Geoscience Reference
In-Depth Information
A1.1
; both elements are subdivided according
to the season of moisture deficit or surplus and
the seasonal concentration of thermal efficiency.
A revised Thornthwaite-type classification is
proposed by J. Feddema (2005).
The system has been applied to many regions,
but no world map has been published. Unlike
the Köppen approach, vegetation boundaries are
not used to determine climatic ones. In eastern
North America, vegetation boundaries do coin-
cide reasonably closely with patterns of
PE
, but
in tropical and semi-arid areas the method is
less satisfactory. Shifts in climatic boundaries
according to Thornthwaite's classification have
been evaluated for the past 111 years for the
United States (Grundstein, 2008).
M. I. Budyko developed a more fundamental
approach using net radiation instead of tempera-
ture (see Chapter 4A). He related the net radiation
available for evaporation from a wet surface (
Ro
)
to the heat required to evaporate the mean annual
precipitation (
L
r). This ratio
Ro/L
r (where
L
=
latent heat of evaporation) is called the
radiational
index of dryness
. It has a value of less than unity in
humid areas and greater than unity in dry areas.
Boundary values of
Ro/L
r are: Desert (>3.0);
Semi-desert (2.0-3.0); Steppe (1.0-2.0); Forest
(0.33-1.0); Tundra (<0.33). In comparison with
the revised Thornthwaite index (
Im
= 100(
r/PE
-
1)), note that
Im
= 100(
L
r/
Ro
- 1) if all the net
radiation is used for evaporation from a wet
surface (i.e., no energy is transferred into the
ground by conduction or into the air as sensible
heat). A general world map of
Ro/L
r has appeared,
but there are few measurements of net radiation
over large parts of the earth.
Energy fluxes were used by Terjung and Louie
(1972) to categorize the magnitude of energy
input (net radiation and advection) and outputs
(sensible heat and latent heat), and their seasonal
range. On this basis, 62 climatic types were
distinguished (in six broad groups), and a world
map was provided. Smith
et al.
(2002) determine
net shortwave and net longwave radiation
criteria for a climate classification with nine global
types that is similar to Köppen's, developed
by Trewartha and Horn (1980).
Table A1.2
summarizes their criteria.
Table A1.2
Radiation budget criteria for major climatic types (from Smith et al., 2002). Units W m
-2
Climatic type
Annual
Annual
Annual range
shortwave
longwave
of shortwave
LAND
Tropical:
> 140
<70
<100
Wet
> 140
<50
<100
Wet/dry
>140
>50
<100
Desert
>90
Steppe
70< L
N
<90
Subtropical
>140
<70
>100
Temperate
100< S
N
<140
<70
Boreal
50 < S
N
<100
<70
Polar
0 <S
N
< 50
<50
OCEANS
Tropical
>210
<140
Convergence and stratus
170 < S
N
<210
<140
Subtropical
> 150
>140
Temperate
80 < S
N
< 150
Polar
0 < S
N
<80
