Geoscience Reference
In-Depth Information
these south or southeast winds bring hot, settled
weather. The lower layers are stable and the air is
commonly hazy, but the upper layers tend to be
unstable and surface heating may occasionally
trigger off a thunderstorm (see southerly cyclonic
type in July,
Figure 10.4
).
60
60
ANTICYCLONIC
40
40
20
20
0
0
60
60
WESTERLY
40
40
20
20
4 Singularities and natural
seasons
0
0
40
40
Popular weather lore expresses the belief that
each season has its own weather (for example,
in England, 'February fill-dyke', and 'April
showers'). Ancient adages suggest that even the
sequence of weather may be determined by the
conditions established on a given date. For
example, 40 days of wet or fine weather are said
to follow St Swithin's Day (15 July) in England;
sunny conditions on 'Groundhog Day' (2
February) are claimed to portend six more weeks
of winter in the United States. Some of these ideas
are fallacious, but others contain more than a
grain of truth if properly interpreted.
The tendency for a certain type of weather to
recur with reasonable regularity around the same
date is termed a
singularity
. Many calendars of
singularities have been compiled, particularly in
Europe. Early ones, which concentrated upon
anomalies of temperature or rainfall, did not
prove very reliable. Greater success has been
achieved by studying singularities of circulation
pattern; Flohn, and Hess and Brezowsky, have
prepared catalogues for Central Europe and Lamb
for the British Isles. Lamb's results are based on
calculations of the daily frequency of the airflow
categories between 1898 and 1947, some examples
of which are shown in
Figure 10.7
. A noticeable
feature is the infrequency of the westerly type
in spring, the driest season of the year in
the British Isles, and also in northern France,
northern Germany and in the countries bordering
the North Sea. The European catalogue is based
on a classification of large-scale patterns of air-
flow in the lower troposphere (
Grosswetterlage
)
over Central Europe. Some of the European
20
20
CYCLONIC ( 5-day mean)
0
0
J
FM
A
M
J
J
A
S
O
N
D
Figure 10.7
The percentage frequency of anti-
cyclonic, westerly and cyclonic conditions over
Britain, 1898-1947.
Source: After Lamb (1950).
singularities that occur most regularly are as
follows:
1
A sharp increase in the frequency of westerly
and northwesterly type over Britain takes place
about the middle of June. These invasions
of maritime air also affect Central Europe,
and this period marks the beginning of the
European 'summer monsoon'.
2
Around the second week in September, Europe
and Britain are affected by a spell of anti-
cyclonic weather. This may be interrupted by
Atlantic depressions, giving stormy weather
over Britain in late September, although
anticyclonic conditions again affect Central
Europe at the end of the month and Britain
during early October.
3
A marked period of wet weather often affects
Western Europe and also the western half of
the Mediterranean at the end of October,
whereas the weather in Eastern Europe
generally remains fine.
4
Anticyclonic conditions return to Britain and
affect much of Europe about mid-November,
giving rise to fog and frost.
5
In early December, Atlantic depressions push
eastward to give mild, wet weather over most
of Europe.
