Geoscience Reference
In-Depth Information
Mountains can function equally in the opposite
sense. For example, Arctic air from the Barents Sea may
move southward in winter over the Gulf of Bothnia,
usually when there is a depression over northern Russia,
giving very low temperatures in Sweden and Finland.
Western Norway is rarely affected, since the cold wave
is contained to the east of the mountains. In conse-
quence, there is a sharp climatic gradient across the
Scandinavian highlands in the winter months.
The Alps illustrates other topographic effects.
Together with the Pyrenees and the mountains of the
Balkans, the Alps effectively separates the Mediterranean
climatic region from that of Europe. The penetration of
warm airmasses north of these barriers is comparatively
rare and short-lived. However, with certain pressure
patterns, air from the Mediterranean and northern Italy is
forced to cross the Alps, losing its moisture through
precipitation on the southern slopes. Dry adiabatic
warming on the northern side of the mountains can
readily raise temperatures by 5 to 6°C in the upper valleys
of the Aar, Rhine and Inn. At Innsbruck, there are
approximately fifty days per year with föhn winds, with
a maximum in spring. Such occurrences can lead to rapid
melting of the snow, creating a risk of avalanches. With
northerly airflow across the Alps, föhn may occur in
northern Italy, but its effects are less pronounced.
Features of upland climate in Britain illustrate
some of the diverse effects of altitude. The mean annual
rainfall on the west coasts near sea-level is about 1140
mm, but on the western mountains of Scotland, the Lake
District and Wales averages exceed 3800 mm per year.
The annual record is 6530 mm in 1954 at Sprinkling
Tarn, Cumbria, and 1450 mm fell in a single month
(October 1909) just east of the summit of Snowdon in
north Wales. The annual number of rain-days (days with
at least 0.25 mm of precipitation) increases from about
165 in southeastern England and the south coast to over
230 days in northwest Britain. There is little additional
increase in the frequency of rainfall with height on the
mountains of the northwest. Hence, the mean rainfall
per rain-day rises sharply from 5 mm near sea-level in
the west and northwest to over 13 mm in the western
Highlands, the Lake District and Snowdonia. This
demonstrates that 'orographic rainfall' here is due
primarily to an intensification of the normal precip-
itation processes associated with frontal depressions and
unstable airstreams (see Chapter 4F.3).
Even quite low hills such as the Chilterns and South
Downs cause a rise in rainfall, receiving about 120 to
130 mm per year more than the surrounding lowlands.
In south Wales, mean annual precipitation increases
from 1200 mm at the coast to 2500 mm on the 500-m
high Glamorgan Hills, 20 km inland. Studies using
radar and a dense network of rain gauges indicate that
orographic intensification is pronounced during strong
low-level southwesterly airflow in frontal situations.
Most of the enhancement of precipitation rate occurs
in the lowest 1500 m. Figure 10.12 shows the mean
enhancement according to wind direction over England
and Wales, averaged for several days with fairly
constant wind velocities of about 20 m s
-1
and nearly
saturated low-level flow, attributable to a single frontal
system on each day. Differences are apparent in Wales
and southern England between winds from the SSW and
from the WSW, whereas for SSE airflow the mountains
of north Wales and the Pennines have little effect. There
are also areas of negative enhancement on the lee side
of mountains. The sheltering effects of the uplands
produce low annual totals on the lee side (with respect
to the prevailing winds). Thus, the lower Dee valley
in the lee of the mountains of north Wales receives
less than 750 mm, compared with over 2500 mm in
Snowdonia.
The complexity of the various factors affecting
rainfall in Britain is shown by the fact that a close
correlation exists between annual totals in northwest
Scotland, the Lake District and western Norway, which
are directly affected by Atlantic depressions. At the
same time, there is an inverse relationship between
annual amounts in the western Highlands and lowland
Aberdeenshire, less than 240 km to the east. Annual
precipitation in the latter area is more closely correlated
with that in lowland eastern England. Essentially, the
British Isles comprise two major climatic units for
rainfall - first, an 'Atlantic' one with a winter season
maximum, and, second, those central and eastern
districts with 'continental' affinities in the form of a
weak summer maximum in most years. Other areas
(eastern Ireland, eastern Scotland, northeast England
and most of the English Midlands and the Welsh border
counties) have a wet second half of the year.
The occurrence of snow is another measure of alti-
tude effects. Near sea-level, there are on average about
five days per year with snow falling in southwest
England, fifteen days in the southeast and thirty-five
days in northern Scotland. Between 60 and 300 m, the
frequency increases by about one day per 15 m of
elevation and even more rapidly on higher ground.
