27 Humid Mesothermal Climates

Dr. Jitender Saroha

Contents

Introduction

Learning Objectives

Humid Mesothermal Climates: Bases and Types

Humid Subtropical Climate

Distribution:

Temperature:

Precipitation:

Natural Vegetation

Subtropical Dry-summer or Mediterranean Climate

Distribution:

Temperature:

Precipitation:

Natural Vegetation:

Marine West Coast Climate

Distribution:

Temperature:

Precipitation:

Natural Vegetation:

Summary and conclusions

Multiple Choice Questions

Answers:

References

Web Links

Introduction

You all very well know that temperature decreases from equator to pole. On the basis of temperature, at macro level, the world is divided into three climatic zones – tropical, temperate and polar. On the basis of temperature and precipitation,Koppen recognized two groups of humid mid-latitude (temperate) climates. The one group, adjoining the tropics, has mild winters (the C climates) and the other, towards the polar region, experiences severe winters (the D climates). In these groups the precipitation exceeds evapo-transpiration. Therefore, these are humid climates. The first group represents humid mesothermal climates and the second represents the humid microthermal climates. The humid mesothermal climates include the following three major types – (i) Moist subtropical climate, (ii) Mediterranean climate and (iii) Marine west coast climate. In this unit our focus is on the humid mesothermal climates.

Learning Objectives

After studying this module, you will be able to:

understand the meaning of humid mesothermal climates,
differentiate between various types of humid mesothermal climates,
describe the distinctive characteristic features of humid mesothermal climates, identify climatic regions of humid mesothermal climate,
establish interrelationship between temperature, precipitation and natural vegetation, highlight the role of variousdeterminants of humid mesothermal climates,and
explain the role of air masses and fronts in determining weather conditions of humid mesothermal climates.

Humid Mesothermal Climates: Bases and Types

Humid mesothermal climates are humid because in these climates precipitation exceeds evaporation. These climates have warm winters. The average temperature of the coldest month is in the range of -30 C to 180 C and the average temperature of the warmest month is above 100 C. Instead of -30 C temperature of the coldest month, Trewartha and many other climatologists have used 00 C for determining humid mesothermal climate. At macro level, the humid mesothermal climates type or group is represented by letter symbol ‘C’. In determining the sub types or second order, seasonal distribution of precipitation is taken into consideration. The areas receiving rainfall throughout the year are represented by symbol ‘f’ and the areas which remain dry during summer but wet in winter, by symbol ‘s’. Further classification is done, on the basis of summer season temperatures, hot summers and cool summers are represented by symbols ‘a’ and ‘b’ respectively. The criteria used by Koppen for the classification of humid mesothermal climates are listed in Table 1.

Table 1. Koppen’s Main Criteria for Humid Mesothermal Climates

Koppenidentified sub types in the humid mesothermalclimate on the basis of temperature, precipitation and seasonal distribution of precipitation. Strahler (1978) used the criteria of air masses and front zones and identified three climatic classes as equivalents to Koppen’s classification of humid mesothermal climates (Table 2). In this unit focus is on the distribution and characteristics of these three major humid mesothermal climatesas proposed by Koppen, and later followed by Trewartha and Strahler. These climates, generally, are located poleward of megathermal climates i.e. tropical climates (Figure 1).

Table2. Humid Mesothermal Climates Classified by KoppenandStrahler

Figure 1: Distribution of Humid Mesothermal Climates in the World

Source: http://slideplayer.com/slide/4558178/15/images/17/Mesothermal+Climates+(C).jpg

Humid Subtropical Climate

The humid subtropical climate is also known as temperate rainy climate with hot summers. It is characterized by hot summers, mild winters and throughout the yearprecipitation.Therefore, in the Koppen’s climatic classification scheme it is symbolized as Cfa. During summer hot and sultry weather conditions of the type expected in the rainy tropics prevail. Besides these, convectional rainfall in summer isanother salientfeature commonly shared with humid tropical climates. On the contrary, during winter, because of change in the overhead position of sun the pressure and wind belts shift and the humid subtropical climate regions are placed under the influence of temperate cyclones and frontal systems.It is noteworthy that the Mediterranean climate occupies the western side of the continents, whereas the humid subtropical climate is located on the eastern sides.

Distribution: The humid subtropical climate is found on the eastern sides of the continents, in the 250 to 400 latitude range in both the hemispheres. As compared to the southern hemisphere land masses are more extensive in the middle latitudes in the northern hemisphere. Therefore, this climate occupies much larger areas in the northern hemisphere. This climate prevails in the southeastern United States and other similarly situated areas: all of Uruguay, southern Brazil and northeastern Argentina in South America, eastern China (particularly south of the Hwang Ho River) and southern most Japan in Asia, and south-east coast of South Africa (Natal region) and eastern coast of Australia. On the basis of its distribution, this climate is also known as China type or Gulf type or Natal type.

Temperature: Temperature values in this climate reflect that the summer temperatures are comparable to the tropics and that winter values are markedly lower (Figure 2). Summer season average temperature range is 160 to 260 C. Humid subtropical regions have mild winters. The average temperatures for winter months vary from 30 C to 130 C. In the coldest month, average temperatures for various stations are as follows: Buenos Aires, Argentina, 100 C; New Orleans, USA, 120 C; Shanghai, China, 3.30C;Nagasaki, Japan, 6.10 C; and Washington D.C., USA, 2.70C.

Figure 2: Distribution of Temperature at Selected Stations in the Humid Subtropical Climate

Source: Frederick K.L and Edward, J.T. (2010), P-436 and 488.

In this climatic region winters are comparatively severe in China as well as United States. In case of China, these areas are invaded by cold polar winds during winter. In case of United States intrusions of cold continental polar air masses result in very severe cold weather. In winter too, the high relative humidity associated with the passage of temperate cyclones and frontal systems makes the cold humid weather uncomfortable. In the southern hemisphere; due to lack of continentality the winters are not that severe.

The annual range of temperature in humid subtropical climate is moderate. However, there are wide variations in annual temperature ranges depending upon the latitudinal position and continentality.The annual ranges at Sydney, Buenos Aires, New Orleans, Charleston, Washington D.C., and Shanghai are 110C, 140C, 150C, 16.50 C, 230C, and 240 C, respectively.

Due to favourable temperature regime, the length of growing season is long (8 to 12 months). Frost is a rare phenomenon in this subtropical climate. But on certain winter nights, frost occurs and damages the more sensitive crops. In the northern hemisphere killing frosts are more frequent than in the southern hemisphere.

Precipitation: In humid subtropical climate, the average annual precipitation is in the range of 75 to 150 cm. Generally, its distribution decreases towards interiors. Precipitation is distributed throughout the year and it exceeds potential evapotranspiration. Generally, precipitation maxima are found in summer but there are considerable variations (Figure 3). For instance, in the United States precipitation in the Gulf States is very uniformly distributed. But northward or toward the drier western margins, much more precipitation takes place in summer. In this climate on-shore trade winds bring precipitation.

Figure 3: Distribution of Precipitation at Selected Stationsin the Humid Subtropical Climate

Source: Frederick K.L and Edward, J.T. (2010), P-436 and 488.

During summer months warm, moist and unstable maritime tropical (mT) air masses move inland from the western portions of the oceanic subtropical anticyclone. As these air masses pass over the heated continent, they become unstable and give rise to convectional rainfall and thunderstorms. These areas experience thunderstorms for 40 to 100 days each year. The frequency is highest during summer season.Sometimes, these storms are very violent and develop into hurricanes or typhoons.Therefore, typhoons (China) and hurricanes (southeast USA) are common in summer.Thus, the late summer precipitation maximum may be the result of combined effect of convectional activity, tropical depressions and the monsoon system of winds.

The winter precipitation is also different in character. Sometimes it is in the form of snow, and most is generated along fronts of the frequent temperate cyclones which sweep over this regions.Winter precipitation is more widespread.

Natural Vegetation

The humid subtropical climate experiencesabundant precipitationand well distributed over the year. It has hot summers and mild winters with long growing season. All such available situations are highly suitable of the growth of natural vegetation. Evergreen oak and magnolia flourish in broadleaf evergreen forest. Towards more humid tropical margins, the natural vegetation consists of evergreen forests while poleward areas with severe winters and lower temperatures and low winter precipitation are covered with deciduous forests. The regions located towards the drier steppe climate have grasslands.Today large areas of former forests have been replaced by agricultural croplands.

Subtropical Dry-summer or Mediterranean Climate

The subtropical dry summer climate is characterized by dry summer, mild rainy winter and abundant sunshine. These characteristics make it ideal as resort climate of the world. Due to dry summer conditions it is known as Cs climate.

Distribution: The dry-summer subtropical climate is typically located along the west sides of continents between 300 to 45 0 latitudes in both the hemispheres. This climate prevails over about 1.7 per cent of the earth’s land area in the following five regions: the borderland of the Mediterranean sea (north of the Mediterranean sea from Portugal to Turkey and beyond in the Iranian Highlands; the southern borderland includes Morocco, northern Algeria, and Tunisia and north of Bengasi in Libya); the central and southern California coast in the United States of America; the central and southern Chile; the Cape town area of South Africa; the southern and southwestern coasts of Australia (Figure 4). Because of the arrangement of the continents and their mountain ranges this climate is limited to coastal areas and inland development occurs only in the Mediterranean basin.

Figure 4: Distribution of Mediterranean Climate

Source:http://www.grabovrat.com/mapsViews/mapsViewsFig/mapsViews800.gif

The most important controlling factor that produces Mediterranean type of climate is the shifting of wind belts. In these areas during the summer, trade winds operate as off-shore winds and dry conditions prevail. During winter due to southward shifting of wind and pressure belts, these areas come under the influence of onshore westerlies and precipitation takes place. This way, middle latitude changeability characterizes the winter and tropical constancy describes the summer.

Temperature: The average temperature of the coldest month in Mediterranean climate is in the range of 70 C to 100C and of the hottest month between 190C and 270 C. The mean annual ranges are between 120C and 170 C. The mean annual range of temperature are less than those of most cool temperate climates and greater than those of dry tropical climates.

The annual range of temperature, however, increases with distance from sea: at San Francisco it is only 90 C, but at Sacramento 170C and at Red Bluff, an inland station in California it is 200 C. Stations that are located farther away from marine influence record highest summer temperatures, whereas coastal locations have low summer temperatures because of the presence of the cold ocean currents which parallel the western coasts in these latitudes. Due to the absence of cold ocean currents in coastal parts of the Mediterranean Sea summer temperatures are much higher. For instance, the average temperature of July month at Rome is 24.40C and at Izmir 280C (Figure 5).

Figure 5: Distribution of Temperature at Selected Stations in the Humid Subtropical Dry Summers Climate

Source: Frederick K.L and Edward, J.T. (2010), P-439 and 489.

Within Mediterranean climate two sub types are identified primarily on the basis of summer season temperatures. The cool summer type (Csb) is represented by San Francisco, Santiago and Chile and is confined to coastal areas. Here cooling effect is intensified by cold ocean currents. In the table the data for Izmir and Sacramento reflects the features of second sub type of warm summer (Csa). In the warm summer sub type the summer temperatures are noticeably higher and winter variation is negligible.

In case of warm summer dry subtropical climate (Csa), along with continentality and absence of cold ocean current influence, subtropical high pressure systems also cause summer temperatures to be higher. Because of clear weather, the average summer temperatures in lower latitudes and inland stations are more than 210 C or even 270 C. Further contributory factors in increasing the temperatures are clear skies, almost vertical rays of the sun and the low relative humidity. Day time maximum temperatures may reach 320C to 380C. Like the dry climates, due to rapid loss of heat by nocturnal radiation the night temperatures record a considerable drop. Thus, the diurnal range of temperature is considerably large, particularly in the drier Mediterranean climates. The diurnal range of temperature for Sacramento in the month of July is about 200 C.

Frost is uncommon in the Mediterranean climate. But there are occasional frosts which are injurious to crops like citrus fruits. Even one night’s frost proves fatal to such crops.

Precipitation: The Mediterranean climate has rainy winter and dry summer. This climate receives moderate to scanty precipitation that is why some climatologists refer it as subhumid climate instead of humid.It is especially true along the equatorward margins. The average annual precipitation is usually between 40 cm and 80 cm. About 75 per cent of the total precipitation is received during winter. Since there is almost complete drought in summer, it is called the dry-summer subtropical climate. As this climate is located between the dry climates found equatorward and the marine west coast climate poleward, it is natural that the amount of precipitation goes on increasing from its equatorward to poleward margins. For instance, Los

Angles receives 38cm, San Francisco (400 km north) 51 cm and further north Portland, Oregon receives 90 cm precipitation. Besides, there is also an increase in precipitation from the inland to coastal locations.

In the northern hemisphere, the Mediterranean climate receives most of its precipitation between December and March, while in the southern hemisphere the rainy season lasts from May to August (Figure 6). During winter the Mediterranean regions are occupied by the west wind belt so precipitation results mainly from the temperate cyclones moving with the westerlies.In coastal regions backed by mountains, orographic precipitation is common.As majority of the precipitation is received during winter the precipitation effectiveness is high. Within the rainy season there are many days of fine and sunny weather. In the Mediterranean climate, snowfall is a rare phenomenon except in highland areas.

Figure 6:Distribution of Temperature at Selected Stations in the Humid Subtropical Dry Summers Climate

Source: Frederick K.L and Edward, J.T. (2010), P-435.

In this climate precipitation is low and variability of precipitation is high. Therefore, the reliability is low. The range of precipitation variability in Chile varies from 18 per cent to 226 per cent. This characteristic of the climate has compelled the farmers to develop means of irrigation for their market crops.Precipitation has a much higher reliability in the Mediterranean basin, in Australia and in South Africa.

Natural Vegetation: The native vegetation of the Mediterranean climate is well adapted to the long summer drought. Trees and scrubs that can survive here are characteristically equipped with small, hard or thick leaves that restrict water loss through transpiration. These plants are known as sclerophylls (hard-leaved). Much of the natural vegetation consists of plantspossessing moisture-conserving devices such as tough surfaces, small, rolled, shiny thick leaves that resist moisture loss and deep roots to be able to suck up soil-moisture.

In the areas having enough precipitation pine, oak, cork oak, cedar, walnut and chestnut trees are also common. In Western Australia, there are vast strands of eucalyptus trees.Low, scrubby bushes growing together in a thick tangle form the most characteristic natural vegetation of this climate. In California, these scrubs are called chaparral which is often threatened by wildfire. Chaparral acts as a check against soil erosion during the periods of rains. This scrub is called maqui in France, mattoral in Chile and mallee in Australia.There is abundance of drought resistant fruits like olive or irrigated citrus fruits. Long summer drought encourages fruit growing. Olive, figs and grapes are the native fruits of the Mediterranean climate which are well adapted to drought conditions. However, peaches, oranges, lemons etc. are grown on irrigated lands.

Marine West Coast Climate

The third major sub type in humid mesothermal climates is the marine west coast climate. Koppen called it temperate rainy climate with warm summer. As this warm temperate climate has precipitation throughout the year and summers are cool, it is represented by symbol Cfb.

Distribution: Marine West Coast climate occupies the windward (western) side of mid-latitude continents between 400 to 650 north and south latitudes. On shore westerlies along with maritime air masses prevail throughout the year. This ensures mild winters, cool summers and ample precipitation throughout the year.

In North America, this marine west coast climate occupies the narrow coastal belt from near the US – Canada border up to southern Alaska, where it borders the subarctic climate of Canada and Alaska. A similar narrow strip extends along the coast in Chile, South America. In these two coastal belts, high mountains run parallel to the coast; therefore, marine climate influence is restricted to the coastal region.

The largest area of this climate is in Europe where it extends along the west coast up to 680 N (Norway). The northward extension of this climate is made possible by the moderating influence of the North Atlantic Drift, a warm ocean current. As there is no mountain barrier blocking the movement of cool maritime air from North Atlantic the climatic conditions prevail far inland.The most extensive development of this climate is found in Western Europe including Great Britain, northwestern France, Germany, Denmark, Holland, Belgium and Norway. The other locations include most of New Zealand, island of Tasmania as well as tiny slivers of South Africa and Australia (Figure 7).

Figure 7: Distribution of Marine West Coast Climate

Source:https://upload.wikimedia.org/wikipedia/commons/3/3e/Koppen_classification_worldmap_CfbCfc.png

Temperature: In this climate, temperatures are modified by marine influence of warm ocean currents. Therefore, there are no significant seasonal extremes of temperature. Keeping in view the latitudinal position, this climate has small annual range of temperature, summers are relatively cool and winters are mild. The annual range of temperature is low (Figure 8). The oceanic control of temperature is also visible in thermal gradients. Although this climate prevails in wide latitudinal zone, temperature changes much more abruptly moving towards continental interiors (east-west) rather than in north-south direction. This is because the transport of heat from oceans is far more than through solar radiation.

Figure 8: Distribution of Temperature at Selected Stations in the Marine West Coasts Climate

Source: Frederick K.L and Edward, J.T. (2010), P-437 and 487-488.

In marine west coast climatesummers are cool. Average summer month temperatures are in the range of 140C to 180C. Mean monthly temperature of the warmest month (July) in the northern hemisphere are as follows: Bergen, Norway, 150C; Dublin, Ireland, 15.60C; Seattle, United States, 170C; London, UK, 180C; Vancouver, Canada, 180C;Paris, France, 190C andBerlin, Germany, 19.40C.

The climate being humid, night temperatures are not as low as in the Mediterranean climate. Therefore, the diurnal range of temperature is low. For instance, in the month of July the diurnal range of temperature at Seattle is 100 C. The ocean is near, so winters are mild and summers relatively cool. Therefore, a low annual temperature range is characteristics of the marine west coast climate.

Taking into consideration the latitudinal position, the winters are mild in this climate region. In the coastal regions of the Western Europe a positive departure of temperature in the range from 110C to 170C is common. In Western Europe, the average temperatures during the coldest month (January) vary from 20C to 100C. At the same time, the continental interiors of Eurasia have average temperatures in the range of -180C to -400C.

The average monthly temperatures of the coldest month for various stations are as follows: Berlin, Germany, -0.50C;Reykjavik, 00C;Bergen, Norway, 1.50C;Vancouver, Canada, 20C; London, UK, 40C;Bordeaux, 50C; and Melbourne, Australia, 9.40C.

In winter season, occasional invasions of continental polar (cP) air masses bring cold spells. These cold spells reduce the temperatures to below freezing. Frost is severe and more frequent towards interiors. However, this climate has a long growing season of nearly 8 months.

Precipitation:The marine west coast climate receives adequate precipitation throughout the year (Figure 9). Depending on topography and distance from sea, the amount of precipitation is characterized by intense spatial variations. In European lowlands the average annual precipitation varies from 50cm to 100cm, but on the windward side of the coastal ranges it is above 250cm here the orographic uplift tends to increase the precipitation. The Andes in Chile and the Rockies in North America get abundant precipitation on their windward (western) sides by temperate cyclones as well as orographic uplift of maritime air masses under the influence of westerlies. Henderson Lake located in British Columbiaprovince (Canada) has a record of 655cm precipitation in a year.

Figure 9: Distribution of Precipitation at Selected Stations in the Marine West Coasts Climate

Source: Frederick K.L and Edward, J.T. (2010), P-437 and 487-488.

Although, there is abundant precipitation well distributed throughout the year, but as compared to summers, winters are foggier, cloudier, stormier and rainier. At many stations in this climate region the reduced summer rainfall is due to the poleward shifting of the oceanic subtropical highs. Although the areas of this climate are too far poleward to be dominated by these dry anticyclones, their influence is sufficient to cause a decline in summer rainfall.

In coastal areas, maximum precipitation takes place during winter but in continental interiors it is highest during summer.Due to marine influence winters are mild and snowfall is less frequent as compared with the other climates of these latitudes.The lowland stations such as Seattle, Paris and London have on an average 10 to 14 days as snow days. However, snowfall days increasepolewards and towards the continental interiors. Heavy snowfall occurs in highland areas and they remain snow covered for several months.

Natural Vegetation: Due to mild winters and abundant precipitation the natural vegetation of marine west coast climate is forest. The density and rate of growth is higher in more humid regions. The forest consists of broad-leaf deciduous and needle-leaf conifers or a mixed forest of both. Pine, cedar, hemlock, fir and spruce constitute the finest timber trees. Deciduous trees are oak, beech, birch, walnut, maple, elm and chestnut. The finest conifer forests, with best type of softwood lumber are located in the northwestern coast of the North America. These are dominated by redwoods and Douglas firs. However, most of these forests in Europe have been cleared due to industrialization, urbanization and agriculture expansion.

Summary and conclusions

Humid Mesothermal climates occur in middle latitudes where the average temperature of the coldest month is less than 180 C but above -30C. The first major sub type in this group of climates is humid subtropical climate with hot summers (Cfa). It prevails on the eastern side of the continents, in the 250 to 400 latitude range.It is characterized by hot summers, mild winters and precipitation throughout the year. During summer, high temperature, high relative humidity and convectional rainfall are the salient features which are commonly shared with humid tropical climates. On the contrary, during winter, because of change in the overhead position of sun the air pressure and wind belts shift and the humid subtropical climate regions are placed under the influence of temperate cyclones and frontal systems.

Mediterranean or Dry summer subtropical climates (Csa, Csb) are typically found along the west sides of continents between latitudes 300 and 450latitudesin both the hemispheres.This climate is characterized by dry summer, mild rainy winter and abundant sunshine.The most important controlling factor that produces Mediterranean type of climate is the shifting of wind belts. During the summer season in these areas trade winds operate as off-shore winds and dry conditions prevail. During winter due to southward shifting of wind and pressure belts these areas come under the influence of onshore westerlies and precipitation takes place. In summer, the regions are dominated by the stable eastern sides of the oceanic subtropical highs. In winter, as the wind and pressure systems shift the regions are under the cyclonic storms of polar fronts.Within Mediterranean climate two sub types are identified primarily on the basis of summer season temperatures – the cool summer type (Csb), and second warm summer type (Csa).

Marine West Coast climate (Cfb) occupies the windward (western) side of mid-latitude continents between 400 to 650 north and south latitudes. On shore westerlies along with maritime air masses prevail throughout the year. This ensures mild winters, cool summers and ample precipitation throughout the year.Temperatures in the marine west coast climate are influenced to a large extent by the oceanic influence and by the warm ocean currents.For their latitudes, the marine west coast climatic regions have relatively cool summers and mild winters and the annual range of temperature is small. The oceanic control of temperature is also visible in thermal gradients. Although this climate prevails in wide latitudinal zone, temperature changes much more abruptly moving towards continental interiors (east-west) rather than in north-south direction.The marine west coast climate receives adequate precipitation throughout the year. Depending on topography and distance from sea, the amount of precipitation is characterized by intense spatial variations.

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Referenes

Barry, R.G. and Chorley, R.J. (1998) Atmosphere, Weather and Climate, Routledge, London. Frederick, K.L and Edward, J.T. (2010) The Atmosphere – An Introduction to Meteorology, PHI Learning Private Limited, New Delhi.
Husain, M. (2002) Fundamentals of Physical Geography, Rawat Publications, Jaipur.
Lal, D.S. (1993) Climatology, Chaitanya Publishing House, Allahabad.
Lal, D.S. (2009) Physical Geography, Sharda PustakBhawan, Allahabad.
Singh, S. (2015) Physical Geography, Pravalika Publications, Allahabad.
Strahler, A.N. and Strahler, A.N. (2001) Modern Physical Geography, John Wiley and Sons, Singapore.
Trewartha, G.T. and Horne, L.H.(1968) An Introduction to Climate, McGraw-Hill, New York.

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