Geology Reference
In-Depth Information
The annual fl uctuations in solar radiation and air temperature create a distinct
freeze-thaw regime. Data for Fenghuo Shan are illustrated in Table 3.1 and in Figure
3.2; these are typical for most stations on the plateau. Although the plateau experiences
approximately the same number of freeze-thaw cycles as other localities of similar
latitude and altitude (e.g. Mont Blanc Station, Peru; see Figure 3.2), the number of
frost-free days is approximately one order of magnitude less on the Plateau than at high
latitudes. For example, the average number of frost-free days at Fenghuo Shan is only
11, while at Tuktoyaktuk, in the Canada Arctic, it is 108, despite the fact that the mean
annual air temperature at Tuktoyaktuk is about 4-5 °C lower than at Fenghuo Shan. This
apparently anomalous situation refl ects the fact that the Tibet plateau has a much smaller
temperature range than the high latitudes, where seasonal rhythms dominate over diurnal
ones.
In summary, the unusual climate of the vast Tibet plateau justifi es its recognition as a
distinct periglacial climate.
3.2.4. Alpine Climates
Alpine periglacial climates are characteristic of tundra regions lying above timberline
(treeline) in mid-latitude locations. They are not as extensive as any of the three previous
climatic environments so far described. In the European Alps and the North American
Rockies, the timberline occurs at elevations varying between 2000 m and 4000 m, but in
sub-arctic localities, such as northern Scandinavia, Iceland, and northern Labrador-
Ungava, the timberline approaches sea level. Irrespective of elevation, however, all these
climatic environments experience a seasonal and diurnal rhythm of both temperature and
precipitation.
None of the alpine climates experiences the severe winter cold of either High Arctic
or continental climates. On the other hand, the diurnal and seasonal rhythm imposed by
their mid-latitude locations results in a higher frequency of temperature oscillations
around the freezing point. Precipitation is also heavy, the result of either orographic or
maritime effects. Total amounts often exceed 750-1000 mm per annum, much of which
occurs as snow. Permafrost is often lacking or discontinuous because of the high mean
annual temperature and the protection given to the ground surface by winter snow
cover.
3.2.5. Climates of Low Annual Temperature Range
There are certain areas of the world which experience not only a mean annual air tem-
perature below
3 °C but also a remarkably small range of temperatures. These rather
unique climatic conditions occur in two types of localities.
The fi rst is a sub-arctic, oceanic location. Here, the surrounding water exerts a moder-
ating infl uence upon temperature. In the northern hemisphere, Jan Mayen and Bear Island
experience this climate, while in the southern hemisphere, the islands surrounding
Antarctica, such as the Falkland Islands, Kerguelen, South Georgia, and the South
Orkneys, are similar. In these climatic environments, the mean annual amplitude of tem-
perature is in the order of only 10 °C (Table 3.1). Not surprisingly, these areas experience
a high frequency of freeze-thaw cycles of short duration and shallow ground penetration.
Because of their maritime locations, these islands have considerable amounts of precipita-
tion, often varying between 1000 mm/year and 2000 mm/year, and unstable cyclonic
weather, with much low cloud and fog.
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