Introduction
The purpose of any classification system is to obtain an efficient arrangement of information in a simplified and generalized form. Thus climatic statistics can be organized in order to describe and delimit the major types of climate is quantitative terms.
Obviously no single classification can serve more than a limited number of purposes of satisfactory and many different schemes have therefore been developed.
Some schemes merely provide a convenient nomenclature system, whereas others are an essential primarily further study.
Many climatic classifications, for instance are concerned with the relationship between climate and vegetation or soils but surprisingly few attempts have been made to base a classification on the direct effects of climate on man.
General classification related to plant growth / vegetation.
Numerous schemes have been suggested d for relating climatic limits to plant growth or vegetation growth rely basically or two basic criteria the degree of aridity and warmth.
Aridity is not simply a matter of low precipitation but of the effective precipitation i.e. precipitation minus evaporation.
The ratio of rainfall/temperature has been used as such an index of precipitation effectiveness or the ground that higher temperatures in cease evaporation.
W. KOPPEN CLASSFICATION
A prime example of this type of classification is W. Koppen published between 1900 and 1936. The key features of Koppen’s final classifications are temperature criteria and aridity criteria.
Temperature Criteria
Five of the six major climatic types are recognized of the basis of monthly mean temperature.
A tropical rainy climatic types are recognized or the basis of monthly mean temperature.
B) Dry months
C) Warm temperature rainy climates coldest month between –3C And 18C warmest month between 10C
D) Cold bored forest climates coldest month – 3C warmest month 10C
E) Tundra climate: warmest month 0C
The arbitrary temperature limits stem from a variety of criteria the supposed significance of selected values being as follows: i) The 10C summer isotherm correlates with the pole ward limit of tree growth ii) The 18C winter isotherm is critical for certain tropical plants. iii) The -3 C isotherm indicates a few weeks of snow cover.
However these correlations are far from precise.
ARIDITY CRITERIA
The criteria implies that with winter precipitation, arid (desert) condition occur where arrival precipitation to mean arrival of temperature ratio 18 least one, semi arid condition where the ratio is between one and two. If rain fails in summer a larger amount is require offsetting evaporation and maintaining an equivalent total of effective precipitation subdivisions of each category are made with reference first to the seasonal distribution of precipitation and second to additional temperature characteristics thus.
Precipitation
i) F no dry season.
M = Monsoon with a short dry season and heavy rains during the rest of the year. iii) S = Summer dry season iv W = Winter dry season
Additional temperature characteristics i) M = mean annual temperatures 18C
K = mean annual temperature below 18C (warmest month ( 18C)

CRITICISM KOPPENS CLIMATE CLASSFICATION
Trewartha (1943) notes that Koppens classification has been criticized from “ various points of view” (Thornthwaite 1931, Jones 1932, Ackerman, 1941). Rigid boundary criteria often lead to large discrepancies between climatic subdivisions and features of the natural landscape. Some boundaries have been chosen largely with natural landscape features in mind (for example, “rainforest”), whilst other boundaries have been chosen largely with human experience of climatic features in mind (for example, “monsoon”).

Trewartha (1943) acknowledges the validity of these criticisms when he writes that “climatic boundaries, as seen on a map, even when precisely defined, are neither better nor worse than the human judgments that selected them, and the wisdom of those selections is always open to debate” he emphasizes, however, that such boundaries are always subject to change “with revision of boundary conditions………..(and that)……. Such revisions have been made by Koppen himself and by other criminologists as well”.
Nevertheless, the telling evidence that the Koppen classifications merits outweigh its deficiencies lies in its wide acceptance. Trewartha (1943) observes that “its individual climatic formulas are almost a common language among climatologists and geographers throughout the world…(and that)…its basic principle have been…widely copied (even) by those who have insisted upon making their own empirical classifications” Trewartha’s (1943) comments are as relevant today as they were half a century ago
In recognition of this, the following changes, which are also summarized in Table I, have been adopted in this work: 1. The former tropical group is now divided into two new groups, an equatorial group and a new tropical group. The Equatorial group corresponds to the former tropical group’s isothermal subdivision. The new tropical group corresponds to that remaining of the former tropical group. 2. The equatorial and tropical group monsoon subdivisions are re-named as rainforest (Monsoonal) subdivisions. This is done because, in these subdivisions, the dry season is so short, and the total rainfall is so great, that the ground remains sufficient wet thought the year to support rainforest. 3. The former dry group is now divided into two new groups, a desert group and a grassland group. The new groups correspond to the former subdivisions of the dry group with the same name. This is believed necessary because of the significant differences between the types of vegetation found in deserts and grasslands. 4. The new desert and grassland winter drought (summer drought)subdivisions now require the additional criterion that there is more than 30 mm in the wettest summer month (winter month) to be so class field. This change is carried out because drought conditions may be said to prevail throughout the year in climates without at least a few relatively wet months. 5. The former temperature group is divided into two new groups, a temperature group and a subtropical group. The new subtropical group corresponds to that part of the former temperature group with annual temperature of a least 18C. The new temperature group corresponds to that part of the former temperature group remaining.
This is done because of the significant differences in the vegetation found in areas characterized by the two new groups, and in order that there is continuity in the boundary between the hot and warm desert and grassland climates where they adjoin rainy climates.

6. For simplicity, the former Kopen cold snowny forest group of climates is re-named as the cold group 7. For the temperate, subtropical and the cold groups, the distinctly dry winter subdivision requires the additional criterion of no more than 30mm in the driest winter month to be so classified. In order that there be consistency between the criteria for the distinctly dry winter and the distinctly dry summer subdivisions, this is thought to be a worthwhile change.

8. Carved out of the temperate, subtropical and the cold group with no distinctly dry season subdivision is the moderately dry winter subdivision. This new subdivision receives at least three times (but less than ten times) the rainfall in the driest winter month. This subdivision has been added in order that there be a match with part of the distinctly dry summer subdivision that was not matched by the distinctly dry winter subdivision.

9. The polar group has added to it the subdivision polar maritime, this subdivision reflecting the climate of the sub-antarctic islands, which otherwise would have been classified (inappropriately) as polar tundra. Polar tundra would be an inappropriate description for climates where the average temperature of the coldest month is -3oC or above. This is because, with the temperature not well below freezing, it is difficult for the ground to become frozen (a characteristic of “polar tundra”).

10. The frequent-fog desert and grassland climates are re-named as high humidity climates. They are also defined in terms of mean annual relative humidity, rather than in terms of fog frequency. This is on account of the dew-fall that results from the high humidity being a significant contributor to plant moisture regions with such climates. They are also restricted to desert climates. This is on account of the dew-fall in grassland climates not being a significant moisture contributor (in comparison with the total rain that falls in grassland climates). Whereas this climate subdivision is found in the desert regions on the west coats of the other two southern hemisphere continents, the relative humidity in the west coast desert region of Australia far too low for the climate to be characterized as high-humidity.

11. Some equatorial and tropical subdivisions (those equatorial climates with an autumn rainfall maximum, those with a high-sun dry season, and those with a hottest month prior to the summer solstice) are considered to be minor and have therefore been absorbed into the other equatorial and tropical subdivisions.

12. Some subtropical and temperate subdivisions (those subtropical and temperate climates that are isothermal, those that have a hottest month prior to the summer solstice and those with a late-spring/early-summer rainfall maximum) are also considered to be minor and have therefore been absorbed into other subtropical and temperate subdivisions.

A.A MILLER CLASSIFICATION (1951)
Miller introduced a somewhat similar classification related to Koppen in 1951 using the following criteria: (i) Bounding of arid conditions. (ii) Bounding of semi-arid conditions.
The thermal units relate to the accumulated temperature which Miller estimated approximately by using month degrees
The excess of mean monthly temperatures above 6oC rather than the usual day degrees based on daily mean temperatures above this limit.
C.W THORNTHWAITE Classification (1931)
C.W Thornwaite introduced a complex empirical classification in 1931.
An expression for precipitation efficiency was obtained by relating measurements of pan evaporation to temperature and precipitation. By determining boundary values for major vegetation regions the following humidity provinces were obtained: - (a) Rain forest (7127) (b) Forest (64-127) (c) grassland (32-63) (d) Steppe (16-31) (e) Desert (<16)
The second element of classification is as index of thermal efficiency (T.E), expressed by the positive departure of monthly mean temperatures from freezing point. Unlike Koppen, Thornwaite makes moisture the primary claificatory factor for a thermal efficiency index.
B. Energy and Moisture Budget classifications
Thornthwaites most important contribution was his second (1948) classification based on the concept of potential evapo-transpiration and moisture budget.
The potential evapo-transpiration (P.E.) is calculated from the mean monthly temperatures (in oC), with corrections for day length for a 30 day month (12 hours days). The monthly water surplus (8) or deficit (D) is determined from a moisture budget assessment taking into account stored soil moisture.
In 1955, the weighting factor (weighting of a deficit of 0.6) was omitted since it was recognized that a deficit can begin as soon as any moisture is removed from the soil by evaporation. The later revision also allows for available soil moisture storage according to vegetation over and permits the evaporation rate to vary with the actual soil moisture content. The climatic types defined by these two factors are: - (a) Megathermal (b) Mesothermal (c) Microthermal (d) Tundra (e) Frost
Criticism of Thornthwaite climate classification
This scheme of climatic classification has been successfully applied to may regions, but because of certain limitations inherent in it no world map has yet been prepared.
This classification system has proved most satisfactory in case of North America where vegetation boundaries nearly coincide with particular PE values. But it is not satisfactory for the tropical and semi arid areas.
However, Thornthwaite does not determine his climatic boundaries ion the basis of vegetation boundaries. In this respect, he differs from Koppen in case of his second classification.
This classification involves a quantitative as well as empirical, but it does not take into account various causative factors of climate. It also employs symbols for designating the climatic types.
The classification involves a lot of calculations, hence it is more difficult to use in determining the climatic type of a particular place or locality. That is why Thornthwaite’s system could not enjoy the same degree of polarity as that of Koppen.
Lastly, Thornthwaite as a climatologist developed the concept of the soil-moisture balance, and he preferred to use it as the foundation of his climate system.
He was convinced that the soil-moisture balance represents availability of moisture for plant, and an assessment of the availability of surplus moisture to supply stream flow and ground water.
This concept implies that precipitation alone does not indicate the amount of water actually available to plants. The amount and availability of soil moisture is also affected, besides other factors, by the losses due to evapo-transpiration.
Thus, moisture requirements of plants, according to Thornthwaite, become higher as temperature increases. Even though this scheme of climatic classification is based on two variables: (1) precipitation, and (2) evapo-transpiration, temperature is by no means ignored in it. Temperature is also accounted for while calculating evapo-transpiration.
If a comparison is made between the amount of water available from precipitation and the water need, it is easy to assess the moisture conditions to determine the seasonal distribution of moisture deficits or surpluses and whether a climate is wet or dry.
The most significant contribution of Thornthwaite has been that the concept of evapo-transpiration has been applied in practical studies of the water balance as regards the problems of water use. The system has bee applied to many regions, although no world map has yet been published. In tropical and semi arid areas the method is not very satisfactory but in Eastern North America for example, vegetation boundaries have been shown to coincide reasonable with particular potential evapo-transpiration (P.E) values. This classification unlike that of Koppen (P.E) and many others does not use vegetation boundaries to determine climatic ones.
M.I. Budyoko developed a similar but more fundamental approach using net radiation rather that temperature. He related the net radiation available for evaporation from a wet surface (R) to the heat required to evaporate the mean annual precipitation (LR). This Ro/LR ratio is called radiational index of dryness.
The boundaries therefore are: (a) Desert (> 3.0) (b) Semi-desert (2.0 – 3.0) (c) Steppe (1.0 2.0) (d) Forest (0.33 – 1.0) (e) Tundra (<0.33)
(C) GENETIC CLASSIFICATION
The genetic basic of macro climates is the atmospheric circulation and this can be related to regional climatology in terms of wind regimes or air masses. A. Hehmer (1931) made an attempt where he incorporated the wind system, rainfall amount and duration, positive relative to the sea and deviation. A more satisfactory scheme was proposed by H. Fiohm (1950). His major categories which are based on the global wind belts and precipitation characteristics are as follows: -

1. Equatorial westerly zone: constantly wet 2. Tropical zone, winter trader: summer rainfall. 3. Sub-tropical dry zone (trades or subtropical winter 4. Subtropical winter-rain zone (Mediterranean type): winter rainfall. 5. Extra tropical westerly zone. 6. Sub polar zone: Limited precipitation throughout the year.
Bored, continental subtype: summer rainfall: limited winter snowfall. 7. High polar zone: meagre precipitation: summer rainfall, early winter snowfall.
However, it should be noted that temperature does not appear explicating in the scheme. A general agreement between these types and those of Koppen is apparent. The bored subtype is restricted to the Northern hemisphere and the subtropical zones do not occur on the east side of a land mass.
Stahler proposed a simple but extremely effective genetic classification of world climates: - 1. Low attitude climates controlled by equatorial and tropical air masses. 2. Middle-latitude climates controlled by both tropical and polar air masses. 3. High latitude climates, controlled by polar arctic air masses.
These are subdivided into fourteen climate regions to which highlands climate is added.
Staller Climate Classification
Group 1. Low Latitude Climates 1. Wet equatorial climate Tropical rainforest climate and
(10o N – 10o S lat) tropical rain forest climate, monsoon type. 2. Trade wind littoral climate
(10o– 25o N and S of Equator 3. Tropical Desert and steppe climates Desert climate hot and
15 o – 35 o N and S of Equator steppe climate , hot.

4. West-coast desert climate
15o – 30 o N and S of Equator

5. Tropical wet dry climate Tropical rainy climate,
15o – 25 o N and S of Equator savanna.

Group 2. Middle latitude climates (controlled by both tropical and polar air masses). 6. Humid sub tropical climate
20o – 35o –N and S of Equator

7. Marine west-coast
Climate 40o – 60o N and S of Equator

8. Mediterranean climate
30o – 45 o N and S of Equator

9. Middle latitude desert & steppe climate
35o – 50 o N and S of Equator

10. Humid continental
35o – 60 o N and of Equator Group 3: High Latitude climates (controlled by polar and arctic air masses) 11. Continental sub arctic climate
50o – 70 o N and of Equator

12. Marine sub-arctic climate
50o – 60o N and 45o – 60o S

13. Tundra climate North of 55o N South of 50o S
Ice cap climate (Greenland and Antarctica) 14. Climate climates.

REFERENCES
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