Geoscience Reference
In-Depth Information
In summer, 'plumes' of warm, moist mT air may
spread northward from the vicinity of Spain into western
Europe. This air is very unstable, with a significant
vertical wind shear and a wet-bulb potential temperature
that may exceed 18°C. Instability may be increased if
cooler Atlantic air is advected under the plume from the
west. Thunderstorms tend to develop along the leading
northern edge of the moist plume over Britain and north-
west Europe. Occasionally, depressions develop on the
front and move eastward, bringing widespread storms
to the region (Figure 10.6). On average, two mesoscale
convective systems affect southern Britain each summer,
moving northward from France (see p. 201).
Continental polar air occasionally affects the British
Isles between December and February. Mean daily
temperatures are well below average and maxima rise
to only a degree or so above freezing point. The air is
basically very dry and stable (see easterly type in
January, Figure 10.4) but a track over the central part of
the North Sea supplies sufficient heat and moisture to
cause showers, often in the form of snow, over eastern
England and Scotland. In total this provides only a
very small contribution to the annual precipitation, as
Table 10.2 shows, and on the West Coast the weather
is generally clear. A transitional cP-cT type of airmass
reaches Britain from southeastern Europe in all seasons,
although less frequently in summer. Such airstreams are
dry and stable.
Continental tropical air occurs on average about one
day per month in summer, which accounts for the rarity
of summer heatwaves, since 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 occasionally surface heating
may trigger off a thunderstorm (see southerly cyclonic
type in July, Figure 10.4).
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 airflow in the lower troposphere
(
Grosswetterlage
) over Central Europe. Some of the
European singularities that occur most regularly are as
follows:
1
A sharp increase in the frequency of westerly and
northwesterly type over Britain takes place in about
mid-June. These invasions of maritime air also affect
central Europe, and this period marks the beginning
of the European 'summer monsoon'.
4 Singularities and natural seasons
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, forty 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
Figure 10.7
The percentage frequency of anticyclonic, westerly
and cyclonic conditions over Britain, 1898 to 1947.
Source
: After Lamb (1950), by permission of the Royal Meteorological
Society.
