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In this article we will discuss about the Koeppen’s, Thornthwaite and G.T. Trewartha classification of climate.
Classification by Koeppen’s:
The German botanist and climatologist Wladimir Koeppen presented his descriptive scheme of the classification of world climates first in 1900 based on vegetation zones of French plant physiologist Candolle presented in 1874. He revised his scheme in the year 1918 wherein he paid more attention to monthly and annual averages of temperature and precipitation and their seasonal distribution. He again modified his scheme in 1931 and 1936.
Koeppen’s original scheme was modified in 1953 by Geigger-Pohi and the revised scheme known as Koeppen-Geigger-Pohl’s scheme of classification of world climates was published. It may be pointed out that the classification of Koeppen is more popular because it is quantitative in nature as numerical values of temperature and precipitation have been used in delineation of boundaries of different climatic types.
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The climates have been named on the basis of alphabets and climates have been determined on the basis of formulae and hence the classification has become difficult to memorise because each alphabet has definite and specific meaning.
Koeppen used five major vegetation zones of the world as identified by Candolle in 1874 (e.g., megatherms, xerophytes, mesotherms, microtherms, and hekistotherms) as the basis of classification of world climates on the belief that the distribution of natural vegetation was the best indicator of the total picture of climate of a region concerned.
Based on these five vegetation zones he divided the world climates into 5 principal types and designated them by capital letters A, B, C, D, and E:
(1) A Climate:
Represents humid tropical climates characterized by winterless season, warm and moist conditions throughout the year and mean temperature always above 18°C.
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(2) B Climate:
Represents dry climates where evaporation exceeds precipitation and there is constant water deficit throughout the year.
(3) C Climate:
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Represents humid mesotherm-mal or middle latitudes warm temperate climates having mild winters, average temperature of the coldest and warmest months between 8° and 18°, and below 18°C respectively.
(4) D Climate:
Includes humid micro-thermal or cold forest climates characterized by severe winters, average temperatures of coldest and warmest months being 3°C and above 10°C respectively.
(5) E Climate:
Includes polar climates characterized by summer less season, average temperature of the warmest month below 10°C.
Besides these capital letters, Koeppen has used the following small (lower) letters in his scheme for specific meaning:
f = precipitation throughout the year, average temperatures of the coldest month being more than 18°C, minimum precipitation of 6cm in every month of a year.
m = monsoon climate, short dry season, average precipitation in driest month less than 6cm.
w = winter dry season.
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S = well defined summer dry season.
Koeppen has divided 5 major climatic types into 11 subtypes on the basis of seasonal regimes of precipitation and nature of aridity and coldness.
(1) Tropical Rainy Climates (A Climates):
A or tropical rainy climate is that where the temperature of the coldest month is above 18°C.
On the basis of periodicity and regime of precipitation this type has been further divided into 4 types:
(i) Af climate:
Humid tropical climate, precipitation in the driest month more than 6cm, seasonal distribution of precipitation more or less uniform throughout the year, very low daily range of temperature.
(ii) Aw climate:
tropical humid and dry climate, winter dry season (w), precipitation of at least one month less than 6cm, high temperature throughout the year.
(iii) Am climate:
monsoon climate, one short dry season but sufficient annual precipitation and thus wet ground throughout the year, dense forest, precipitation of at least one month less than 6cm.
The boundary between Aw and Am climates is demarcated on the basis of annual precipitation and the precipitation of the driest moth as per formula given below:
a = 3.94-r/25
where a = precipitation of driest month
r = annual precipitation
If the precipitation of the driest month of a place is less than the value of a, it will be Aw climate, if it is more than the value of a, it will be Am climate.
Example = If the annual precipitation of a place is 50 inches, then Am/Aw boundary would be = 3.94 – 50/25 = 1.94 inches. If the precipitation of the driest month of that place is 2 inches (this should be always less than 2.4 inches, otherwise it would be Af climate), it would be Am climate. On the other hand, if the precipitation of the driest month of that place is 1.8 inches, it would be Aw climate.
(iv) As climate-dry summers, rarely found:
The aforesaid Af, Am and Aw climatic types as identified by Koeppen are generally similar to equatorial rainforest climate (Af), monsoon climate (Am), and savanna climate (Aw) respectively.
Koeppen has further identified finer details in A climates and has used the following lower letters to indicate them:
w’ maximum precipitation in autumn
w” two seasons of maximum precipitation separated by two dry seasons
s dry summers
i difference of temperature of the warmest and the coldest month less than 5°C.
g hottest season preceding precipitation
(2) Dry climates (B climates):
Evaporation exceeds precipitation, precipitation not sufficient to maintain permanent stable water table of ground water.
B climates are divided into two types on the basis of annual temperature and the rainiest month of the year e.g.:
(i) Dry desert climate (BW), and
(ii) Semi-arid or steppe climate (BS).
The boundary between BW and BS climates is determined on the basis of the following formula:
r =0.44t – 8.5 /2
Where
r = annual precipitation (inches)
t = temperature (O0F)
If the annual precipitation of a given place is more than the value of r, the climate of that place will be BS but if it is less then r, the climate will be B W.
For example, if the temperature of a place is 80°F, the annual value of precipitation for dividing boundary between BS and BW climates will be 13.3 inches as given below:
r = 0.44 × 80 – 8.5/2 = 13.3 inches
B climates are further differentiated on the basis of annual temperature. When the mean annual temperature is more than 18°C (64.4°F), the climate is indicated by h letter but if the mean annual temperature is less than 18°C, it is indicated by k letter.
Thus, B climates are divided into the following four types:
(i) BWh tropical desert climate, average annual temperature more than 18°C (64.4°F)
(ii) BSh tropical steppe climate, mean annual temperature above 18°C
(iii) BWk middle latitude cold desert climate, mean annual temperature below 18°C
(iv) BSk middle latitude cold steppe climate, mean annual temperature below 18°C
Koeppen has identified further details in B climates and has used the following letters to indicate them:
k mean annual temperature below 18°C
h mean annual temperature above 18°C
a summer dry, three times more precipitation in the wettest month of winter season than the driest month of summer season
w winter dry, 10 times more precipitation in the wettest month of summer season than the driest month of winter season
n minimum fog
(3) Humid mesothermal or warm temperate rainy climates (C climates):
Average temperature of the coldest month above 3°C but below 13°C, precipitation in all seasons.
Based on seasonal distribution of precipitation C climates have been divided in to 3 climatic types:
(i) Cf climate:
precipitation throughout the year, precipitation more than 1.2 inches in the driest month of summer season. This climate represents Western Europe type of climate. This is further divided into two second order sub-divisions e.g. Cfa (humid subtropical) and Cfb (marine west coast type)
(ii) Cw climate:
Dry winters, 10 times more precipitation in the wettest month of summer season than the driest month of winter season. This represents China type of climate.
(iii) Ca climate:
Dry summers, three times more precipitation in the wettest month of winter season than the driest month of summer season, precipitation of the driest month of summer season less than 1.2 inches. This represents Mediterranean type of climate.
Koeppen has identified further minor details in C climates and has used a few explanatory small letters as given below:
a warm summers, temperature of the warmest month above 22°C (71.6°F).
b cold winter, temperature of the warmest month below 22°C.
c cold short summer season
i, n, g = as explained above.
(4) Humid microthermal or cold snow forest climates or humid cold climates (D climates):
Temperature of the coldest month below – 3°C (26.6°F) but of the warmest month above 10°C (50°F), ground surface covered with snow for several months of a year.
This climate has been divided into three types:
(i) Df Climate:
Humid cold climate, no dry season.
This is further divided into:
(a) Dfa (long warm summers, continental),
(b) Dfb (long and cool summers), and
(c) Dfc (short cool summer-subarctic).
(ii) Dw Climate:
Humid cold climate, dry winters, further divided into:
(a) Dwa-continental climate with long cool summer,
(b) Dwb- cool short summer (sub-arctic type), and
(c) Dwc-cold winters, d = temperature of the coldest month below – 38°C, f, a, w, b, c as explained above.
(5) Polar climates (E climates):
Temperature of the warmest month less than 10°C (50°F), further divided into:
(i) ET and
(ii) EF climates.
(i) ET climate-tundra climate, temperature of the warmest month below 10°C but above 0°C.
(ii) EF climate-permanent snow fields, temperature in all months below 0°C.
Evaluation of Koeppens’s Scheme:
Koeppen used two easily measurable weather elements e.g., temperature and precipitation as the basis for statistical parameters for the delineation of different climatic regions. In fact, temperature and precipitation are most widely and most frequently used effective weather elements as representatives of the effects of climatic controls. His scheme of climatic classification is primarily based on the relationship between floral types and their characteristics, and climatic characteristics of a given place or a region. He also paid due consideration to the loss of moisture through evaporation as he included effective precipitation, which depends on the rate of potential evapotranspiration, in his scheme.
It may be pointed out that it is not the total annual precipitation which matters more for vegetation community rather it is the effective precipitation (amount of precipitation which is actually available to plants) which is more important for flora. Koeppen’s scheme appealed more to geographers because the scheme recognized association between vegetation types and climatic types. Besides, this scheme is descriptive, generalized and simple and hence it was widely acclaimed.
Inspite of several merits as referred to above the Koeppen’s scheme also suffers from some serious drawbacks. Koeppen gave undue significance to mean monthly values of temperature and precipitation in his scheme of climatic classification and neglected other weather elements such as precipitation intensity, amount of cloudiness and number of rainy days, daily temperature extremes, winds etc.
He made his scheme more descriptive and generalized and ignored the consideration of causative factors of climate. He did not include the characteristics of different airmasses in his classification. The use of different letter symbols to indicate different climatic types and their secondary and tertiary subtypes makes the scheme very difficult to memorise.
Classification by Thornthwaite:
C. W. Thornthwaite, an American climatologist, presented his first scheme of classification of climates of North America in 1931 when he published the clamatic map of North America. Later he extended his scheme of climatic classification for world climates and presented his full scheme in 1933. He further modified his scheme and presented the revised second scheme of classification of world climates in 1948. His scheme is complex and empirical in nature.
(i) 1931 Classification:
Like Koeppen Thornthwaite also considered natural vegetation of a region as the indicator of climate of that region. He accepted the concept that the amount of precipitation and temperature had paramount control on vegetation but he also pleaded for inclusion of evaporation as important factor of vegetation and climate. This is why Thornthwaite used two factors, e.g. precipitation effectiveness and temperature effectiveness, for the delimitation of boundaries of different climatic regions.
(ii) Precipitation Effectiveness:
Precipitation effectiveness or precipitation efficiency refers to only that amount of total precipitation which is available for the growth of vegetation. He used precipitation efficiency ratio for the calculation of this amount of water available to vegetation.
Precipitation efficiency ratio (P/E ratio) is calculated by dividing total monthly precipitation by monthly evaporation and precipitation efficiency index (P/E index) is derived by summing the precipitation efficiency ratios for 12 months of a year.
Since it is difficult to obtain data of evaporation for every centre and hence Thornthwaite suggested the following formulae for the calculation of precipitation efficiency ratio and index:
Where:
r = mean monthly rainfall in inches
t = mean monthly temperature in °F
He identified 5 humidity zones on the basis of P/ E Index and boundary values for the major vegetation zones:
Thornthwaite further subdivided each humidity zone into 20 sub-humidity zones on the basis of seasonal distribution of precipitation:
where r = adequate rainfall in all seasons
s = rainfall deficient in summer
w = rainfall deficient in winter
d = rainfall deficient in all seasons
(ii) Thermal Effectiveness:
He believed that temperature had important contribution in the growth of vegetation.
He thus, devised an index of thermal efficiency or temperature effectiveness, expressed by positive departure of monthly mean temperatures from freezing point, and suggested the following formulae:
(i) Thermal Efficiency Ratio
(T- E Ratio) = (t-32) /4
(ii) Thermal Efficiency Index
where t – mean monthly temperature in °F. It is apparent that T-E Index is the sum of thermal efficiency ratios for 12 months.
On the basis of T-E index Thornthwaite divided the world into 6 temperature provinces:
Thus, on the basis of precipitation effectiveness, thermal efficiency, and seasonal distribution of rainfall there may be 120 probable combinations and hence climatic types on theoretical ground but he depicted only 32 climatic types on the world map as given below:
1. A A’r:
Tropical wet climate with rainfall adequate in all seasons
2. A B’r:
Mesothermal wet climate with adequate rainfall in all seasons
3. A C’r:
Microthermal wet climate with adequate rainfall in all seasons
4. B A’r:
Tropical humid climate with adequate rainfall in all seasons
5. B A’w:
Tropical humid climate with rainfall deficient in winter
6. B B’r:
Mesothermal humid climate with adequate rainfall in all seasons
7. B B’w:
Mesothermal humid climate with rain fall deficient in winter season.
8. B B’s:
Mesothermal humid climate with rain fall deficient in summer season.
9. B C’r:
Microthermal humid climate with adequate rainfall in all seasons.
10. BC’s:
Microthermal humid climate with rain fall deficient in summer season.
11. C A’r:
Tropical subhumid climate with adequate rainfall in all seasons.
12. C A’w:
Tropical subhumid climate with deficient rainfall in winter seasons.
13. C A’d:
Tropical sub-humid climate with rainfall deficient in all seasons.
14. C B’r:
Mesothermal subhumid climate with adequate rainfall in all seasons.
15. C B’w:
Mesothermal sub-humid climate with rainfall deficient in winter season.
16. C B’s:
Mesothermal sub-humid climate with rainfall deficient in summer season.
17. C B’d:
Mesothermal sub-humid climate with rainfall deficient in all seasons
18. C C’r:
Microthermal sub-humid climate with rainfall in all seasons.
19. C C’s:
Microthermal sub-humid climate with rainfall deficient in summer season.
20. C C’d:
Microthermal sub-humid climate with rainfall deficient in all seasons.
21. D A’w:
Tropical semiarid climate with deficient rainfall in winter season.
22. D A’d:
Tropical semiarid climate with rainfall deficient in all seasons.
23. D B’w:
Mesothermal semiarid climate with rain fall deficient in winter season.
24. DB’s:
Mesothermal semiarid climate with rain fall deficient in summer season.
25. D B’d:
Mesothermal semiarid climate with rainfall deficient in all seasons
26. D C’d:
Microthermal semiarid climate with rainfall deficient in all seasons
27. E A’d:
Tropical arid climate with rainfall deficient in all seasons.
28. E B’d:
Mesothermal arid climate with rainfall deficient in all seasons.
29. E C’d:
Microthermal arid climate with rainfall deficient in all seasons.
30. D’:
Taiga type climate
31. E’:
Tundra type climate
32. F’:
Permanently snow-covered polar climate.
(2) 1948 Classification:
After making sizeable modifications Thorthwaite presented his modified scheme of climatic classification in 1948. Though he again used previously devised three indices of precipitation effectiveness, thermal efficiency and seasonal distribution of precipitation in his second classification but in different way.
Instead of vegetation, as done in 1931 classification, he based his new scheme of climatic classification on the concept of potential evapotranspiration (PE) which is in fact an index of thermal efficiency and water loss because it represents the amount of transfer of both moisture and heat to the atmosphere from soils and vegetation (evaporation of liquid or solid water, and transpiration from living plant leaves) and thus is a function of energy received from the sun.
It may be pointed out that potential evapotranspiration is calculated (and not directly measured) from the mean monthly temperature (in °C) with corrections for day length (i.e., 12 hours).
The PE (Potential Evapotranspiration) for a 30-day month (a day having only the length of sunshine i.e. 12 hours) is calculated as follows:
PE (in cm) = 1.6(10t/I)a
where PE = Potential Evapotranspiration
I = the sum for 12 months of (t/5)1 .514
a = a further complex function of I
t = temperature in °C
Thornthwaite developed four indices to determine boundaries of different climatic types e.g.:
(i) Moisture index (Im),
(ii) Potential evapotranspiration or thermal efficiency index (PE),
(iii) Aridity and humidity indices, and
(iv) Index of concentration of thermal efficiency or potential evapotranspiration.
(i) Moisture Index (Im):
Moisture index refers to moisture deficit or surplus and is calculated according to the following formula:
Im = (100s – 60D)/PE
Where Im = monthly moisture index
S = monthly surplus of moisture
D = monthly deficit of moisture
The sum of the 12 monthly values of Im gives the annual moisture index.
(ii) Thermal Efficiency Index:
Thermal efficiency is simply the potential evapotranspiration expressed in centimetres as expressed above. It is, thus, apparent that the thermal efficiency is derived from the PE value because PE in itself is a function of temperature. The method of the calculation of PE is given above.
(iii) Aridity and Humidity Indices:
These indices are used to determine the seasonal distribution of moisture adequacy.
These are calculated as follows:
Aridity Index = in moist climates annual water deficit taken as a percentage of annual PE becomes aridity index.
Humidity Index = in dry climates annual water surplus taken as a percentage of annual PE becomes humidity index.
(iv) Concentration of Thermal Efficiency:
Concentration of thermal efficiency refers to the percentage of mean annual potential evapotranspiration (PE) accumulating in three summer months.
On the basis of moisture index (Im) Thornthwaite identified 9 moisture or humidity provinces:
On the basis of thermal efficiency (potential evapotranspiration) 9 thermal provinces were recognized:
On the basis of summer concentration of thermal efficiency the world was further divided into 8 provinces:
On the basis of seasonal moisture adequacy 2 major and 10 sub-climatic types were identified:
The climate of a place, thus, is determined by combining the aforesaid elements of the climatic classification e.g.. moisture index, thermal efficiency index, summer concentration of thermal efficiency, and seasonal moisture adequacy (aridity and humidity indices). Thus, the climate of a place is represented by four letters.
For example- A A’ a’r climate = Perhumid (A) megathermal (A’) climate with summer concentration of annual thermal efficiency (PE in cm) of less than 48 per cent (a’) and little or no water deficit (r) etc. On the basis of above indices the classification system becomes so complex due to large number of climatic types that it becomes difficult to represent them carto- graphically.
Evaluation of Thorthwaite’s Schemes:
In many aspects the 1931 classification scheme of Thornthwaite was almost similar to Koeppen’s scheme because both had a few common points e.g.:
(i) Like Koeppen’s scheme his scheme is also empirical as well as quantitative as the boundaries of different climates are determined on the basis of quantitative parameters derived from precipitation and temperature,
(ii) Vegetation is made as the basis for the identification of climatic zones, (iii) Various letter combinations are used to designate different climatic types etc.
The Thornthwaite’s scheme differs from the Koeppen’s scheme in that the former used two indices of precipitation efficiency and thermal efficiency for differentiation of different climatic types but the delimitation of climatic boundaries on the basis of these two indices becomes difficult and vague.
Moreover, the Thornthwaite’s scheme yielded the number of major climatic types (32) three times greater than Koeppen’s climatic types. Like Koeppen’s scheme Thornthwaite’s scheme also became popular among zoologists, botanists and geographers but it was not appreciated by meteorologists and climatologists because this scheme did not include the causative factors of climates into the classification of world climates in different types.
This scheme also suffers from a serious problem of non-availability of the data of evaporation for all the places. Thus, the lack of adequate climate data makes it difficult for the precise demarcation of climatic boundaries.
Though 1948 scheme of climatic classification of Thornthwaite was thoroughly revised and modified and was based on 4 important indices of moisture index, thermal efficiency or potential evapotranspiration index, seasonal moisture adequacy (aridity and humidity indices), and summer concentration of thermal efficiency but no world map of different climatic types could he prepared. It may he pointed out that it becomes very difficult to cartographically represent a large number of climatic types identified quantitatively on the basis of aforesaid indices.
Moreover, the data of potential evapotranspiration are not available for all the places for a worldwide classification of climates. The complex empirical formulae divised by Thornthwaite require regular data but these are not always forthcoming. They also involve a lot of calculations for determining the climatic type of a particular place. This is why his scheme could not get more popularity and recognition.
Classification by G.T. Trewartha:
G.T. Trewartha, an American climatologist, made several revisions and modifications in the scheme of climatic classification of Koeppen since 1930s and ultimately presented his simple scheme of climatic classification having a blending of both empirical and genetic schemes of classification of world climates.
In fact, Trewartha’s basic aim was to present a simple, generalized and unambiguous scheme of the classification of world climates so that the major climatic types at world level could be easily and realistically identified and cartographically represented on world map.
Thus, Trewartha’s scheme is a compromise between purely empirical and genetic methods of climatic classification. He was fully convinced that the schemes should not be cumbersome and complex as were the schemes of Kooppen and Thornthwaite.
He was also opposed to produce large number of climatic types on the basis of statistical and quantitative parameters. That is why he recognized only a limited number of major climatic types. According to him, if required, several second and third-order subdivisions may be added within each major climatic type. Like other scientists he also made precipitation and temperature as the basis for his scheme of climatic classification.
He identified 6 major climatic types of first order at world level and designated them as A,B,C,D,E,F climates out of which B climates were determined on precipitation criteria while others were determined on the basis of temperature criteria.
(1) Tropical Humid Climates (A Climates):
A climates or tropical humid climates are found in those low latitudes on either side of the equator which are characterized by high temperature and adequate rainfall throughout the year and absence of winter season.
On the basis of variations in precipitation A climates are subdivided into:
(i) Af,
(ii) Aw, and
(iii) Am climates.
(i) Af climate is tropical wet climate which extends up to 5° to 10° latitudes on either side of the equator and is characterized by adequate rainfall throughout the year. This is also known as tropical rainforest climate. There is no winter season as it is characterized by uniformly high temperature all the year round.
(ii) Aw climate is a tropical wet and dry climate characterized by uniformly high temperature throughout the year but there are more than two dry months. This climate is also known as savanna climate which is dominated by dry trade winds or subtropical anticyclones during winter season and by equatorial westerlies and inter-tropical convergence during summer season.
(iii) Am climate is monsoon climate which receives more than. 80 per cent of annual rainfall during four summer monsoon months.
(2) Dry Climates (B Climates):
The boundaries of B (dry) climates have been determined on the basis of precipitation variations. They extend from the outer boundary of A climates to the middle latitudes. B climates are characterized by high evaporation, loss of moisture through evapotranspiration exceeding the annual receipt of water gain from precipitation, large annual and daily ranges of temperature, extreme seasonal temperatures, very low and highly variable annual precipitation, extremely low relative humidity, abundant sunshine and clear sky.
On the basis of aridity and annual average precipitation B (dry) climates have been divided into two climatic types e.g.:
(i) Arid or desert climate- BW climate, and
(ii) Semi-arid or steppe climate- BS climate.
On the basis of temperature variations arid (B W) and semiarid (BS) climates have been divided into 4 climatic types as follows:
(i) BWh climate:
Tropical-subtropical hot desert climate.
(ii) BWk climate:
Middle latitudes or temperate and boreal cold dry climate.
(iii) BSh climate:
Tropical-subtropical steppe or semiarid climate.
(iv) BSk climate:
Middle latitudes or temperate and boreal steppe climate.
The boundary between hot dry and cold dry climates is determined on the basis of 32°F (0°C) isotherm of the coldest month. Tropical-subtropical dry (BWh) and steppe (BSh) climates are dominated by dry trade winds and subtropical anticyclones resulting into constant dry conditions.
At least 8 months of a year record average temperature above 10°C. On the other hand, temperate cold dry (BWk) and cold steppe (BSk) climates are located on the leeward sides of the mountains in the interior of the continents and are dominated by cold anticyclones during winter season.
(3) Middle Latitudes Wet Climates (C Climates):
The isotherm of 18°C of the coldest month forms the equator-ward boundary of C climates.
On the basis of seasonal distribution of precipitation C climates are divided into 3 types e.g.:
(i) Cs climate (subtropical sub-humid climate with dry summer, also known as Mediterranean climate),
(ii) Ca climate (subtropical humid climate), and
(iii) Cb climate (middle latitude marine climate).
Cs climate, located on the western sides of the continents on the tropical margins of the middle latitudes, is affected by subtropical anticyclonic conditions in summers and by wet westerlies in winters. Ca climate (Cfw of Koeppen), located on the eastern sides of the continents, receives precipitation in all seasons but summer months receive more rainfall than winter months (this climate is known as China type of climate). Cb climate is affected by westerlies throughout the year.
(4) Microthermal or Temperate Climates (D Climates):
These climates are found in the areas of high middle latitudes which are affected by westerlies in summers and by polar winds in winters. The poleward and equatorward boundaries are determined by average temperatures of 10°C for 4 months in the case of the former and for 6 months in the case of the latter.
On the basis of temperature variations ‘D’ climates have been divided into 4 types e.g.:
(i) Da climate (continental humid climate with temperature of the warmest month above 25°C),
(ii) Db climate (continental humid climate with temperature of the warmest month below 22°C),
(iii) Dc climate (subpolar climate, short summer season), and
(iv) Dd climate (temperature of the coldest month less than -38°C).
(5) Boreal Climate (E Climate):
Boreal climate is located in the higher middle latitudes and is characterized by short and cool summer season, long and very cold winter season, very short frost free season, one to three months of a year having average temperature of 10°C or more etc.
(6) Polar Climate (F Climate):
Summer season is absent. Polar winds dominate throughout the year. These climates are found in the northern hemisphere only. No month of the year records average temperature above 10°C.
On the basis of temperature variations ‘F’ climates are divided into:
(i) Tundra climate (Ft climate) and
(ii) Icecap climate (Ff climate).
Departures from Koeppen’s Classification:
The Trewartha’s scheme of climatic classification registers the following departures from the scheme of Koeppen:
(1) In B climates Koeppen used isotherm of 18°C average annual temperature to differentiate the boundary between hot dry and cold dry (h/k boundary) climates while Trewartha used an isotherm of 32°F (0°C) of the coldest month for the determination of h/k boundary.
(2) Koeppen used the isotherm of -3°C (26.6°F) temperature of the coldest month for determining boundary between B and C climates while Trewartha selected isotherm of 32°F (0°C) for the purpose.
(3) Koeppen divided C climates on the basis of seasonal distribution of precipitation into 3 types e.g:.
(i) Cs (summers dry),
(ii) Cw (winter dry), and
(iii) Cf (no dry season)
But Trewartha divided C climates into:
(i) Cs,
(ii) Ca, and
(iii) Cb types.
(4) Koeppen divided D climates on the basis of precipitation into:
(i) D w and
(ii) Df types
While Trewartha divided them on the basis of summer temperature into:
(i) Da,
(ii) Db, and
(iii) Dd types.
Evaluation:
As stated earlier Trewartha’s scheme of climatic classification is very simple, unambiguous and a mixture of both empirical and genetic methods of climatic classification. It uses only two weather elements i.e. precipitation and temperature and avoids vigorous statistical and mathematical calculations in determining climatic type of a place and demarcating boundaries between two different climatic types. This scheme also includes the effects of land and water surfaces on the climate of an area. Trewartha’s scheme became more popular among geographers because of its simplicity.