Those of us who live west of the Cascade mountain range are not strangers to wet weather. While the Northwest as a whole gets a bum rap for being excessively rainy and dreary, the description is apt for the land between the Pacific coast and the Cascades. This region, incidentally, is where most of the large cities are, which is likely why members of the general public picture rain clouds when they think of Oregon or Washington.
The map to the right illustrates my point. The areas of bright blue and purple are those that receive the most rain. The first bright vertical stripe on the left is the coast range of Washington and Oregon. The bright blue to the right of that is the Cascade range, which is so effective at wringing moisture out of the atmosphere that it creates a rain shadow to its east, indicated by the orange and yellow areas in the central portion of the map. These two thick stripes of heavy precipitation are interrupted only briefly by the lowlands of the Puget Sound, Willamette Valley, and other parts of the I5 corridor which, despite their lower relative rainfall, still get more cloudy days than even brooding Portland hipsters care to deal with.
Surprisingly though, it’s not necessarily the quantity of precipitation that gives Portland and Seattle their reputations, but the extended period of time in which it falls or threatens to fall. We have a lot of grey days on the west side of the mountains, but the rain falls intermittently, lightly, and (on some overcast days) not at all.
Annual rainfall in major US cities:
63.96 in. – Mobile
|To the left is a list of major U.S. cities that, on average, receive more annual rainfall than Seattle, Washington or Portland, Oregon. You can see that the list is pretty extensive and that the first west coast city to show is Seattle at number 36.So why the damp reputation? Well, as I mentioned, it’s the number of days of rain that people are responding to, not simply the volume of precipitation, and it’s there that Seattle and Portland lead the pack with an average 155 and 152 days per year(respectively) with at least 0.1 inch of rain. Of large cities in the contiguous states, they rank #1 and 2.To further their soggy reputation, these two Pacific Northwest cities reign supreme in number of days with overcast skies: an average of 226 days per year for Seattle and 222 for Portland. That’s damn near 2/3 of the year spent under a blanket of grey.
Take a look at this full continental U.S. precipitation map:
It’s pretty obvious from the above image that the areas of greatest precipitation are in the Pacific Northwest; Why then is there comparatively little rain in the cities themselves? Well, the cities lie in the valleys and lowlands between the coast range and the Cascades. It’s the orographic effect of the mountains lifting and cooling the air which wrings the moisture out of the lower atmosphere so that, when the air descends on the other side, the relative humidity is significantly lower.
As moisture-laden winds from over the Pacific Ocean reach the coast, the coast range mountains begin the first stage of dehumidification. The air is forced upward to cool and the peaks along the coast get drenched (especially the Olympic range which ascends quickly from the coast to great heights). In the valleys, we are spared much of this hard rain as our warmer climate increases the ability of the air to hold water. By the time the air is making a second pass over the high Cascades, it has already lost much of its original moisture, but that which remains rains or (more likely) snows onto the mountain peaks. Behind the Cascades, the rain shadow spreads far to the east as the air has been thoroughly sapped of it’s moisture.
So there you have it: a quick an easy explanation for why the Pacific Northwest has the reputation it does. Those of us who live here year round learn to deal with the cloudiness by taking vitamin D supplements (likely placebos) and by just learning to enjoy the weather or ignore it. There is usually a significant increase in whininess from the populace in the late spring, though, as we’re all anxious for the start of our sun-blessed summers of unparalleled beauty. Slogging through the eight months of wet weather makes the summer and early-fall all the more joyous.