Geology Reference
In-Depth Information
3.2. PERIGLACIAL CLIMATES
The concept of a periglacial climate was fi rst proposed by C. Troll (1944) in his global
survey of frost-action conditions. It was subsequently incorporated into a scheme of
morphogenetic regions by L. C. Peltier (1950), who characterized the periglacial climate
as having a mean annual air temperature of between
1 °C, precipitation of
between 120 mm and 1400 mm per annum, and “intense frost action, strong mass move-
ment, and the weak importance of running water.” This defi nition masks a wide range of
climatic conditions and implies a relative uniformity to the periglacial climate. This is
misleading because it is far more realistic to stress the variety of cold climates that exist
in which frost-action processes are important.
Some of the fi rst to appreciate this were the French geomorphologists J. Tricart and A.
Cailleux (Tricart, 1963; Tricart and Cailleux, 1967, pp. 45-67). They distinguished between
three types of periglacial climatic environments. The fi rst (Type I: Dry climates with
severe winters) experiences seasonal and deep freezing while the third (Type III: Climates
with small annual temperature range) experiences shallow and predominantly diurnal
freezing. The second (Type II: Humid climates with severe winter) is intermediate. Per-
mafrost is characteristic of the fi rst, is irregular in occurrence and distribution in the
second, and absent in the third. A problem with this three-fold division is that the fi rst
two types are identifi ed primarily in terms of humidity while the third is identifi ed in terms
of temperature. As a result, Type I includes the rather different climatic environments of,
for example, the Canadian High Arctic and sub-arctic central Siberia. Moreover, an arctic
subtype of category II, as typifi ed by Spitsbergen, is more similar to the environments of
other high latitudes than to a mountain variety, which is the other subtype of that
category.
In a more pragmatic fashion, D. Barsch (1993) suggests that periglacial environments
simply exist in areas with polar climates. In other words, they exist in those areas domi-
nated by E (Ef and ET) climates of Köppen (1923). As regards alpine periglacial climates,
the boundaries are similar but problems arise in arid to semi-arid mountains, where the
upper timberline (i.e. the lower limit of periglacial conditions) does not exist. As regards
the upper limit of periglacial conditions, the existence of glaciers and permanent snow
and ice for both the polar and alpine periglacial environments is relatively unambiguous.
Unfortunately this approach fails to recognize the important boreal forest component of
the periglacial domain, and the fact that relict permafrost can be just as important as
contemporary frost action in determining the character of the periglacial landscape.
In the second edition of this topic fi ve broad categories of periglacial climates were
proposed using the criteria of insolation, temperature, and elevation. This was to incor-
porate the relatively unique climatic conditions of the Qinghai-Xizang (Tibet) plateau.
Basic to the classifi cation is the fact that radiant energy from the sun infl uences both air
and ground temperatures. For example, Figure 3.1(A) shows potential insolation, expressed
as a percentage of the amount received at the equator, as a function of latitude. If one
assumes potential insolation at the equator to be 3.12
−
15 °C and
−
10
5
ly/year (Dingman and Koutz,
1974), then the potential insolation of a horizontal surface on the Qinghai-Xizang plateau
at latitude 29-38° N is approximately 2.68
×
10
5
ly/year. This can be compared to the com-
parable value for the Mackenzie Delta, Canada (latitude 68-69° N) of 1.58
×
10
5
ly/year.
Elevation also infl uences the amount of solar radiation received. For example, Fenghuo
Shan, on the Tibet plateau, is at the same latitude as, but 3500 m higher than, Xi'an in
central China. However, Figure 3.1(B) indicates the actual solar radiation received at
Fenghuo Shan is much higher than at Xi'an. This is because the water vapor and aerosol
content in the atmosphere decrease with increasing elevation, which, in turn, cause less
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