Spokane's location, between the Cascades Range to the west and the Rocky Mountains to the east and north, protects it from weather patterns experienced in other parts of the Pacific Northwest. The Cascade Mountains form a barrier to the eastward flow of moist and relatively mild air from the Pacific Ocean in winter and cool air in summer. As a result of the rain shadow effect of the Cascades, the Spokane area has average annual precipitation, less than half of Seattle's . Precipitation peaks in December, and summer is the driest time of the year. The Rockies shield Spokane from some of the winter season's coldest air masses traveling southward across Canada.