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Continental polar air masses that move down out of the snow-covered regions of northern Canada and Alaska are often in the picture when bitterly cold winter weather visits the United States (see Figure 3-2). Winds of this frigid, dry air blows over the northern plains and through the Midwest and the Northeast and occasionally reaches as far south as Texas and Florida. Out west, the Rockies, Sierra Nevada, and Cascade Mountains usually protect the Pacific Coast. As these air masses move southward, their extreme cold tends to modify, protecting the southern states from the most extreme cold. The barrier of the Appalachians sometimes protects the cities of the East Coast from the worst of it.

Maritime polar air masses regularly sweep over the West Coast from out of the northern Pacific Ocean, bringing cool, moist air that dumps heavy snow in the western mountains. By the time this air crosses east of the Rockies, it is relatively dry. It warms as it blows down the eastern slopes of the mountains and sometimes brings fair weather and warming temperatures to the plains. Maritime polar air also originates in the North Atlantic and occasionally sweeps southwestward into New England and the mid-Atlantic states, bringing rain and snow.

FIGURE 3-2: The map shows the different air masses that affect weather in the continental United States and helps explain why the nation gets so much dramatic weather.