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