The western half of the state is close to the ocean with warm ocean currents, so you get a lot of moisture. However the Cascade Mountains split the state N and S. Windward side is Seattle side, so all the rain stays there.

Eastern Washington then is in the rain shadow. Actually I think it's high desert - there's very little annual cloud cover.

The Cascades go all the way to California so this pattern applies to Washington, Oregon, and maybe Nevada.

In the context of my original comment, I can only speak from an experiential sense. I think it's the strength or frequency of the sunlight, maybe the UV, is what makes the difference.