Geoscience Reference
In-Depth Information
the east between 22 January and 22 February and
even daytime temperatures rose little above
freezing point. Snow fell in some part of Britain
every day from 22 January to 17 March 1947, and
major snowstorms occurred as occluded Atlantic
depressions moved slowly across the country.
Other notably severe winter months - January
1881, February 1895, January 1940 and February
1986 - were the result of similar pressure
anomalies with pressure well above average to the
north of the British Isles and below average to
the south, giving persistent easterly winds.
The effects of winter blocking situations over
northwest Europe are shown in
Figures 10.10
and 10.11
. Precipitation amounts are above
normal, mainly over Iceland and the western
Mediterranean, as depressions are steered around
the blocking high following the path of the upper
jet streams. Over most of Europe, precipitation
remains below average and this pattern is repeated
with summer blocking. Winter temperatures
are above average over the northeastern Atlantic
and adjoining land areas, but below average over
Central and Eastern Europe and the Mediter-
ranean due to outbreaks of cP air (
Figure 10.11
).
The negative temperature anomalies associated
with cool northerly airflow in summer cover most
of Europe; only northern Scandinavia has above-
average values.
The exact location of the block is of the utmost
importance. For instance, in the summer of 1954
a blocking anticyclone across Eastern Europe
and Scandinavia allowed depressions to stagnate
over the British Isles, giving a dull, wet August,
whereas in 1955 the blocking was located over the
North Sea and a fine, warm summer resulted.
Persistent blocking over northwestern Europe
caused drought in Britain and the continent
during 1975-1976. Another, less common
location of blocking is Iceland. A notable example
was the 1962-1963 winter, when persistent high
pressure southeast of Iceland led to northerly and
northeasterly airflow over Britain. Temperatures
in central England were the lowest since 1740,
with a mean of 0
H
H
H
H
H
Figure 10.9
Frequency of occurrence of blocking condi-
tions for the 500mb level for all seasons. Values were
calculated as five-day means for 381
×
381km squares for the
period 1946-78.
Source: From Knox and Hay (1985). By permission of the Royal
Meteorological Society.
February 1963. Central Europe was affected by
easterly airstreams with mean January tempera-
tures 6
°
C below average.
6 Topographic effects
In various parts of Europe, topography has a
marked effect on the climate, not only of the
uplands themselves but also of adjacent areas.
Apart from the more obvious effects on tempera-
tures, precipitation amounts and winds, the major
mountain masses also affect the movement of
frontal systems. Frictional drag over mountain
barriers increases the slope of cold fronts and
decreases the slope of warm fronts, so that the
latter are slowed down and the former accelerated.
The Scandinavian mountains form one of
the most significant climatic barriers in Europe
as a result of their orientation with regard to
westerly airflow. Maritime air masses are forced to
rise over the highland zone, giving annual precipi-
tation totals of over 2500mm on the mountains
of western Norway, whereas descent in their lee
°
C for December 1962 to
