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Europe's particular distribution of land and sea-which includes several major inland seas such as the Mediterranean, the Baltic, and the Black Sea-and its long coastline facing the eastern North Atlantic ocean are shaping factors of the numerous regional climates of the continent. The presence of numerous regions of high mountains, which act as a physical barrier to atmospheric flows, is responsible for substantial regional differences in precipitation patterns.
Although much of Europe lies in the northern latitudes, the relatively warm seas that border the continent give most of central and western Europe a temperate climate, with mild winters and summers. European climate is determined essentially by the interactions among three pressure centers: the Icelandic Low, the Azores High, and continental highs (which predominate in winter) and lows (which generally are confined to summer months). Indeed, in recent years, there has been increasing interest in the relative strengths of these systems, as well as their persistence. This is exemplified particularly by the North Atlantic Oscillation index (e.g., Hurrell and van Loon, 1997), which is a measure of the strength of atmospheric flows over the North Atlantic and their links to temperature and precipitation patterns over Europe. The prevailing westerly winds, warmed in part by their passage over the North Atlantic ocean currents (the Gulf Stream), bring precipitation throughout most of the year. The strength of these winds varies, partly in response to the North Atlantic Oscillation. In the Mediterranean area (i.e., Spain, southern France, Italy, southern Croatia, Montenegro, Macedonia, Albania, and Greece), the summer months usually are hot and dry; almost all rainfall in this area occurs in the winter. From central Poland eastward, the moderating effects of the seas are reduced; consequently, drier conditions prevail, accompanied by a greater amplitude of annual variation of temperatures (i.e., hot summers and cold winters). NorthWestern Europe is characterized by relatively mild winters, with abundant precipitation along the Scottish and Norwegian coasts and mountains, and much colder winters and generally drier conditions in Sweden and Finland. In mountain regions such as the Alps, winters generally are cool, and snow remains on the ground for several months of the year; summers typically are cool and moist. The mountains intercept precipitation driven by frontal systems and can trigger convective rainfall (summer thunderstorms) in the absence of major synoptic disturbances.
Table 5-1 provides some insight into the variety of European climates-which are determined not only by latitude or altitude but by proximity to the ocean or to one of the numerous inland seas. Annual temperature ranges vary from some 10�C in coastal regions of the United Kingdom and Ireland to about 30�C in Finland and Russia. Annual precipitation totals range from as low as 200 mm in southern Spain and Greece to more than 2,000 mm in coastal regions of Scotland and Norway and at some locations in the Alps.
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