Answer: It occurs because of the rain shadow effect
Explanation: Air expands and cools when air mass moves from a low elevation to a high elevation. Cool air can hardly hold moisture, and this leads to cloud formation. As the cool air rises up a mountain, it leads to rainfall.
After the air mass crosses over the peak of the mountain and starts down the other side, the air warms up and the clouds dissipate, which in turn leads to less rainfall and snowfall.
