Geoscience Reference
In-Depth Information
the study of vegetation groups defined on a
physiological basis
(
i.e., according to the internal
functions of plant organs).
A new analysis of world climate data has been
used by Peel
et al
. (2007) to map the distribution
of Köppen-Geiger climate types for each continent
and the world. The land areas covered by the
major classes, (in percent) are as follows: arid (B)
30.2; cold (D) 24.6; tropical (A) 19.0; temperate
(C) 13.4; and polar (E) 12.8.percent.
The Köppen climatic classification has proved
useful in evaluating the accuracy of GCMs in
simulating current climatic patterns, as a con-
venient index of recent climatic change, and for
climate scenarios projected for CO
2
doubling.
C. W. Thornthwaite introduced a further
empirical classification in 1931. An expression for
precipitation efficiency
was obtained by relating
measurements of pan evaporation to temperature
and precipitation. The second element of the
classification is an index of
thermal efficiency
,
expressed by the positive departure of monthly
mean temperatures from freezing point. Distribu-
tion for these climatic provinces in North America
and over the world have been published, but the
classification is now largely of historical interest.
Aridity criteria
The criteria imply that, with winter precipitation,
arid (desert) conditions occur where
r/T
< 1,
semi-arid conditions where 1 <
r/T
< 2. If the rain
falls in summer, a larger amount is required to
offset evaporation and maintain an equivalent
effective precipitation.
Subdivisions of each major category are made
with reference, first, to the seasonal distribution
of precipitation. The most common of these are:
f
= no dry season;
m
= monsoonal, with a short, dry
season and heavy rains during the rest of the year;
s
= summer dry season;
w
= winter dry season.
Second, there are further temperature criteria based
on seasonality. Twenty-seven subtypes are recog-
nized, of which 23 occur in Asia. The ten major
Köppen types each have distinct annual energy
budget regimes, as illustrated in
Figure A1.1
.
Figure A1.2A
illustrates the distribution of the
major Köppen climate types on a hypothetical
continent of low and uniform elevation. Experi-
ments using GCMs with and without orography
show that, in fact, the poleward orientation of
BS/BW climatic zones inland from the West Coast
is largely determined by the western cordilleras. It
would not be found on a low, uniform-elevation
continent.
B ENERGY AND MOISTURE
BUDGET CLASSIFICATIONS
Thornthwaite's most important contribution was
his 1948 classification, based on the concept of
potential evapotranspiration and the moisture
budget (see Chapters 4C and 10B.3c). Potential
evapotranspiration (
PE
) is calculated from the
Table A1.1
Thornthwaite's climatic classification
PE
Im (1955 system)*
cm
in
Climatic type
>100
Perhumid (A)
>114
>44.9
Megathermal (A
′
)
20 to 100
Humid (B
1
to B
4
)
57 to 114
22.4 to 44.9
Mesothermal (B
′
1
to B
′
4
)
0 to 20
Moist subhumid (C
2
)
28.5 to 57
11.2 to 22.4
Microthermal (C
′
1
to C
′
2
)
-33 to 0
Dry subhumid (C
1
)
14.2 to 28.5
5.6 to 11.2
Tundra (D
′
)
-67 to -33
Semi-arid (D)
<14.2
<5.6
Frost (E
′
)
-100 to -67 Arid (E)
Note: *Im = 100(S - D)/PE is equivalent to 100(r/PE - 1), where: r = annual precipitation (cm); T = mean annual
temperature (
C).
°
