While discussing the climatic classification scheme of Trewartha in an earlier chapter, it was pointed out that certain climatic groups are based only on temperature criteria. The micro-thermal D group is one of them. Do and Dc are its two types.

Again, Dc climate is divided into two subtypes based primarily on temperature contrasts: one with a warm summer (Dca), called humid continental hot summer climate, and the other with a cool one (Deb) – humid continental cool summer climate.

Location:

Humid continental hot summer climate occupies the more southerly parts of the humid continental type and, therefore, is warmer and less severe. Remember that the general Dc
climate is confined to the Northern Hemisphere because this climatic type is associated with the huge landmasses in middle latitudes.

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Dca climate occupies an extensive region in the United States of America, where it extends from the semi-dry steppe regions of the American Midwest to the Atlantic seaboard in the east.

In the south the following states share this type of climate: Kansas, Missouri, Illinois, Indiana, West Virginia and Maryland. The northern boundary of this climate runs through the states of North Dakota, Minnesota, Wisconsin, Michigan, New York, and Massachusetts.

The “Corn Belt’ of the United States has this type of climate ; that is why it is also know as ‘Corn Belt climate’. Thus, the humid continental hot summer climate is located on the leeward side of the continent in North America extending from 35°N to 45° N.

In Europe this climatic type is found farther pole-ward, but it is located on the windward side of the interior regions. It is confined to certain parts of Romania and Bulgaria, occupying the lower Danube Valley which is also famous for growing maize.

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In eastern Asia, this climatic region includes North China bordering the Yellow Sea, North and South Korea, and northern Honshu, the main island of Japan.

In North America and Asia, humid continental hot summer climate is bounded by subtropical humid climate in the south. Towards its pole-ward margins, this climate gradually passes into a more severe humid continental cool summer climate.

In Europe, however, the equator-ward boundary of this climate is set by dry climates or Mediterranean climate. Like other regions, towards the north this climate in Europe, too, passes into a more severe type of humid continental climate.

Temperature:

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Average summer month temperatures vary from 21°C to 24°C. Summer is long, warm, and humid. Most of the time, tropical maritime air mass predominates. Because of the high relative humidity, summer days are rather uncomfortable and weather oppressive and sultry much of the time.

However, occasional polar air masses that visit this climatic region during warm months bring down temperatures by a few degrees.

This climatic region in North America and Asia has summer temperature comparatively higher than its counterpart in Europe.

Average temperatures for the month of July at certain stations in North America and Asia are the examples: Galesburg (Illinois) 25°C; New York, 23.3°C; Tienstsin, 27°C; Mukden (Manchuria) 25°C.

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In European Dca climatic region July average temperature at Bucharest is 22.8°C. In the event of heat waves, summer temperatures may rise to about 38°C.

Winter temperatures in this climatic region average -4° to 1.7°C for a period of 3 to 5 months. But during the long spells of cold weather brought about by the invasion of polar continental cold air mass, subfreezing temperatures may be recorded for several days. The areas open to marine influence may have warm summers and mild winters.

The average annual temperature ranges are fairly large-more than 22°C. At Pittsburgh, Omaha, Mukden, and Bucharest the average annual ranges of temperature are 24.4°C, 30.5°C, 38.5°C, and 25.5°C, respectively.

The diurnal ranges of temperature are highly variable. They may be relatively lower than those in drier regions in the same latitude. However, during the cold spells much larger diurnal range of temperature is recorded.

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Precipitation:

Since this climatic region lies in a frontal zone where polar and tropical air masses interact, precipitation is received throughout the year with its maximum during summer. However, the amount and distribution of precipitation may vary from one year to another and from place to place.

Annual average precipitation varies from 60 to 90 cm. This climate lacks a distinct dry season. In the eastern part of Asia, this climate has dry winter because of the development of monsoon system of winds.

Usually precipitation registers a decrease towards the interior of land masses: New York receives an average of 112 cm. of precipitation, whereas Indianapolis has 100 cm. Hannibal 90 cm and Grand Island 58 cm of precipitation.

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Much of the interior locations have convectional precipitation during the warm season when intense surface heating causes the maximum convective activity. Much of the summer precipitation results from the cumulonimbus clouds.

Thundershowers of very short duration are common in the warmer part of the year. There are occasional cyclonic or frontal precipitations during summer.

During winter, the cyclonic precipitation is the rule when there is maximum interaction between the polar and tropical air masses along the cold and warm fronts. In autumn and spring too, it is the cyclonic precipitation which predominates.

Natural vegetation:

Because of the fact that precipitation maximum occurs during summer when temperature is high, this climate is favourable for tree and grass growth, but the winter precipitation is less effective because the ground remains frozen and the vegetation goes through a dormant state.

The growing season lasts for about 150 to 200 days. During the growing period, high temperature combined with precipitation maximum favours rapid vegetative growth.

In more humid regions with good soil, natural vegetation consists of broad-leaf deciduous forest or mixed broad-leaf deciduous-coniferous forest. Where soil is sandy or acidic, needle-leaf coniferous forest predominates.

In semiarid steppe lands, forest is replaced by the prairie with tall grasses. However, in this climate most of the forests and prairie regions have been cleared for agriculture, pastures and vineyards. But forests are still there in mountainous areas like the Appalachians.