Koppen and Thornthwaite Classification on climate change || Climatology

Koppen and Thornthwaite Classification on climate change

1. INTRODUCTION
Climate change
Climate change, periodic modification of Earth’s climate brought about as a result of
changes in the atmosphere as well as interactions between the atmosphere and various other
geologic, chemical, biological, and geographic factors within the Earth system.
The atmosphere is a dynamic fluid that is continually in motion. Both its physical properties
and its rate and direction of motion are influenced by a variety of factors, including solar
radiation, the geographic position of continents, ocean currents, the location and orientation
of mountain ranges, atmospheric chemistry, and vegetation growing on the land surface. All
these factors change through time. Some factors, such as the distribution of heat within
the oceans, atmospheric chemistry, and surface vegetation, change at very short timescales.
Others, such as the position of continents and the location and height of mountain ranges,
change over very long timescales. Therefore, climate, which results from the physical
properties and motion of the atmosphere, varies at every conceivable timescale.
Climate is often defined loosely as the average weather at a particular place, incorporating
such features as temperature, precipitation, humidity, and windiness. A more specific
definition would state that climate is the mean state and variability of these features over
some extended time period. Both definitions acknowledge that the weather is always
changing, owing to instabilities in the atmosphere. And as weather varies from day to day, so
too does climate vary, from daily day-and-night cycles up to periods of geologic
time hundreds of millions of years long. In a very real sense, climate variation is
a redundant expression—climate is always varying. No two years are exactly alike, nor are
any two decades, any two centuries, or any two millennia.
Köppen climate classification
Köppen climate classification, widely used, vegetation-based, empirical climate
classification system developed by German botanist-climatologist Wladimir Köppen. His aim
was to devise formulas that would define climatic boundaries in such a way as to correspond
to those of the vegetation zones (biomes) that were being mapped for the first time during his
lifetime. Köppen published his first scheme in 1900 and a revised version in 1918. He
continued to revise his system of classification until his death in 1940. Other climatologists
have modified portions of Köppen’s procedure on the basis of their experience in various
parts of the world.
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2. Fig 1. The major climatic types are based on patterns of average precipitation, average
temperature, and natural vegetation. This map depicts the world distribution of climate types
based on the classification originally invented by Wladimir Köppen in 1900. 
System
Köppen’s classification is based on a subdivision of terrestrial climates into five major types,
which are represented by the capital letters A, B, C, D, and E. Each of these climate types
except for B is defined by temperature criteria. Type B designates climates in which the
controlling factor on vegetation is dryness (rather than coldness). Aridity is not a matter
of precipitation alone but is defined by the relationship between the precipitation input to
the soil in which the plants grow and the evaporative losses. Since evaporation is difficult to
evaluate and is not a conventional measurement at meteorological stations, Köppen was
forced to substitute a formula that identifies aridity in terms of a temperature-precipitation
index (that is, evaporation is assumed to be controlled by temperature). Dry climates are
divided into arid (BW) and semiarid (BS) subtypes, and each may be differentiated further by
adding a third code, h for warm and k for cold.
As noted above, temperature defines the other four major climate types. These are
subdivided, with additional letters again used to designate the various subtypes. Type A
climates (the warmest) are differentiated on the basis of the seasonality of precipitation: Af
(no dry season), Am (short dry season), or Aw (winter dry season). Type E climates (the
coldest) are conventionally separated into tundra (ET) and snow/ice climates (EF). The mid-
latitude C and D climates are given a second letter, f (no dry season), w (winter dry), or s
(summer dry), and a third symbol (a, b, c, or d [the last subclass exists only for D climates]),
indicating the warmth of the summer or the coldness of the winter.
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3. Although Köppen’s classification did not consider the uniqueness of highland
climate regions, the highland climate category, or H climate, is sometimes added to climate
classification systems to account for elevations above 1,500 metres (about 4,900 feet).
The Köppen classification has been criticized on many grounds. It has been argued that
extreme events, such as a periodic drought or an unusual cold spell, are just as significant in
controlling vegetation distributions as the mean conditions upon which Köppen’s scheme is
based. It also has been pointed out that factors other than those used in the classification, such
as sunshine and wind, are important to vegetation. Moreover, it has been contended that
natural vegetation can respond only slowly to environmental change, so that the vegetation
zones observable today are in part adjusted to past climates. Many critics have drawn
attention to the rather poor correspondence between the Köppen zones and the observed
vegetation distribution in many areas of the world. In spite of these and other limitations, the
Köppen system remains the most popular climatic classification in use today.
World distribution of major climatic types
The following discussion of the climates of the world is based on groupings of Köppen’s
climatic types. It should be noted that the highland climate (H) is also included here.
Type A climates
Köppen’s A climates are found in a nearly unbroken belt around the Earth at low latitudes,
mostly within 15° N and S. Their location within a region in which available net solar
radiation is large and relatively constant from month to month ensures both
high temperatures (generally in excess of 18 °C [64 °F]) and a virtual absence of
thermal seasons. Typically, the temperature difference between day and night is greater than
that between the warmest and the coolest month, the opposite of the situation in mid-
latitudes. The terms winter and summer have little meaning, but in many locations annual
rhythm is provided by the occurrence of wet and dry seasons. Type A climates are controlled
mainly by the seasonal fluctuations of the trade winds, the intertropical convergence
zone (ITCZ), and the Asian monsoon. Köppen specifies three A climates:
Wet equatorial climate (Af)
Tropical monsoon and trade-wind littoral climate (Am)
Tropical wet-dry climate (Aw)
Type B climates
Arid and semiarid climates cover about a quarter of Earth’s land surface, mostly between 50°
N and 50° S, but they are mainly found in the 15–30° latitude belt in both hemispheres. They
exhibit low precipitation, great variability in precipitation from year to year, low relative
humidity, high evaporation rates (when water is available), clear skies, and intense solar
radiation. Köppen’s classification recognizes three B climates:
Tropical and subtropical desert climate (BWh, part of BWk)
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4. Mid-latitude steppe and desert climate (BSh)
Tropical and subtropical steppe climate (BSk, part of BWk)
Type   C   and   D   climates
Through a major portion of the middle and high latitudes (mostly from 25° to 70° N and S)
lies a group of climates classified within the Köppen scheme as C and D types. Most of these
regions lie beneath the upper-level, mid-latitude westerlies throughout the year, and it is in
the seasonal variations in location and intensity of these winds and their associated features
that the explanation of their climatic character must be sought. During summer, the polar
front and its jet stream move poleward, and air masses of tropical origin are able to extend to
high latitudes. During winter, as the circulation moves equatorward, tropical air retreats and
cold polar outbreaks influence weather, even within the subtropical zone. The relative
frequency of these air masses of different origins varies gradually from low to high latitude
and is largely responsible for the observed temperature change across the belt (which is most
marked in winter). The air masses interact in the frontal systems commonly found embedded
within the traveling cyclones that lie beneath the polar-front jet stream. Ascent induced
by convergence into these low-pressure cells and by uplift at fronts induces precipitation, the
main location of which shifts with the seasonal circulation cycle. Other important sources of
precipitation are convection, mainly in tropical air, and forced uplift at mountain barriers.
Monsoon effects modify this general pattern, while the subtropical anticyclone plays a role in
the explanation of climate on the western sides of the continents in the subtropics. Köppen’s
classification identifies six C climates and eight D climates:
Humid subtropical climate (Cfa, Cwa)
Mediterranean climate (Csa, Csb)
Marine west coast climate (Cfb, Cfc)
Humid continental climate (Dfa, Dfb, Dwa, Dwb)
Continental subarctic climate (Dfc, Dfd, Dwc, Dwd)
Type   E   and   H   climates
Köppen’s type E climates are controlled by the polar and arctic air masses of high latitudes
(60° N and S and higher). These climates are characterized by low temperatures and
precipitation and by a surprisingly great diversity of subtypes. In contrast, type H climate
contains all highland areas not easily categorized by other climate types. Although this
category was not part of Köppen’s original system, some later climate systems include it as
part of Köppen’s climate classification. Köppen’s two E climates and the H climate are listed
below:
Tundra climate (ET)
Snow and ice climate (EF)
Highland climate (H)
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5. Thornthwaite Climatic Classification
C. W. Thornthwaite, an American climatologist, presented his first scheme of classification
of climates of North America in 1931 when he published the climatic 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. In his 1948 concept; gave the potential evapotranspiration
concept. His scheme is complex and empirical in nature.
In 1931, his classification looked similar to Koeppen. Like Koeppen, Thornthwaite also
thought that vegetation is the indicator of climate type.
Two basic features of this classification are
1.Precipitation Effectiveness; (P/E, where P is the total monthly precipitation and E is
the total monthly evaporation)
2.Temperature Efficiency.
On the basis of these two indicators, Thornthwaite divided the world into five humidity
regions.
A: Very Humid Rain Forest
B: Humid Forest
C: Semi Humid Grassland
D: Semi-Dry Steppe
E: Dry Desert
Each region had its own special type of vegetation as shown in the table below:
Fig 2. Humidity region
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6. Design of Thornthwaite Climatic Classification
Thornthwaite’s design of climate classification is a combination of three letter alphabets.
1.The first alphabet used in the major climatic classification is any one of the English
capital letters from A to E.
2.The second letter used in the climatic classification is also an English capital
alphabet superscript with a dash. It denotes thermal provinces.
3.The third letter in a combination of alphabets is denoted by a set of 8 small
English alphabets.
Precipitation effectiveness
Plants’ growth is not only dependent on precipitation but precipitation effectiveness.
Precipitation effectiveness P/E ratio=total monthly precipitation/ Evapotranspiration P/E
index= sum of 12 month P/E ratio.
Based on the P/E index, Thornthwaite classified five humidity region:
A: ( P/E index>128) – Wet-Rainforest.
B: ( P/E Index 64 to 127) – Humid-Forest
C: ( P/E index 32 to 63) – Subhumid-Grassland.
D: (P/E index 16-32) – Semi Arid-Steppe
E: (P/E index less than 16) – Arid-Desert
On the basis of precipitation effectiveness, thermal efficiency, and seasonal distribution of
rainfall there may be 120 probable combinations and hence climatic types on the
theoretical ground but he depicted only 32 climatic types on the world.
On the basis of the distribution of seasonal rainfall the above types of humidity
regions were further divided into the following subdivisions:
r = Heavy rainfall in all seasons
s = Scarcity of rainfall in the summer season
w = Scarcity of rainfall in the winter season
d = Scarcity of rainfall in all seasons
Aridity index for humid climates
oMoisture deficit acute during winter = w2
oMoisture deficit acute during summer =s2
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7. Humidity index for arid climates
oMoisture surplus abundant during winter =s2
oMoisture surplus abundant during summer = w2
Fig 3. Humidity region based on P/E index
Temperature efficiency
Temperature efficiency is calculated mean average temperature of through years.
Based on Temperature efficiency – Thornthwaite has divided the world into six thermal
provinces. They are expressed as:
A’ — tropical: (T/E index more than 128).
B’ — Subtropical: (T/E index 64-127).
C’ — Temperate: (T/E index 32 – 63)
D‘ — Taiga: (T/E index 16-31)
E’ — Tundra: (T/E index 1-15).
F’ — Frost: (T/E index 0).
Thornthwait was being criticized for making climatic classification complex. To make it
simple,  Thornthwait gave the evapotranspiration concept to derive a climatic region in 1948.
Evapotranspiration: Combined, evaporation from the soil and transpiration from vegetation is
called Evapotranspiration.
The modified Thornthwaite system  (1948) is based on the concept of potential
evapotranspiration (Potential ET), which approximates the water use of plants with an
unlimited water supply.
Though he again used previously devised three indices of precipitation effectiveness, thermal
efficiency, and seasonal distribution of precipitation in his second classification but in a
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).
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8. 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.
Index in modified method
Aridity Index (Ia)
Humidity Index (Ih)
Soil Moisture Index (Im)
if: PET>Precipitation =SoilMoisture0/-ve
if: Precipitation > PET  = Soil Moisture +ve
Moist Climate determined by Aridity Index (variability in summer and winter)
Dry climate determined by Moisture Index
Fig 4. Aridity index
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9. Fig 5. Thermal Efficiency Index
Fig 6. Moisture Index
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10. Fig 7. Climatic type based on Thornthwaite moisture index
Criticism of the Thornthwaite Climatic Classification
1.Thornthwaite’s classification of world climates is improved qualitatively.
However, classification seems to have ignored the role of prevailing winds,
relative humidity, air pressure, and air masses.
2.The classification system has proved most satisfactory in the case of North
America where vegetation boundaries nearly coincide with particular P/E values. But
it is not satisfactory for tropical and semiarid areas.
3.The calculation of soil moisture balance for different natural regions and
vegetation zones poses a basic problem. Several combinations at local and regional
levels increase complexity obscuring the clarity of classification.
4.Availability of data for all the meteorological variables over time and space is a
serious problem.
5.Despite being an improved classification qualitatively, it is being less used and of
limited application because of its complex nature.
6.The classification of climate seems to have ignored the role of relief, the position
of the sun with reference to the incidence of solar radiation on the earth.
7.Current issues of global warming, climate change, and increasing incidence of
extreme events do not find a place in Thornwaite’s classification of world climates.
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11. Thornthwaite climatic division of India
The following are the climatic division of India as per the Thornthwaite concept of
Evapotranspiration.
Per Humid( A) region of India:
Western Ghats
Most parts of the NorthEastern States
Humid(B) region of India:
Adjoining region of the Perhumid region
Moist Sub Humid(C1) climatic region
Narrow belt Adjoining region of the humid region of Western Ghats.
Eastern India comprises of West Bengal and Orissa
Dry Sub Humid(C2) regions:
Northern Narrow belt of the Ganga basin.
Part of Uttar Pradesh, Bihar, MP, Chhattisgarh, Jharkhand
Western Maharastra and Southern Gujarat
Semi-Arid(D) climatic region:
Part of Punjab and Haryana
Eastern part of Rajasthan, Maharashtra, Karnataka, Lenangna
Western Pat of Tamilnadu.
Arid climatic( E) region of India:
Western Rajasthan
Western Himalayan 
Rainshadow zone of western Ghats
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