Geoscience Reference
In-Depth Information
24
environments
It may appear that there are few climatological or
biogeographical reasons for linking polar and alpine
areas together, as the processes producing their respec-
tive climates and vegetation differ. However, they both
represent cold parts of Earth's environments. In polar
regions, radiation inputs decline polewards, are small and
strongly seasonal, so that temperatures are normally low.
In alpine regions, temperatures decrease with height, so
that even in equatorial regions such as Kenya glaciers can
survive because of the high altitude and low temperatures.
In some respects the increase of altitude is like an increase
in latitude. As a result, these two regions are often grouped
together, as they also exhibit similarities in their
geomorphology. Because of this we will examine the
nature of their climates first before moving on to other
environmental features.
by a cool ocean dominated by westerly winds. Let us look
at these areas in turn.
Arctic climate
Although summer is brief, radiation inputs can be high;
in the Canadian Arctic net radiation reaches about 110 W
m
-2
day
-1
in July, compared with 133 W m
-2
day
-1
at 49
N
on the Canada-US border. The contrast between eight to
eleven months with a large negative radiation balance and
one to four months with a large positive balance is an
important environmental control. As a result, tempera-
tures rise above freezing for only two to four months per
year, and the average temperature is below 10
C in the
warmest month (
Figure 24.1
).
Annual precipitation in the Arctic 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; relative humidity may be high, but
absolute humidity is always low. In addition, the number
of occasions when air is able or is forced to rise sufficiently
for precipitation to form is limited. About 60 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 all polar stations are notoriously unreliable because
of the difficulty of measuring snowfall accurately.
Generally precipitation declines at higher latitudes, where
temperatures are colder, and where air masses from
temperate latitudes are less frequent.
Treeless polar climates have traditionally been delim-
ited by the isotherm for 10
C for the warmest summer
POLAR CLIMATES
The polar regions are characterized by low inputs of
energy with a strong seasonal cycle between the prolonged
darkness of the winter season and the continuous day-
light of summer to give a unique environment. They are
dominated throughout the year by cold, dry air with
occasional incursions, in the northern hemisphere, of
warmer air from the oceans or, in summer, from the
continental areas. Although both poles do have climatic
features in common, their geographic setting means that
there are marked differences between them. The Arctic is
a frozen sea surrounded by continents whilst Antarctica
is largely a continental area at high elevation surrounded
