When moving in the western direction, the moist and warm pacific air cause rain and snowfall in the windward side. In the east side of the mountain, however, the air loses the capacity to cause rainfall since it compresses and thin out the clouds.
Rainshadow in the eastern side is evident by the extreme dryness in the south-central interior of the valley in British Columbia. The area experiences a variable rainfall with the highest point on the windward side receiving 1200 mm - 2000 mm annual rainfall while the driest west side receives 500 mm.
