The North American continent spans nearly 60° of latitude and, not surprisingly, exhibits a wide range of climatic conditions. Unlike Europe, the West Coast is backed by the Pacific Coast Ranges rising to over 2750 m, which lie across the path of the mid-latitude westerlies and prevent the extension of maritime influences inland. In the interior of the continent, there are no significant obstructions to air movement and the absence of any east-west barrier allows airmasses from the Arctic or the Gulf of Mexico to sweep across the interior lowlands, causing wide extremes of weather and climate. Maritime influences in eastern North America are greatly limited by the fact that the prevailing winds are westerly, so that the temperature regime is continental. Nevertheless, the Gulf of Mexico is a major source of moisture supply for precipitation over the eastern half of the United States and, as a result, the precipitation regimes differ from those in East Asia.
We look first at the characteristics of the atmospheric circulation over the continent.
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