Ocean currents. The Pacific ocean currents off the west coast flows from North to South hence cooler water along the coast. Since the ocean is cooler, less moisture makes it to the atmosphere overall. It’s the opposite in the East coast as the Atlantic ocean currents flow from the warm gulf of mexico and the equator up along the coast. In addition, there are relatively few cold fronts (mainly in the Southeast) so warmer air equals more moisture equals more humidity.
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