Central North America has the typical climate of a continental interior in mid-latitudes, with hot summers and cold winters (Figure 10.17), yet the weather in winter is subject to marked variability. This is determined by the steep temperature gradient between the Gulf of Mexico and the snow-covered northern plains; also by shifts of the upper wave patterns and jet stream. Cyclonic activity in winter is much more pronounced over central and eastern North America than in Asia, which is dominated by the Siberian anticyclone (see Figure 7.9A). Consequently there is no climatic type with a winter minimum of precipitation in eastern North America.
The general temperature conditions in winter and summer are illustrated in Figure 10.18, showing the frequency with which hourly temperature readings exceed or fall below certain limits. The two chief features of all four maps are (1) the dominance of the meridional temperature gradient, away from coasts, and (2) the continentality of the interior and east compared with the 'maritime' nature of the west coast. On the July maps, additional influences are evident and these are referred to below.
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