Here's why the Pacific Northwest has such a wide divergence of climate: Warm, moist ocean air moving inland cools, turns to fog and releases moisture as it rises over the obstructing mountains. The dry air then has very little precipitation to deposit on the mountains' eastern slopes, leaving that region semi-arid.Thus, within only a few miles, the character of the land changes rapidly from a forested, mild coast to sub-artic plains and bush.