The vast landmasses of North America and Asia, separated by the North Atlantic and North Pacific oceans, exert such a powerful control over pressure conditions in the northern hemisphere that the belted arrangement typical of the southern hemisphere is absent.
Land areas develop high-pressure centres at the same time that winter temperatures fall far below those of adjacent oceans. In summer, land areas develop low-pressure centers, at which season land-surface temperatures rise sharply above temperatures over the adjoining oceans. Ocean areas show centres of pressure opposite to those on the lands, as seen in the January and July isobaric maps.
In winter, pressure contrasts are greater, just as temperature contrasts are greater. Over north central Asia is developed the Siberian high, with pressure average exceeding 1035 mb. Over central North America is a clearly defined, but much less intense, ridge of high pressure, called the Canadian high. Over the oceans are the Aleutian low and the Icelandic low, named after the localities over which they are centered. These two low-pressure areas have much cloudy, stormy weather in winter, whereas the continental highs characteristically have a large proportion of clear, dry days.
In summer, pressure conditions are exactly the opposite of winter conditions. Asia and North America develop lows, but the low in Asia is more intense. It is centered in southern Asia where it is fused with the equatorial low-pressure belt. Over the Atlantic and Pacific oceans are two well-developed cells of the subtropical belt of high pressure, shifted northward of their winter position and considerably expanded. These are termed the Azores (or Bermuda) high and the Hawaiian high respectively.
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