Environmental Engineering Reference
In-Depth Information
DISTRIBUTION OF POLAR LANDSCAPES
The harsh polar environment is typified by the treeless tundra biome. In high latitudes the
lack of summer warmth does not allow trees or large woody shrubs to grow, and the
vegetation consists of grasses, sedges, low shrubs, mosses, lichens, and highly specialized
flowering plants. About 10 million km
2
of tundra are located in the Arctic regions of
Eurasia and North America, but only about 50,000 km
2
around the southern Antarctic
latitudes. A further 10 million km
2
of mountain tundra is found at high elevations
throughout the world, though it is not discussed in this chapter.
Polar climates are dominated throughout the year by cold, dry Arctic (cA) air masses,
with some incursions of humid, milder polar maritime (mP) air masses in summer; this
effect is especially noticeable in north-western North America (Alaska and north-west
Canada), where summer warmth is advected inland from the warm waters of the Japanese
current in the Pacific, and likewise in north-west Europe (Iceland and Scandinavia) under
the influence of the North Atlantic Drift in the Atlantic. Temperatures rise above freezing
for only two to four months per annum, and the average temperature is below 10° C in
the warmest month. Incoming solar radiation and length of day are the two elements
which give greatest contrast between winter and summer. Although the summer is brief,
net radiation can be high; in the Canadian Arctic it is about 109 W m
−2
day
−1
in July,
compared with 133 W m
−2
day
−1
at 49° N on the Canada-United States border. The
contrast between eight to eleven months with a large negative radiation balance and one
to four months with a large positive radiation balance is an important environmental
control.
Annual precipitation in polar regions is low, hence the label 'cold desert'. Most polar
regions will receive less than 250 mm of precipitation annually, as the cold air is able to
hold little moisture, and though relative humidity may be high, absolute humidity is
always low. Sixty per cent of precipitation occurs as snow. Throughout polar regions lack
of available water may be as limiting an ecological factor as extreme cold, exacerbated
by soil water being frozen for much of the year. However, precipitation figures for polar
stations are notoriously unreliable, owing to the difficulty of measuring snowfall
accurately. Generally precipitation declines at higher latitudes, where temperatures are
colder, and where air masses from temperate latitudes have more difficulty in penetrating.
Treeless polar climates have traditionally been delimited by the isotherm for 10° C for
the warmest summer month, normally July in the northern hemisphere and January in the
southern hemisphere. For the Arctic this isotherm reflects latitude, ocean currents and
continentality (Figure 24.1). It includes most of the northern coasts and islands of Alaska,
Canada, Scandinavia and Russia, all of Greenland and Svalbard, and the northern two-
thirds of Iceland. Anomalies occur where it is pushed
