The map Bern shared is a good graphic for understanding the role of high mountain ranges and maritime wind effects on North America. Right now I'm in the blue-to-green zone at around 20 F in western Oregon, and would not be that cold were it not for the presence of the Columbia River Gorge, cutting the Cascade Range, through which continental winds bring deep, sudden cold. The Pacific coastline is about 120 miles west of us, with its mild winter temperatures in spite of the offshore cold Japan Current. The arctic air mass so prominent on the map is able to extend all the way to the Atlantic coast because there are no continuous high mountains east of the Rocky Mountain range.