North America and Asia are, of course, the areas in this region. The location is intermediate between the source region of polar continental air masses on the north and maritime or continental tropical air masses on the south and southeast. In this polar-front zone, maximum interaction between polar and tropical air masses can be expected along warm and cold fronts associated with east moving cyclones. In winter, the continental polar air masses dominate and much cold weather prevails; in summer, tropical air masses dominate and high temperatures prevail. Thus, strong seasonal temperature contrasts must be expected in this region. Precipitation is ample throughout the year, because the region lies in a frontal zone. Strong contrasts in air masses result in strong frontal activity and highly changeable weather. This climate may be described as both humid and continental in properties.
The climate picture drawn above applies well to the north-central and northeastern United States and southeastern Canada as well as to northern China, southern Manchuria, Korea and northern Japan. Very similar climatic conditions also hold for much of central and eastern Europe, the Balkan countries, and Russia, but this third region differs from the first two in that it is influenced by a great source region of continental tropical air masses lying to the south and southeast, instead of the oceanic source regions of maritime tropical air masses.
Four varieties of climate, according to Koppen, are found in the regions that we are grouping together as having the humid continental climate. Dfa and Dfb are cold, snowy, forest climates, the first with a hot summer, the second with a warm summer. The precise definitions of a and b are as previously given. Dwa and Dwb climates, found in eastern Siberia, Manchuria, and northern Korea, are cold, snowy forest climates with a dry winter. Letters a and b refer to hot and warm summers, respectively.
The winter dryness, typical of all climates of eastern Asia, is explained below. We are also including here under the humid continental climate the northern part of Koppen's Cf climate region in the eastern United States. Recall that Koppen used as his CD boundary the isotherm of 26.6°(-3°C) of the coldest month. Thus, for example, Koppen places New Haven and Cleveland in the same Cfa climate as New Orleans and Tampa, despite obvious contrasts in January mean temperatures, soil groups, and natural vegetation between these northern and southern zones.
Precipitation records of the four stations illustrated show certain marked dissimilarities, which require explanation. A summer maximum is apparent in all four but is very weakly defined in New York City because maritime air masses, both polar and tropical, have ready access to the eastern seaboard at all times of the year. Omaha and Moscow have well-defined summer maxima and winter minima, reflecting the predominance of tropical air masses in summer and continental polar air masses in the winter. Moscow, being farther north, near to the source region of polar continental air masses, has less precipitation than the other stations. Peiping shows a very strong summer maximum and a winter drought. This contrast is characteristic of the east Asiatic middle latitude stations and reflects the powerful Monsoon control, whereby dry continental air masses dominate in winter.
The summer moisture deficiency is small, while winter recharge leads only to a small spring surplus.
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