Earth Sciences / 83of dry climates is defined by aridity. These main types were then subdivided into more detailed
categories, allowing for climates with or without dry and rainy seasons, monsoon climates, and
others. The relationship between temperature and plant distribution is imprecise, however, so the
categories are somewhat arbitrary, with many exceptions, and his classification is rather crude, despite
its popularity.
Thornthwaite adopted a different approach derived from the water required by farm crops (ALLABY,
1992a, p. 109) and based on precipitation efficiency and thermal efficiency. Both of these can be
calculated. Precipitation efficiency is measured for each month as the ratio of precipitation to
temperature to evaporation (as 115(r/t -10)10/9, where r is the mean monthly rainfall in inches and t is
the mean monthly temperature in °F), the sum of the 12 monthly values giving a precipitation efficiency
(P-E) index. Thermal efficiency is calculated each month as the extent to which the mean temperature
exceeds freezing (as (t -32)/4); the thermal efficiency (T-E) index is the sum of the monthly values.
The major change Thornthwaite introduced to his scheme in 1948 concerned the importance of
transpiration by plants. Combined with evaporation (in practice the two cannot be measured separately
in the field) this is evapotranspiration or, if water is available in unlimited amounts, ‘potential
evapotranspiration’ (PE). It is calculated in centimetres from the mean monthly temperature in °C,
corrected for changing day length.
Using his three indices, Thornthwaite defined nine ‘humidity provinces’ and nine ‘temperature
provinces’, the respective index value doubling between each province and the next in the hierarchy.
He then added further subdivisions to reflect the distribution of precipitation through the year, leading
to 32 distinct climate types. The humidity provinces, with their denoting letters, are: perhumid (A);
humid (B 4 , B 3 , B 2 , B 1 ); moist subhumid (C 2 ); dry subhumid (C 1 ); semi-arid (D); and arid (E). The
temperature provinces are: frost (E’); tundra (D’); microthermal (C’ 1 , C’ 2 ); mesothermal (B’ 1 , B’ 2 ,
B’ 3 , B’ 4 ); and megathermal (A’). This classification makes no assumptions about the distribution of
plants, but is based wholly on recorded data.
These classifications are described as ‘empirical’, because they are based on data. Their disadvantage
arises from the fact that divisions among sets of continuous variables are inevitably arbitrary, so the
number of categories is potentially huge, and the more regional variations a scheme recognizes the
more unwieldy it becomes. ‘Genetic’ classifications, derived from seasonal patterns of insolation
and precipitation or the dominant air masses, are not widely used, but there are several of them.
Indeed, there are many classificatory systems (HIDORE AND OLIVER, 1993, pp. 263–264), but
those of Köppen and Thornthwaite remain the most popular.
Thornthwaite devised a scheme to classify climates independently of the vegetation each type
supports, but the historical association between climate classification and plant distribution
is close. Up to a point the link is obvious. Tropical rain forests flourish in the humid tropics,
cacti and succulents in arid climates, conifer forests in high latitudes, and tundra vegetation
borders the barren polar regions. Clearly, plants occur only where the climate suits them;
bananas do not grow in Greenland (at least, not in the open). Although plant distribution is
linked to climate, however, other factors also influence it. Continental drift has separated
what were once adjacent landmasses supporting similar plants, producing very discontinuous
distributions. The southern beeches (Nothofagus), for example, occur in Australasia and western
South America, and pepper bushes (Clethra) in China and South-East Asia and from the south-
eastern United States to the northern and central regions of South America, but with fossil
remains in Europe. Major climate changes alter vegetation patterns, but often leave remnants
of the former pattern surviving as isolated relicts. The strawberry tree (Arbutus unedo) belongs
to a pattern of plants known as Lusitanian; these occur in south-western Europe, but also, as
relicts, in southern Ireland and Brittany.