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and a wet-bulb potential temperature that may
exceed 18
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 precipi-
tation, as
Table 10.2
shows, and on the West Coast
the weather is generally clear. A transitional cP-cT
type of air mass 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
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 northwest 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.
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
°
(A)
(B)
H
H
H
L
1010
L
H
1200 UTC
19 AUG
0000 UTC
20 AUG
(C)
(D)
L
L
1010
L
L
1010
L
1200 UTC
20 AUG
0000 UTC
21 AUG
Figure 10.6
Distribution of thunderstorms over Western Europe during the period 19-21 August 1992 (storms
shown for the four-hour period preceding the times given). A small depression formed over the Bay of Biscay and
moved eastward along the boundary of the warm air, developing a strong squall line.
Source: Blackall and Taylor (1993). Reprinted from the Meteorological Magazine(Crown copyright ©) by permission of the Controller of Her
Majesty's Stationery Office.
