow. If the Olympics weren't there, Seattle would probably get closer to 50-60 inches of rain a year.
The shadow does an even better job of protecting the greater Everett area -- many times bringing less rain there on our big rainy days than elsewhere.
But Everett's annual rainfall is about equal to Seattle's despite getting more of the shadow's benefits, and that's because of the Puget Sound Convergence Zone. It turns out that Everett gets more rainy days due to the Convergence Zone than Seattle and Bellevue, but on other days when we get our wettest storms, the shadow brings a lighter rain to the convergence zone areas.
More info: The Great Convergence Zone Trade-Off
Cascade Rain Shadow
And of course, almost all of Eastern Washington is in the Cascade Rain Shadow.
The physics are the same -- the Cascades squeeze out most of the moisture and leave very little to make it over to Eastern Washington. That's why it's so dry over there. Meanwhile, rainfall totals pick up once again as you head into the western Cascade foothills.
Great Resource for Olympic Rain Shadow Stats
Looking for more weather data on the rain shadow? Check out David Britton's Olympic Rain Shadow website.
More stories on the Olympic Rain Shadow