In this essay we will discuss about the methods used for classification of world climate.
Essay # 1. Koppen Classification:
Vladimir Koppen (1846-1940) related climate with vegetation.
His classification is based on:
a. Mean annual precipitation.
b. Mean monthly temperature.
He recognized five major categories of climate, each denoted by a capital letter (A, B. C, D, E) as indicated below:
A — Tropical rainy climates, hot all seasons.
B — Dry climate.
C — Warm temperate rainy climates, mild winters.
D — Cold forest climates, severe winters.
E — Polar climates.
A, C, D, E categories are based on temperature characteristics and the B on precipitation characteristics. Each of these categories has been subdivided on the basis of precipitation and temperature characteristics with small symbols.
Such subdivisions for Tropical rainy climates-A are indicated below:
Af— Tropical rainy climates (A), no dry season (J).
As — Tropical rainy climates (A), summer dry season (5).
Aw — Tropical rainy climates (A), winter dry season (w).
i. This method provides a scientific basis for classification which has been modified subsequently using more elements.
ii. This classification has the advantage of statistical manipulation and interpretation with respect to temperature and precipitation.
iii. The relationship between heat and moisture is well established in depicting the vegetative types.
iv. The climatic groups, subgroups and further sub-divisions are easy to follow through use of letters.
i. This method has not taken the effect of rainfall intensity and duration to depict the important vegetation zones of the world.
ii. It has not considered the effect of wind, cloudiness and daily temperature to avoid clumsiness in sub-grouping the elements.
Essay # 2. Thornthwaite Classification:
Thornthwaite (1931-33) modified the classification system of Koppen. He also considered that plant is the nature meteorological instrument for measuring climatic parameters.
Thornthwait introduced the following indices in his climatic classification:
i. Precipitation effectiveness index.
ii. Thermal efficiency index.
iii. Concept of potential evapotranspiration and water balance.
iv. Moisture adequacy index.
i. Precipitation Effectiveness Index:
He conceived that tropical rainforest grows most rapidly and has the densest vegetation because of constantly high temperature and abundant rainfall. As such, their efficiency must be maximum in rainforests.
Precipitation effectiveness decreases from maximum in tropical rainforest to the minimum in tropical deserts and the temperature efficiency reduces from maximum in tropical climate to minimum in climate of perpetual forest.
where, P is total monthly precipitation and E is total monthly evaporation. This ratio is summed over 12 months of the year and multiplied by 10 to avoid fractions.
On the basis of PIE ratio, he recognised 5 humid provinces and characteristic vegetations.
ii. Thermal Efficiency Index:
Calculation of thermal efficiency index (TIE ratio) is similar to that of P/E ratio as indicated above, except that T (monthly temperature) replaces P (monthly precipitation).
Based on TIE ratio, he recognized the following 6 vegetations:
iii. Concept of Potential Evapotranspiration and Water Balance:
In 1948, Tornthwaite introduced the concept of potential evapotranspiration (PET) and water balance (WB) to revise his classification of 1931. He developed the concept of thermal efficiency. According to him, thermal efficiency is PET (cm).
Thermal efficiency is an index of energy in terms of water depth. It can be derived as TIE (T is monthly average temperature and E is monthly evaporation). On the basis of annual thermal efficiency index and its summer concentration, 9 thermal provinces were recognised.
iv. Moisture Adequacy Index:
Thornthwaite revised his classification further in 1955, by introducing four more climatic criteria: moisture adequacy index (Im), thermal efficiency, humidity index (Ih,) and aridity index (la) based on PET and precipitation.
Most commonly used moisture adequacy index is given by:
The moisture regimes according to this revised classification are:
Essay # 3. Troll Classification:
Troll (1965) classified the tropical climate in the basis of humid months in a year. He defined humid month as the month having precipitation more than PET.
Troll classification has been used by ICRISAT as a workable system of defining the geographical extent of semiarid tropics (SAT). Using Troll system, areas having 2 to 7 humid months in the warm season (5 to 10 arid months) are classified as semiarid.
Two further subclasses are proposed : dry savanna belt having 4.5 to 7 humid months, generally, referred to as wet-dry semiarid tropics and the thorn savanna belt with 2 to 4.5 humid months called dry tropical climate areas.
According to Troll approach, Bikaner and Jodhpur are classified as arid and other 8 stations (Ludhiana, New Delhi, Agra, Allahabad, Ahmedabad, Hyderabad, Mumbai and Sholapur) in India as semiarid. It is important to note that Mumbai, which receives a mean annual rainfall of 2,099 mm, is also classified as SAT.
The SAT of India as classified by Troll shows that most of the areas of Rajasthan, Gujarat and part of Madhya Pradesh are falling in 2 to 4.5 months humid zone dry SAT. This area constitutes 26 per cent of the total area of tropical India and represents 47 per cent of the SAT India as defined by Troll.
The wet-dry zone having 4.5 to 7 humid months forms a 200-500 km wide strip running from northwest to south of India around the dry semiarid zone. It constitutes 30 per cent of total area of tropical India and 53 per cent of semiarid area of the country.
The total SAT area works out to be 56 per cent of the total tropical India, a figure very close to the 54 per cent estimate made by economic group. The IMD in 1967 published 30 year (1931- 60) normals for over 200 meteorological stations. Monthly PET values were published in 1971.