Why is the land on the eastern side of the cascade range drier than land on the western side?

The Cascade Range acts as a barrier to moisture-laden air coming from the Pacific Ocean, causing air to rise and release precipitation on the western side. By the time this air descends on the eastern side, it has lost much of its moisture, leading to drier conditions. This rain shadow effect is the main reason for the difference in moisture levels between the two sides of the range.