Geology Reference
In-Depth Information
3.2.2. Continental Climates
Large areas south of the treeline in sub-arctic and continental locations experience a much
wider range of temperatures than High Arctic areas. Here, although mean annual air
temperature is approximately the same or slightly higher than in areas north of the
treeline, the mean value disguises excessively low temperatures in winter and remarkably
high temperatures in summer. Yakutsk, in central Siberia, and Dawson City, in Yukon
Territory, illustrate this phenomenon (Table 3.1). For example, at Yakutsk, January tem-
peratures of
20 °C are common, and the annual average
temperature range is 62 °C. The latter value can be compared to Sachs Harbour, where
the annual range is only 36 °C.
Besides being hotter, summers are longer than in High Arctic climates, and above-
freezing temperatures occur for 5-6 months of the year. Typically, seasonal thaw may
penetrate to depths in excess of 2.0-3.0 m. Precipitation amounts are also greater than in
High Arctic regions because disturbances associated with the Arctic and polar fronts are
more frequent at these latitudes. Between 250 mm and 600 mm per annum are typical,
with the majority falling during the summer months. However, evaporation rates are also
high during the summer and there is usually a soil moisture defi cit.
The continental climatic regime supports a northern or boreal forest. This is termed
taiga in Siberia. In North America, the boreal forest is composed largely of spruce (
Picea
glauca
and
Picea
mariana
) whereas in Siberia it is composed predominantly of pine (
Pinus
silvestris
) and tamarack (
Larix
dahurica
). In northern Eurasia, birch (
Betula
pubescens
)
is more common. The trees, the high summer air temperatures, and the dryness of the
summer months constitute a favorable environment for forest fi re. According to Viereck
(1973b), wildfi res burn an average area of 400,000 ha/year in Alaska alone. Some ecolo-
gists regard the northern boreal forests as fi re-climax communities. Fires constitute,
therefore, a distinct feature of the continental climatic environment. They are rare, but
not unknown, in the southern tundra but have no counterpart in the treeless and cooler
High Arctic environments. Winter snowfall is another differentiating characteristic of
continental climates since amounts are considerably greater than in High Arctic climates.
Together, snowfall and tree cover interact to determine whether permafrost forms or not.
Beneath trees, where snow depth is less, there is deeper frost penetration and a greater
probability of permafrost occurrence (Viereck, 1965).
−
40 °C and July temperatures of
+
3.2.3. Qinghai-Xizang (Tibet) Plateau
Because the Tibet plateau has low latitude and high elevation, it experiences both diurnal
and seasonal rhythms. The mean annual air temperature ranges between
−
2.0 °C and
−
6.0 °C and the annual range of monthly air temperature is relatively low when compared
to continental and High Arctic climates. In contrast to alpine environments, however,
precipitation is low, being more similar to high latitudes. Because evaporation rates of
between 1200 mm/year and 1800 mm/year are typical and annual total precipitation
ranges between 200 mm and 600 mm, most of it concentrated in the June-September
period, the plateau has a marked water-budget defi cit. As a consequence, vegetation is
steppe-tundra in nature and more barren then the Low Arctic tundra. Snow cover is also
generally thin,
6 cm on average. It often disappears two to three days after deposition
when air temperatures rise above 0 °C in mid-afternoon, or when evaporation and/or
sublimation occurs. In general, the mean annual air temperature on the plateau changes
with both latitude and altitude: lapse rates range between 0.44 °C and 0.55 °C
per 100 m.
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