The climate is most important factor controlling the environment. The type of soil and the native vegetation in a given region are basically the product of the climate of the area. The scientific study of climate requires a scheme of climate classification. Such schemes aim to categorize all the variations in the climates found in different areas into several clearly defined and easily distinguishable groups. Many useful systems of climate classification can be devised by taking different weather elements such as temperature, pressure, winds, precipitation as the basis of classification. the distribution of natural vegetation and soils may suggest still other types of classification schemes.
Temperature as basis of climate classification
The general parallelism of isotherms with parallels of latitude was perhaps the first basis of climate classification. With intelligent application, temperature has been made a fundamental factor in most of the schemes. Three major climate groups are clearly recognizable on the basis of this criterion:
- Equatorial tropical group: characterized by uniformly warm temperature throughout the year and absence of a winter season.
- Middle-latitude group: characterized by alternating summer and winter seasons.
- Polar-arctic group: characterized by the absence of true summer season.
A common boundary between polar-arctic climates and middle-latitude climates is 10 degrees celsius (50 degrees F) isotherm of the warmest month (i.e. july in northern hemisphere). The boundary between middle-latituse and equatorial climates is marked by 18 degrees celsius (64.4 degrees F) isotherm of the coldest month.
The use of temperature alone as the basis of climate classification is unsatisfactory because humid and desert regions are not distinguished in such scheme.
Precipitation as basis of climate classification
The climate map in such a scheme would be same as the mean annual rainfall map. Such system may be refined by subdividing classes according to distribution of precipitation throughout the year, whether uniform or seasonal. However, such classification scheme fails because it groups cold arctic climates together with hot deserts of low latitudes controlled by air temperature. The cold climates, in general, are effectively humid with same meager precipitaion that produces very dry deserts in hot subtropics and tropics.
Vegetation as basis of climate classification
Different plant types require special condition of temperature and precipitation for their survival. Thus, plants form an index of climate and limits of growth of key plant types provide meaningful boundaries of climstic zones. There is much merit in such vegetational zones based climate classificstion scheme. However, vegetation is an effect rather than a primary cause of climate. Therefore, it can not give so satisfactory a climate classification as the one based on the primary cause(s) of climate.
Koppen climate classification system
After various attempts of classification schemes based on some single characteristic feature, it became evident that a meaningful system should devise climate classes that combine temperature and precipitation characteristics but also set limits and boundaries that fit into obviously known vegetational distributions. Dr. Wladimir Koppen (1918) devised such a system that was later revised and extended by him and his students. this Koppen climate classification system has become most widely used system for biogeographical purposes.
Koppen system is strictly empirical in nature. Each climate group is defined according to fixed values of temperature and precipitation, computed according to averages of the year or of individual months. No attention is paid to the causes of climate in terms of pressure belts, wind belts, air masses, air fronts or storms. A given place can be assigned to a particular climate subgroup solely on the basis of the records of temperature and precipitation of that area provided that the records are long enough to yield meaningful averages. Since air temperature and precipitation are most easily obtainable surface weather data, the system has great advantage that the areas covered by each subtype of climate can be computed or estimated for large regions of the world. This system thus, incorporates an empirical-quantitative approach.
Koppen system has a shorthand code of letters disignating major climate groups (A, B, C, D), subgroups within major groups(S, W, f, m, s, w) and further subdivisions to distinguish particular seasonal characteristics of temperature and precipitation (a, b, c, d, h, k).
Air mass source regions and frontal zones as basis of climate classification
Increasingly detailed studies of vsrious charateristics of weather elements, global circulation, air mass properties, source regions and cyclonic storms have yielded many principles related to the causes of weather patterns and their seasonal variations at global scale. Therefore, such knowledge has also been incorporated in the explanatory-descriptive system of climate classification based on the cause and effect. Thus, such system is based on the location of air mass source regions and nature and movement of air masses, fronts and cyclonic storms. Koppen code symbols have been incorporated into this system by interrelating both systems. This system simply provides a reasonable scientic explanation for the existence of Koppen’s climate groups.