Geoscience Reference
In-Depth Information
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 Mediterranean due to
outbreaks of cP air (Figure 10.11). The negative tem-
perature 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 to 1976. Another, less common
location of blocking is Iceland. A notable example was
the 1962 to 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°C for
December 1962 to February 1963. Central Europe was
affected by easterly airstreams with mean January
temperatures 6°C below average.
Figure 10.9
Frequency of occurrence of blocking conditions for
the 500-mb level for all seasons. Values were calculated as five-
day means for 381
381-km squares for the period 1946 to
1978.
Source
: From Knox and Hay (1985), by permission of the Royal
Meteorological Society.
diverted northeastward towards the Norwegian Sea
or southeastward into southern Europe. This pattern,
with easterly flow around the southern margins of the
anticyclone, produces severe winter weather over
much of northern Europe. In January to February 1947,
for example, easterly flow across Britain as a result
of blocking over Scandinavia led to extreme cold and
frequent snowfall. Winds were almost continuously
from 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 north-
west 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,
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 temperatures, 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
airmasses are forced to rise over the highland zone,
giving annual precipitation totals of over 2500 mm on
the mountains of western Norway, whereas descent in
their lee produces a sharp decrease in the amounts. The
upper Gudbrandsdalen and Osterdalen in the lee of the
Jotunheim and Dovre Mountains receive an average of
less than 500 mm, and similar low values are recorded
in central Sweden around Östersund.
