Useful notes on Subtropical Humid Climate (Cf) or China Type of Climate

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The subtropical humid climate is characterized by hot summers and mild winters, though some of the regions in this climatic type have rather cold winters. This type of climate is, however, marked by variations within it.

For example, precipitation regime (seasonal distribution) is not the same everywhere. In some areas, the summer has the maximum precipitation, while in others there is a distinct winter precipitation maximum due to cyclonic storms and frontal activity. However, in humid subtropical climate more or less rain falls throughout the year.

Location:

The humid subtropical climate is found on the eastern side of the continents between 25° and 35°/40°N and S. Thus, it is located within almost the same latitudes as the Mediterranean or dry summer subtropical climate.

It is noteworthy that the Mediterranean climate occupies the western sides of the continents, whereas the humid subtropical climate is located on the eastern sides.

Since in the Northern Hemisphere land masses are more extensive in the middle latitudes than those in the Southern Hemisphere, this climate covers much larger areas in the Northern Hemisphere.

The subtropical humid climate is developed in the following regions: Southeastern United States of America, eastern Asia, particularly China, south of the Hwang Ho River, eastern and central South America (in Paraguay, Southern Brazil, Uruguay, and northeastern Argentina), cast coasts of South Africa and Australia.

The latitudinal location of humid subtropical climate clearly shows that this climate occupies a transitional position. During summer its climatic conditions are similar to those of the humid tropical climates.

Like humid tropical climate, these regions are dominated by maritime tropical (mT) air masses, and have convectional showers and thunder-storms.

Besides, high temperatures combined with high relative humidity and convectional rainfalls are other salient features which are commonly shared by humid subtropical as well as humid tropical climates during summer.

On the contrary, during winter, because of the equator-ward migration of the pressure and wind belts, the humid subtropical climate regions are placed under the influence of extra-tropical cyclones and frontal systems.

Another salient feature of the location of this climate is the fact that it is located on the western side of the oceanic high- pressure cell subsidence, and the stability of air is weak. This characteristic favours heavy rainfall.

Still another vocational characteristic of this type of climate is that there are warm ocean currents found along the eastern coasts of the continents which raise air temperatures during winter, and produce instability in the air during summer months by warming the lower atmosphere.

Furthermore, the East Asia and southeastern United States develop monsoonal tendencies which cause abundant summer precipitation because of warm and moist tropical air blowing on­shore.

In fact, these are the controlling factors that bring about humid subtropical climate on the eastern sides of the continents in latitudes 25° to 40°N and S in both the hemispheres.

Temperature:

Humid subtropical regions have mild winters. The average temperature for winter months may vary from 4.4°C to 12.8° C. In the coldest month, average temperatures for various stations are as follows: Buenos Aires, 9.4°C, New Orleans, 11.7°C, Shanghai, 3.3°C, Nagasaki, 6.1°C.

The annual range of temperature in the humid subtropical climate is not very large. However, the annual range is marked by wide variation in accordance with the latitudinal location of a particular station and the size of the land mass.

The annual ranges at Buenos Aires, Sydney, Montgomery and Shanghai are 12.8°C, 10.5°C, and 24°C respectively. In fact, the larger land mass has larger annual range of temperature.

During winter, the advection of extremely cold polar air from the continental interior and higher latitudes to the southeastern United States brings about very severe winter there. This region, therefore, has the lowest average winter temperatures after China.

Both in China as well as the southeastern United States winters are more severe. In the southeastern United States, because of the absence of any east-west mountain barrier, extremely cold continental polar air mass sweeps from the higher latitudes to the Gulf of Mexico which results in very severe cold weather.

The coastal areas of the northern Gulf States record nearly -12°C temperature. China, on the other hand, in invaded by the cold polar winds expelled by the central Asiatic winter anticyclone developed to the north. That is why; winter temperatures are too low in those latitudes.

In winter too, the higher relative humidity associated with the passage of extra-tropical cyclones and frontal systems makes the cold damp weather uncomfortable in this climate.

In the Southern Hemisphere, because there is no severe continental climate on the pole-ward margins of the subtropical humid climate, there is no invasion of cP air masses, so that the winters are not as severe as in China or the Gulf States.

Another characteristic feature of the humid subtropical climate is the irregular spells of warmer and colder weather brought about by the passage of winter cyclones with their southerly and northerly winds.

Because of the particular temperature regime, the growing season in humid subtropical climate is long -7 to 12 months. Frost is a rare phenomenon in this climate.

But on certain winter nights, frost may occur damaging the more sensitive crops. In the Northern Hemisphere where winters are severe, killing frosts occur more frequently than in the Southern Hemisphere.

Precipitation:

The humid subtropical climate has abundant precipitation, the annual average varying from 75 cm to 150 cm. In certain favoured locations the total precipitation may be as much as 250 cm. Precipitation exceeds potential evapotranspiration throughout the year.

Nonetheless, precipitation amount decreases inland towards the western margins which gradually merge into the multitude steppe climate.

During summer, rainfall occurs from cumulus clouds which result from convectional currents due to the intense heating of the land masses. Unstable tropical maritime air masses yield abundant precipitation under favourable conditions.

There are unusually large numbers of thunder­storms along the coastal areas of the Gulf States in the United States of America. However, most of the precipitation is associated with the extensive but weak atmospheric disturbances.

These thundershowers are organized in belts. However, despite strong summer heat, no rains fall every day. There are also random convectional rainfalls.

On the contrary, precipitation in winter is associated with the winter cyclones coming along the westerlies. Winter precipitation is more widespread.

Land surface in the cool season is colder than the maritime tropical air moving towards the land areas; thus cooling from below makes the air more stable and incapable of rising upward.

Such an air mass resists uplift unless forced upward by physical barriers or by frontal uplift. However, the cyclonic or frontal precipitation does not show belted character.

Monsoonal system of winds which develop over China, and to some extent over the southeastern part of the United States, give maximum precipitation in summer.

In this warm season, heating from below causes the unstable hot and moist oceanic air masses to rise, resulting in heavy convectional showers in late afternoons when the surface is heated to the maximum.

Since during the high sun period, the pressure and wind belts shift a few degrees towards the poles, the subtropical humid regions are visited by tropical storms. Sometimes these storms are very violent and develop into hurricanes or typhoons.

Thus, the late summer precipitation maximum may be the result of the combined effect of convectional activity, tropical depressions and the monsoon system of winds. Remember that most of the hurricanes and typhoons develop in late summer or early autumn.

Even though summer is the rainy season, there is plenty of sunshine in the humid subtropical climate.

In the humid subtropical regions occasional snowfall occurs when an intense cyclonic storm follows a path nearer the equator. Snow in this climate does not remain on the ground for more than a few days.

In the southeastern margins of the United States snow falls on very rare occasions. Towards the pole-ward margins the number of days with snowfall varies from 5 to 15 days, and during this period the ground remains snow-covered.

Natural vegetation:

The humid subtropical climate, with abundant precipitation, high temperatures and long growing season is highly favourable for the growth of natural vegetation.

Towards the more humid tropical margins the natural vegetation consists of evergreen forests, while pole-ward areas with severe winters and lower temperatures are covered with deciduous forests. The regions located towards the drier steppe climate have grasslands.

Animal life:

The humid subtropical climatic regions have various kinds of animals which include deer, rabbits, foxes, squirrels etc. There are also different kinds of birds.


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