Subgroups within the five major groups are designated by a second letter according to the following codes:
S: Steppes climate: A semi-arid climate with about 15 to 30 in (38 to 76 cm) of rainfall annually at low latitudes. Exact rainfall boundary determined by formula taking temperature into account.
W: Desert climate: Arid climate. Most regions included have less than 10 in (25 cm) of rainfall annually. Exact boundary with steppe climate determined by formula.
(The letters, S and W are applied only to the dry B climates, yielding two combinations, ES and EW.)
f Moist. Adequate precipitation in all months. No dry season. This modifier is applied to A, C, and D groups.
w Dry season in winter of the respective hemisphere (low sun season).
s Dry season in summer of the respective hemisphere (high-sun season).
m Rain forest climate despite short, dry season in monsoon type of precipitations cycle. Applies only to A climates.
From combinations of the two letter groups, eleven distinct climates emerge as follows:
Af Tropical rain forest (Also Am, a variant of AD.
Aw Tropical savanna.
ES Steppe climate
EW Desert climate
CW Temperate rainy (humid mesothermal) climate with dry winter.
Cf Temperate rainy (humid mesothermal) climate moist all seasons.
Cs Temperate rainy (humid mesothermal) climate with dry summer.
Df Cold snowy forest (humid microthermal) climate moist in all seasons.
Dw Cold snowy forest (humid microthermal) climate with dry winter.
ET Tundra climate.
Ef Climates of perpetual frost (ice caps).
To differentiate still more variations in temperature or other weather elements. Koppen added a third letter to the code group. meanings are as follows:
a With hot summer, warmest month over 71.6°F (22°C) (C and D climates).
b With warm summer, warmest month below 71.6°F (22°C) (C and D climates).
c With cool, short summer; less than four months over 50°F (10°C) (C and D climates).
d With very cold winter, coldest month below - 36.4°F (-38°C) (D climates only).
h Dry-hot; mean annual temperature over 64.4°F (18°C) (B climates only).
k Dry-cold; mean annual temperature under 64.4°F (18°C) (B climates only).
As an example of a complete Koppen climate code, Bwk would refer to cool desert climate, Dfc would refer to cold, snowy forest climate with cool, short summer.
Advantages of Koppen's classification. Koppen used the temperature and precipitation statistics in his classification of the climate. These two weather elements are easy to measure. Because of this quality, these elements are most widely and most frequently used. Since Koppen's classification is based on statistical parameters, each climatic region can be precisely defined.
Besides, the temperature and precipitation are the two most effective weather elements that exhibit the effects of climatic controls more clearly than any other weather elements. These elements affect other aspects of our physical environment more directly than any other element. Obviously, the system of classification devised by Koppen is directly related to those aspects of environment, which are clearly visible to us. His climatic classification system is based on the relationship between the types of plants at a particular place and the climatic characteristics of the place. Thus, his scheme is not a mere abstraction. Further, Koppen introduced the concept of effective precipitation which depends on the rate of potential evapotranspiration. Potential evapotranspiration is largely controlled by temperature. Thus, in Koppen's classification the relationship between heat and moisture factor gets due recognition. The moisture requirement of plant varies with the rate of evapotranspiration. For example, an amount of precipitation sufficient to support coniferous forest in the cool temperate zone may support little plant life in a low-latitude desert.
Another advantage of this classification is that it is possible to assign a given place to a particular climatic sub-group only on the basis of certain easily acquired statistics about an area's temperature and precipitation. Besides, this system of climatic classification is descriptive and generalized and therefore, it is more useful to geographers.
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