Geoscience Reference
In-Depth Information
Appendix 1
Climate classification
The purpose of any classification system is to obtain an
efficient arrangement of information in a simplified and
generalized form. Climate statistics can be organized in
order to describe and delimit the major types of climate
in quantitative terms. Obviously, any single classifica-
tion can serve only a few purposes satisfactorily and
many different schemes have therefore been developed.
Many climatic classifications are concerned with the
relationships between climate and vegetation or soils
and rather few attempt to address the direct effects of
climate on human beings.
Only the basic principles of the four groups of the
most widely known classification systems are summa-
rized here. Further information may be found in the
listed references.
extensively in geographical teaching. The key features
of Köppen's approach are temperature criteria and
aridity criteria.
Temperature criteria
Five of the six major climate types are based on monthly
mean temperature values.
1
Tropical rainy climate: coldest month >18°C.
2
Dry climates.
3
Warm, temperate, rainy climates: coldest month
between -3° and
18°C, warmest month >10°C.
4
Cold boreal forest climates: coldest month <-3°,
warmest month >10°C. Note that many American
workers use a modified version with 0°C as the C/D
boundary.
A GENERIC CLASSIFICATIONS
RELATED TO PLANT GROWTH
OR VEGETATION
5
Tundra climate: warmest month 0 to 10°C.
6
Perpetual frost climate: warmest month <0°C.
The arbitrary temperature limits stem from a variety
of criteria. These are as follows: the 10°C summer
isotherm correlates with the poleward limit of tree
growth; the 18°C winter isotherm is critical for certain
tropical plants; and the -3°C isotherm indicates a few
weeks of snow cover. However, these correlations are
far from precise. De Candolle determined the criteria
in 1874 from the study of vegetation groups defined
on a physiological basis (i.e. according to the internal
functions of plant organs).
Numerous schemes have been suggested for relating
climate limits to plant growth or vegetation groups.
They rely on two basic criteria: the degree of aridity and
of 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 is
often used as an index of precipitation effectiveness,
since higher temperatures increase evaporation.
W. Köppen developed the pre-eminent example of such
a classification. Between 1900 and 1936, he devised
several classification schemes that involve considerable
complexity in their full detail. The system has been used
