The rain shadow effect leads to wet conditions on the west side of the mountains and dry conditions on the east side.

The rain shadow effect occurs when moist air from the Pacific Ocean is blocked by mountain ranges, such as the Cascade Range in Washington and Oregon. As the air rises to pass over the mountains, it cools, causing moisture to condense and fall as precipitation on the windward (west) side of the mountains. This results in wet conditions and often creates temperate rainforests, especially in regions like western Washington. On the leeward (east) side, the air descends and warms, causing the air to lose moisture, creating dry conditions. This phenomenon is responsible for areas like Eastern Washington and Oregon having arid climates despite being close to the ocean.