Shifting attention now toward higher latitudes, and using the information about air masses, fronts, and cyclones, we might expect to find that western coasts in the belt of cyclonic storms would receive ample rainfall from maritime polar air masses and would have rather moderate temperature variations because of the proximity to the oceans from which the air masses tend to drift landward. These conditions are fulfilled in west coasts of landmasses lying between 40° or 45° and 60° latitude. Situated too far poleward to be dominated by the dry influence of the subtropical oceanic high-pressure cells, these climates lack a dry season. Because continental polar air masses tend to drift eastward, they rarely move westward to visit the west coasts, hence severe dry-cold conditions are uncommon.
Koppen classifies the marine west coast climate as Cfb, a temperate rainy climate with warm summers. The average temperature of the warmest month is under 71°F (22°C) and at least four month average 50°F (l0°C) or more. The cool summer variety Cfc has fewer than four months with averages over 50°F(10°C).
Precipitation is well distributed throughout the year, but shows a distinct reduction during the summer months. This feature is also found on rainfall graphs for coastal stations on the Pacific Northwest coast of the United States. Why should this reduction occur? In summer, the oceanic subtropical high is most strongly developed and moves farthest north, bringing its arid influence to bear just enough to make a distinct decrease in summer rainfall. In other words, this is a manifestation of the same mechanism that causes a west coast desert in low latitudes.
The cooler air temperatures reduce evaporation and produce a very damp, humid climate with much cloud cover. Nearness to the ocean makes for a small annual temperature range, just as it does in lower latitudes on the west coast deserts. Mild winters and relatively cool summers are characteristic. Winters, which are severely cold at the same latitudes in mid-continent and east continent positions, are by contrast surprisingly mild on the west coasts.
The soil-moisture budget of the marine west coast climate shows a strong tendency to follow the Mediterranean regime in that the cycles of precipitation and evaportranspiration are exactly in opposite phase. Instead, there is a very large water surplus.
The influence of coastal ranges upon rainfall is extremely marked in middle latitudes. Whereas low-lying coasts, such as in northern France or southern England, receive only 30 to 40 in (75-100 cm) of precipitation annually, mountainous coasts of British Columbia, Alaska, Norway, and Chile get 60 to 80 in (150-200 cm) and over. This has been a major factor in the development of fords along the seacoasts. Heavy snows m the glacial period nourished vigorous valley glaciers which descended to the sea, scouring deep troughs below sea level at their lower ends.
Natural vegetation of the marine west coast climate is forest, but of widely different classes from one world locality to another. Western Europe has a summer-green deciduous forest whereas that of west coast North America is of the needle leaf class. On Coast Ranges of the Pacific Northwest, Douglas fir, red cedar, and spruce grow in magnificent forests. Forests of the marine west coast climate belts of Chile and New Zealand are of the temperate rain forest class.
Soils of the marine west coast climate regions bearing needle leaf forests are of a strongly leached type, the podzols, and are acid in nature. Under cool temperatures, bacterial activity is slow, in contrast to the warm tropics, so that vegetative matter is not consumed and forms a heavy surface deposit. Organic acids from the decomposing vegetation react with the soil compounds, resulting in removal of such bases as calcium, sodium, and potassium. Soils of Western Europe are of the gray-brown podzolic group, typically associated with deciduous forests in middle latitudes. This soil is not so strongly leached and is suited to diversified forms of agriculture.
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