The foregoing concepts of devising climate classes that combine temperature and precipitation characteristics, but of setting limits and boundaries fitted into known vegetation and soil distributions, were actually carried out in 1918 by Dr. Wladimir Koppen. The classification was subsequently revised and extended by Koppen.
The Koppen system is strictly empirical. This is to say that each climate is defined according to fixed values of temperature and precipitation, computed according to the averages of the year or of individual months. It is possible to assign a given place to a particular climate subgroup solely on the basis of the records of temperature and precipitation of that place, provided, of course, that the period of record is long enough to yield meaningful averages. A climate system based on these data has a great advantage, in that the areas covered by each subtype of climate can be delineated for large regions of the world.
The Koppen system features a shorthand code of letters designating major climate groups, subgroups within the major groups, and further subdivisions to distinguish particular seasonal characteristics of temperature and precipitation.
Five major climate groups are designated by capital letters as shown in the following figures. Groups A, C, and D have sufficient heat and precipitation for growth of high-trunk trees (e.g., forest and woodland vegetation).
A Tropical climates. Average temperature of every month is above 64.4°F (18°C). These climates have no winter season. Annual rainfall is large and exceeds annual evaporation.
B Dry climates. Potential evaporation exceeds precipitation on the average throughout the year. No water surplus; hence no permanent streams originate in B climate zones.
C Warm temperate (mesothermal) climates. Coldest month has an average temperature under 64.4°F (18°C), but above 26.6°F (-3°C); at least one month has an average temperature above 50°F (10°C). The C climates thus have both a summer and a winter season.
D Snow (microthermal) climates. Coldest month average temperature under 26.6°F (-3°C). Average temperature of warmest month above 50°F (10°C), that isotherm coinciding approximately with poleward limit of forest growth.
E Ice climates. Average temperature of warmest month below 50°F (10°C). These climates have no true summer.
Note that four of these five groups (A. C, D, and E) are defined by temperature averages, whereas one (B) is defined by precipitation-to-evaporation ratios. This procedure may seem to be a fundamental inconsistency.
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