28 Humid Microthermal Climates

Dr. Jitender Saroha

   Contents

Introduction

Learning Objectives

Humid Microthermal Climates: Bases and Types

Humid Continental Hot Summer Climate

Distribution

Temperature

Precipitation

Natural Vegetation

Humid Continental Mild Summer Climate

Distribution

Temperature

Precipitation

Natural Vegetation

Boreal or Subarctic climate or Taiga climate

Distribution

Temperature

Precipitation

Natural Vegetation

Summary and conclusions

Multiple Choice Questions

Answers

References

Web Links

Introduction

Humid microthermal climates represent the continental climates of North America and Eurasia. The term microthermal is used to describe the low temperature conditions which prevail in these climates, specially,during the winter season. The characteristic feature of the microthermal climates is cold winters. In these climates winters are cold enough to ensure that surface remains snow covered for a fixed period of time every year. Conceptually, an average temperature of 0°C or colder is assumed to be necessary to bring this snow cover.Therefore, this climate is determined on the basis of average temperature of the coldest month below 0°C. Koppen used the criterion of average temperature of the coldest month below -3°C, but it was modified to 0°C by Trewartha. Majority ofclimatologists follow this modified criterion for determining the boundary between mesothermal and microthermal climates.For the warmest month the average temperature has to be 100C and more to call the climate type to be mesothermal. As precipitation exceeds evapotranspiration the climate is humid. In this module, the focus is on the humid microthermal climates.

Learning Objectives

After studying this module, you will be able to:

• understand the meaning of humid microthermal climates,
• identify the climatic regions of main humid microthermal climates, differentiate between various types of humid microthermal climates,
• describe the distinctive characteristic features of humid microthermal climates,
• describe the precipitation form, regime and causative factors in this climate,
• establish interrelationship between temperature, precipitation and natural vegetation,
• identify and explain the role of factors influencing the climatic conditions, and explain the role of air masses and fronts in humid microthermal climates.

Humid Microthermal Climates: Bases and Types

Humid Microthermal Climates are known as humid because precipitation is more than potential evapotranspiration. The term microthermal means low temperature and it, specially, indicates the temperature conditions of winter season. In humid microthermal climates, the winters are cold. These climates are differentiated from humid mesothermal climates on the basis of 0°C isotherm (Koppen used -3°C). At macro level, humid microthermal climates are represented by letter symbol ‘D’. On the basis of distribution of precipitation throughout the year the symbol ‘f’ is added to make it ‘Df’. These climates are typically subdivided into three categories based on the temperature characteristics of the summer season.

The southernmost of the threeis frequently referred to as the Humid Continental Hot Summer Climate or Cold Snowy-Forest, hot summers (Dfa) climate. The letter symbol ‘a’ represents hot summers, the warmest month average temperature is 22°C or above and at least four months average temperatures are more than 10°C. In the middle zone lies the second major type of humid microthermal climate known as Humid Continental Mild Summer Climate or Cold Snowy-Forest Climate, warm summers (Dfb). In this the letter symbol ‘b’ represents that no summer month has an average temperature as warm as 22°C, but at least four months have average 10°C or higher. The northernmost of the three microthermal zones is the subarctic or borealzone. This part climate is known as Cold Snowy-Forest Climate, cool summers or Sub arctic climate or Taiga climate (Dfc). The letter symbol ‘c’ means there are only one to three months with average temperatures of at least 10°C and the warmest month average temperature is less than 22°C (Table 1).

Table1. Humid Microthermal Climates Main Criteria (Koppen)

These criteria place all of the world’s microthermal climates in the northern hemisphere, as the absence of broad land masses at upper-middle latitudes in the southern hemisphere precludes the existence of such temperature conditions there (Figure 1). On the basis of Koppen’s classification and its simplification by Trewartha and use of criteria of air masses and fronts by Strahler following three main types of the humid microthermal climates are identified:

(i) Humid Continental Hot Summer Climate or Cold Snowy Forest Climate, hot summers (Dfa),

(ii) Humid Continental Mild Summer Climate or Cold Snowy Forest Climate, warm summers (Dfb),

(iii) Cold Snowy Forest Climate, cool summers or Sub arctic climate or Taiga climate (Dfc)

Figure 1: Distribution of Humid Microthermal Climates in the World

Source: http://astro.wsu.edu/worthey/earth/html/im-atmosphere/climate-zone-de.gif

Humid Continental Hot Summer Climate

Distribution: This climate is also known as Cold Snowy Forest Climate, hot summers (Dfa). Itis confined to the southerly parts of the humid microthermal type. Thus, within humid microthermal climates it is warmer and less severe.This climate is a product of huge landmasses in mid-latitudes. As continentality is a key feature, this climate does not occur in the southern hemisphere, where the mid-latitudes are dominated by the oceans.

This climate prevails over an extensive area in the United States of America where it extends from the semi-dry Prairie regions of the American Midwest to the Atlantic coast in the east. The ‘corn belt’ of the United States has this type of climate; that is why it is also known as ‘corn belt climate’. Thus, the humid continental hot summer climate (Dfc) is located on the leeward side of the continent in North America extending from 350 to 450 N. Itsunusual extent to eastern sea coast is because of westerlies. These winds are so persistent that incursions of maritime influence from the east are insignificant and continental character is maintained.

In Europe, this climate type is found slightly northward in the latitudinal range of 430-500 N. In case of Europe,it is located on the windward (westerlies) side of the interior regions. It is confined to parts of Romania and Bulgaria and occupies the lower Danube Valley.

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

Temperature: In humid continental climate with hot summers the average summer month temperatures vary from 180 C to 250 C. Summer season is long, warm and humid. Most of the time, tropical maritime (mT) air masses predominate. Due to high relative humidity, summer days are rather uncomfortable. However, occasional intrusions of polar air masses bring down temperatures by a few degrees during summer months.

Within this climate region the summer temperatures are higher in North America and Asia as compared to Europe. Average temperatures for the month of July at certain stations in North America and Asia are the examples; New York, 23.30C; Peoria 23.930C;Omaha 250C;Galesburg (Illinois) 250C;Urumchi 23.9 0C; Mukden (Manchuria) 25 0C;and Tienstsin 270 C. In Europe, the average temperature of July at Bucharest is 22.80 C. In the case of occasional events of heat waves, summer temperatures may be over 350Cat many places.

Winter temperatures in this climate region average -50C to 1.50 C for a period of 3 to 5 months. In cold winter season, due to invasion of polar continental (cP) cold air masses, subfreezing temperature conditions prevail for several days. In the coastal parts, due to marine influence summers are warm and winters are mild, for instance, in New York.

The average annual temperature ranges are fairly large in this climate because the winter and summer temperatures are relatively severe. The average annual range of temperature is normally more than 220C. For instance, atNew York, 240C; Pittsburgh,24.40C;Bucharest,25.50C;Peoria 28.30C; Omaha,30.50C;Mukden, 38.50C, and at Urumchi, about 400C. From these examples, it can be concluded that annual temperature ranges also vary substantiallywithin this climate. The range of temperature is, generally, increasing from south to north and from coast toward the interior.

The summer temperature gradients are weak and winter temperature gradients are steeper (Figure 2). Shifts in wind direction during the cold season often result in sudden large temperature changes but summers are more or less uniform. Like annual ranges of temperatureas mentioned above, the diurnal ranges of temperature are also highly variable.

Figure 2: Mean MonthlyTemperature at Selected Stations

Source: Frederick K.L and Edward, J.T. (2010), P-440, 486 and 487.

Precipitation: This climate region extends in the frontal zone of interaction of polar and tropical air masses. Therefore, due to atmospheric instability precipitation is received throughout the year. It is normally highest during the summer season. However, there are spatial and temporal variations of the amount and distribution of precipitation. For instance, average annual precipitation at Urumchi, Harbin, Omaha, Des Moines, Peoria, Chicago, Lake Station and New York is 28, 58, 65, 76, 89, 92, 94 and 107 cm, respectively. Annual average precipitation varies from 50 to 90 cm.

Harbin, Manchuria shows the most pronounced summer maximum and a winter drought (Figure 3). This is the characteristic of the eastern part of Asia in the mid-latitudes and reflects the dominating role of the monsoon. Precipitation in this climate region generally decreases towards continental interiors(New York receives 107 cm and Omaha 65 cm average annual precipitation) and from south to north, mainly due to increasing distance from the sources of maritime tropical (mT) airmasses.

Intense heating during summer season results into convectional activity and cumulonimbus clouds bring convectional precipitation along with thunderstorms. There is occasional cyclonic or frontal precipitation also during summer.

Figure 3: Mean Monthly Precipitation at Selected Stations

Source: Frederick K.L and Edward, J.T. (2010), P-440, 486 and 487.

During winter, the cyclonic precipitation is the rule. It is result of frontal interaction of tropical and polar air masses along the cold, warm and occluded fronts. Part of this precipitation is in the form of snowfall and its proportion increases with latitude. The summer rains are often in the form of relatively short convective showers, whereas winter snows are more prolonged. In autumn and spring, the cyclonic precipitation dominates.

Natural Vegetation: The high temperature and high summer season precipitation provides suitable conditions for the growth of vegetation. In winter season, as ground remains frozen for some days the vegetation has adapted to a dormant phase. The growing season in this climate extends to about 150 to 200 days. During growing season due to favourable temperature and moisture conditions the vegetation growth rate is fast. In wetter regions with better soil, natural vegetation consists of broad-leaf deciduous forest or mixed broad-leaf deciduous-coniferous forest. Throughout much of the northeastern United States and southeastern Canada mixed coniferous and deciduous forest was the native type. It grades southward into broadleaf deciduous forest, which was found in a large area of the eastern United States. This type of vegetation was common in central and Eastern Europe also. Dairy farming and extension of crops have changed the landscape. Cereals grown in this climatic region on extensive scale in North America and Europe are corn, wheat and oats. Soybeans are intensively cultivated in the mid-western United States and in northern China and Manchuria, but very little in Europe. Beet sugar is an important product of this climate region in Europe. Rice is common in South Korea and Japan.

Humid Continental Mild Summer Climate

Distribution: This climate is characterized by a longer, more severe winter and a shorter, more moderate summer. It extends mainly between 500 N and 65 0 N latitudes. This sub type of humid continental climate has more severe winters as compared with Dfa. It is distributed north of humid continental hot summer climate.In the dry continental interiors of Eurasia towards equator it merges with steppe regions. Its poleward boundary is delimited by subarctic climate with far more severe winters.

In North America this climate extends in the mid latitudes mainly in south Canada and north eastern United States east of the 1000 W longitude upto the coast. The Great Lake region is part of this climate. In Europe, it extends over Poland, eastern Germany, southern Sweden and a large part of former USSR. In Asia, central and northern Manchuria, south east Siberia and Hokkaido island of Japan experience this type of climate.

Temperature: As this climatic subtype is located in higher latitudes, temperature is usually lower than in humid continental hot summer climate. Winters are more severe and summers are milder as compared to the Dfa climate. It is notable that the difference in temperature between these two subtypes is much larger in winter season as compared to summer. The coldest winter months have an average temperature of -150C to -50C, whereas the hottest summer month’s average is 160C to 200C.

The hottest month (July) average temperatures for a few selected placesof this climatic region are as follows: Goose Bay (Canada), 16.40C;Calgary (Canada), 16.60 C; Stockholm (Sweden), 17.80C;Marquette (USA), 180C;Moscow (Russia), 190C;Portland (USA), 200C;Winnipeg (Canada), 200 C; Sapporo (Japan),20.60C; Vladivostok (Russia), 20.60 C; and

Toronto(Canada),20.60C. Average coldest month temperatures for the same stations are as follows: Stockholm, -2.90C; Portland, -5.60C; Sapporo, -60C;Toronto, -60C;Marquette, – 8.90 C; Moscow, -9.90C; Calgary, -10.40 C; Vladivostok, -150C; Goose Bay, -17.80C;and Winnipeg, – 180C. Thus, it is evident that at these stations there is little difference in the average temperatures for the hottest month. Within these selected stations this difference is only 4.20C. On the contrary, there is greater variation in the average temperatures for the coldest month (January). Within these selected stations this difference is 150C. Therefore, July witnesses less steep temperature gradients than January (Figure 4).

Figure 4: Mean Monthly Temperature at Selected Stations

Source: Frederick K.L and Edward, J.T. (2010), P-440, 486 and 487.

Heat waves are also common during summer season but they are not as prolonged as in Dfa climate. In winter season cold spells are frequent and last longer than in Dfa climate. During these cold spells subzero temperatures are common. The annual range of temperature is high. It is on an average 250C to 400C.The highly dynamic daily weather condition is another important characteristic feature of this climate. Like Dfa climate in this climate also non-periodic temperature fluctuations are common and temperature changes within a short span of time. Within a day many places register a change of temperature of about 200 C. In the western part of Europe, due to marine influence the winters are milder and summers are cool.

Precipitation: In humid continental climate with warm summers the precipitation tends to be moderate in amount. It is mostly concentrated in summer.This climate region receives relatively less precipitation than Dfa climate. Precipitation tends to decrease from the coastal region towards continental interiors. For instance, in Canada, precipitation received at Montreal, Toronto, Winnipeg, Calgary and Edmonton is 98, 79, 51, 46 and 43 cm, respectively. Precipitation also maintains its declining trend towards the pole. The spatio-temporal variations of precipitation are common in this climate (Figure 5).However, the average annual precipitation is in the range of 30 to 80 cm.

Figure 5: Mean MonthlyPrecipitation at Selected Stations

Source: Frederick K.L and Edward, J.T. (2010), P-440, 486 and 487.

Precipitation is distributed throughout the year but summer season maxima are common.The convectional precipitation of summer is far less as compared to Dfa climate. In the form of precipitation rainfall dominates in summer and snowfall in winter. Winter precipitation is associated with fronts and cyclones. Although this climate receives lesser precipitation as compared to Dfa climate but it receives greater snowfall. The snow cover is also thicker and lasts for more than 3 months.

Natural Vegetation:In this humid continental mild summer climate or cold snowy-forest climate the humid regions have forests as natural vegetation. Towards the polar margins as winter is longer and more severe conifer trees dominate. The major species in these areas are fir, pine and spruce. In southern parts of this climatic region along with conifer, mixed broadleaf deciduous forests are common.

Boreal or Subarctic Climate or Taiga Climate

Distribution: The subarctic climate is found only in North America and Eurasia between 500 or 550 N to 650 or 700N latitudes. These two continents have large land mass in the higher middle latitudes to experience this harsh or extreme climate. It covers broad, uninterrupted expanses in North America from western Alaska to Newfoundland and in Eurasia from Norway to the Pacific coast of Russia. Taiga term is used for the subarctic lands by the Russians. This term is now used in North America as well as in Eurasia for this climate.

Northward,taiga climate (Dfc) merges into polar tundra climate (ET).Therefore, the poleward limit of this climate is determined by the 100C isotherm of mean temperature of the warmest month. On its southern margin, this climate grades into Dfb climate. The marine west coast climate (Cfb) prevails in the western side. In North America, the western segment of the taiga climate extends a little beyond the Arctic Circle. On the contrary, it extends farther southwards in the eastern sides of both North America and Eurasia continents.

As compared to North America, the distribution of taiga is more wide spread in Eurasia. The Eurasian taiga climate is more harsh or extreme. It is mainly due to the following two factors –

(i) continentality – as the size of Eurasian continent is much larger, the effect of continentality is higher; (ii) further the effect of continentality is reduced in North America by the presence of vast water body of the Hudson Bay in Canada.

Temperature: The taiga regions in the higher middle latitudes of North America and Eurasia continents are the major source regions for the continental polar (cP) air masses.The outstanding feature is the dominance of winter. The continental situationoftaiga climate is ideal for intense heating and cooling of the land. Due to its high-latitudinal situation, the winter cooling is more intense than summer heating and it operates for longer duration. The length of winter season is about 8 months. Therefore, these climates have long and bitterly cold winters, whereas summers are very short and cool,and extent for only one to three months (Figure 6).

Figure 6: Mean Monthly Temperature at Selected Stations

Source: Frederick K.L and Edward, J.T. (2010), P-444, 486 and 487.

The average annual temperatures are very low in taiga climate. For instance, it is -3.30 C at Fairbanks, -50 C at Yukon, -100 C at Yakutsk and -15.20C at Verkhoyansk. Mean temperature of coldest month(January) for a number of inland places may give an idea of the winter conditions in this climate: Fairbanks, Alaska, -23.90 C; Eagle, Alaska -260C; Dawson, Canada, -30.60C; Okhotsk, Russia, -240C; Yakutsk, Russia, -430C and Verkhoyansk, Russia, -46.80 C.

The Siberian taiga climate has the lowest recorded minimum winter temperatures outside the ice caps of Greenland and Antarctica. In fact, for many years the world’s coldest temperature was attributed to Verkhoyansk in east-central Siberia, where the temperature dropped to – 680 C on February 5 and 7, 1892. Over period of about two decades, this place had an average monthly minimum of – 620 C during January. Although, exceptional, these temperatures reflect the severe cold that prevails in the taiga winter.The lowest temperature ever recorded in North America subarctic region is -62.80 C at Snag, Yukon. In Alaska and Canada, winters are relatively less severe because of the lesser continentality and the moderating influence of the Hudson Bay.

The long winter nights in taiga region further drop the already low temperatures. Besides, long lasting snow cover because of high albedo reflects the small amount of radiation that reaches it and negligible is absorbed. The cooling effect of long duration snow surface results into development of high atmospheric pressure conditions.Further, the extremely low temperatures of subarctic region during the long winter months cause vast areas to be permanently frozen to great depths. This is known as ‘permafrost’.

In contrast, taiga summers are significantly warm, despite their short duration. During summer season, the temperature rises rapidly. July, the warmest month, has an average temperature around 160C. In June and August, the mean monthly temperatures are in therange of 100 to 150 C. In the months of May and September this range is between 40 and 80C.

The longer duration of sunshine compensates the shortness of the summer.At 600N latitude, the length of the days during summer solstice is 18.8 hours and it further increases poleward. In fact, at the time of summer solstice the amount of insolation received at the surface at latitude 600N and at equator is almost same.The long summer days allow large quantities of heat and light to reach subarctic surface. Despite the long hours of daylight, the Sun never rises very high in the sky, so the solar radiation is not intense. During this time daily maximum of 280 C or above is common.

Summer days in this climate are generally warm or cool. Due to severe weather conditions the growing season in this climate is very short ranging from 50 to 75 days. Midsummer frosts are also frequent in the subarctic region.

The extremely cold winters and relatively warm summers of the taiga combine to produce the greatest annual temperature ranges on earth. The annual ranges of temperature in this climate are very large. Yakutsk holds the distinction of having the greatest average temperature range in the world, 630C. In North America, this range on maximum side is about 450C. The North American subarctic is relatively less severe. Diurnal ranges of temperature in this climate are also high.

Precipitation: This climate prevails in northerly continental interiors which are the source regions of continental polar (cP) air masses. Therefore, only limited moisture is available throughout the year. The Taiga climate is characterized by meager precipitation, seldom exceeding 50 cm. In Siberia, the precipitation averages about 38 cm, while in Canada it varies from 35 cm to 50 cm. However, most of the precipitation is received during summer (Figure 7). At this time the polar anticyclone is weaker and atmospheric instability is induced by increased moisture holding capacity of the air due to high temperatures.

There are several factors responsible for the meager precipitation in the Taiga region: (1) low temperature conditions reduce the moisture-holding capacity of the air. (2) Interior or leeward locations of the continentskeep them mostly away from the maritime influences. (3) the most dominant factor for stability is the influence of the polar high pressure system. By far the greatest precipitation comes as rain from scattered summer convectional and frontal showers.

The Taiga region receives lesser snowfall as compared to other humid continental climates, yet there is illusion of more. The reasons for this illusion are – (i) as no melting takes place for months at a time, so the entire winter season snow surface of upto 1 meter is visible all at once; and (ii) during blizzards, high velocity winds swirl the dry, powdery snow into high drifts, givinga wrong notion of more snow fall than actual.Extremely low temperatures cause the winter precipitation to fall as fine, dry snow. Snow cover stays on the ground for a long period of 7 or 8 months.

Figure 7: Mean MonthlyPrecipitation at Selected Stations

Source: Frederick K.L and Edward, J.T. (2010), P-444, 486 and 487.

   It is noteworthy that because of low temperature even the meager amount of precipitation is more than what can be evaporated. Therefore, the subarctic regions are humid enough for forest growth.

Natural Vegetation: Coniferous forest is the natural vegetation of the sub-arctic climate. In Russia this forest is called Taiga which means ‘snow-forest’. These needle-leaf coniferous forests are composed of evergreen spruce, fir and pine. These forests are valuable for the supply of fire wood and wood pulp. In the central and eastern Siberia, the boreal forest is dominated by the larch, which sheds its needles in winter and is thus a deciduous tree. Associated with the needleleaf trees are stands of aspen, balsam poplar, willow and birch. Along the northern fringe of boreal forest lies a zone of woodland in which low trees, such as black spruce, are spaced widely apart. The open areas are covered by a surface layer of lichens and mosses. Taiga is not found beyond the 100C isotherm for the warmest month.This isotherm also represents the tree line i.e. the poleward limit of trees distribution. Thus the northern extent of the Taiga climate is the poleward limit of forest growth as well. Although scrawny, the spruce, pine, fir, larch and birch trees in Snowy-Forest (Taiga) or boreal represent the largest stretch of continuous forest on earth.

Summary and Conclusions

Humid microthermal climates are climates of mid-latitude and adjoining high-latitude regions. Moist continental climate and Boreal forest climate are the two major types of climates within humid microthermal group. Moist continental climate includes – (i) Humid continental hot summer climate or Cold Snowy-Forest climate, hot summer (Dfa) and (ii) Humid continental mild summer climate or Cold Snowy-Forest climate, warm summer (Dfb). These climates prevail in North America and Eurasia mainly in the latitudinal range of 300 to 550 (Europe 450 to 600). It means they prevail over central and eastern parts of North America and in mid latitudinal zone of Eurasia.

The climate Dfa is located mainly south of the Dfb climate. In both, seasonal contrasts are strongly established. The day to day weather conditions are highly variable. Precipitation is distributed throughout the year with maxima in summer. In North America, the maritime tropical air mass is more active and in case of Europe, maritime polar air masses coming under the influence of westerlies from the North Atlantic Ocean play the more significant role. In North America,from eastern coast to central part, climate changes from per-humid in coastal areas, to humid in Lake Region and to sub-humid in central part (95 to 100 E). In case of Europe it becomes humid to sub-humid from west to east towards the interiors. In Asia (north China, North and South Korea and Japan) it remains dominantly humid.

Boreal Forest Climate or Taiga or Cold Snowy-Forest climate, cool summers (Dfc) is the most extreme type of microthermal climates. The poleward limit of this is set by the 100 C isotherm of the average temperature of the warmest month. This climate is distributed across the continents in North America and Eurasia in the latitudinal belt of 500 – 700 N. Winter in this climate extends for over 8 months and permafrost conditions prevail. Although much of the boreal forest climate is humid, with precipitation of 50 – 100 cm, large areas in Western Canada and Siberia have annual precipitation totals of less than 40 cm. The precipitation is cyclonic and frontal in nature. The annual range of temperature is very high due to continentality. Extremely low temperatures cause the winter precipitation to fall as fine and dry snow. The surface and trees remain snow covered for majority part of the year. This climate is associated with conifer vegetation. The major tree species belong to fir, spruce and pine. In eastern part of Siberia, winter season remains stable and negligible precipitation in received in this season. This part climate is identified as Cold Snowy-Forest climate, winter dry (Dw). In this part of Siberia, the Taiga forest is dominated by larch trees, which shed their needles in winter.

References

• 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.

Web Links

• http://astro.wsu.edu/worthey/earth/html/im-atmosphere/climate-zone-de.gif