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The Rain Shadow Effect in the Pacific Northwest

The Pacific Northwest can be roughly divided into two strikingly different landscapes: wet and dry. Or call them green and brown. It’s damp and green to the west of the Cascades and relatively arid and brown to the east.

The PNW is famous for its rain, but what about the dry side? Why is there a desert over there?

It’s because of the rain shadow effect. Many dry places around the world exist because of this phenomenon. Here are the basics of how the rain shadow effect works in Washington and Oregon:

Rain Shadow Effect in the Pacific Northwest

The prevailing wind direction is shown as an orange arrow in the illustration above. Winds tend to blow from west to east, from the Pacific Ocean over the the Coast Range and the Olympic Mountains, across the lowland valleys, and finally over the high peaks of the Cascade Range.

1.

Winds carry wet air in from the Pacific and push it up against the coastal mountains. The air is forced up to higher altitudes, where the atmospheric pressure is lower. The air expands and cools and can’t hold as much water vapor. Tiny water droplets coalesce and clouds form. If there is a lot of moisture in the air, and it gets cool enough, the droplets become rain drops and they fall to Earth.

The amazing temperate rainforests blanketing the west sides of the Coast Range and Olympic Mountains exist because of this effect.

2.

The air warms up again as it continues on down the east side of the coastal mountains and into the Puget Lowland or Willamette Valley.  The air gets warmer because it is under higher pressure at the lower altitudes. Warm air can hold more water vapor than cold air. So even if there is still moisture in the air as it descends into the valleys, the moisture is less likely to condense into rain– it stays “trapped” in the warm air.

This is why the valleys are a bit drier and warmer than the coast– they are in the rain shadow that is cast by the coastal mountains.

The Olympic Mountains block so much rain that the region around the town of Sequim on the northeast coast of the Olympic Peninsula is practically a desert. Cactuses and oak trees grow there in the rain shadow of the Olympics.

The rain shadow of the Coast Range in Oregon (and their Washington counterparts, the Willapa Hills) is not as strong as those cast by the Olympic Mountains or the Cascades. The smaller the mountains, the weaker the rain shadow effect, and vice versa.

3.

The process is repeated when winds carry moisture-laden air eastwards to the Cascades. The air is forced upwards, it cools, clouds form, and water falls as rain. Or snow, if it’s cold enough.

The Cascades are a formidable barrier, like the Olympics, and so much rain is dropped from air hitting their western flanks that very little moisture is left when the air reaches the other side.

4.

When the air falls to lower altitudes on the east side of the Cascades it warms up, which means it isn’t going to release much of whatever moisture it still has.

The city of Bend, just east of the Cascades in Oregon, averages about 300 sunny days a year because it sits right in the rain shadow of the Cascades.

Satellite image of Oregon rain shadow (NASA)

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4 comments… add one

  • Michael Kauffmann June 20, 2011, 8:16 PM

    Ivan- great post. The amazing part about the rain shadow, when coupled with localized soils, topography, etc (cumulatively known as phytogeography) is how it all comes together to affect plant distributions. Northwest California is close to my heart so I have attempted to interpret how these factors create dynamic conifer distribution at the southern tip of the Pacific Northwest and northern tip of the California Floristic Provence:
    http://conifercountry.com/nwCalifornia/phytogeography.html

    -Michael

    • Ivan Phillipsen June 22, 2011, 5:00 PM

      Thanks, Michael. The graphic you linked to is awesome! I assume that is going to be in your book?

      Phytogeography (that was a new word for me when I first saw it on your website) is really cool, indeed. The stories of why plants live where they live is a lot more complicated and interesting than, “They grow where it rains.” This is something I want to learn more about.

  • Kelsey March 28, 2012, 10:20 PM

    Hi Ivan,

    It seems that you know a ton about this region and it weather. Maybe you can help me understand more about the prevailing wind patterns here. You see, there is a special bicycle route I’d like to travel this summer called the Sierra Cascades Bicycle Route (http://www.ziligy.com/googleMaps/displayGPX.html?gpx=SierraCascades.gpx). But the problem is that I don’t know which way to ride in order to experience the least amount of head wind.

    Could you help me understand where the winds will be moving more from? The northwest or southwest? Why?

    • Ivan Phillipsen April 6, 2012, 7:11 PM

      Hi Kelsey,

      Thanks for the question and sorry it has taken me awhile to get back to you.

      From what I understand, the prevailing winds will be coming from the northwest in the summer. There is a high pressure zone that builds up in the eastern Pacific during the summer and the jet-stream is pushed to the north.

      I hope this helps you a bit. Sounds like a great adventure– have fun!

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