Geoscience Reference
In-Depth Information
Table 10.1
General weather characteristics and air masses associated with Lamb's 'Airflow Types' over
the British Isles
Type
Weather conditions
Westerly
Unsettled weather with variable wind directions as depressions cross the country. Mild and
stormy in winter, generally cool and cloudy in summer (mP, mPw, mT).
Northwesterly
Cool, changeable conditions. Strong winds and showers affect windward coasts especially, but
the southern part of Britain may have dry, bright weather (mP, mA).
Northerly
Cold weather at all seasons, often associated with polar lows. Snow and sleet showers in winter,
especially in the north and east (mA).
Easterly
Cold in the winter half-year, sometimes very severe weather in the south and east with snow or
sleet. Warm in summer with dry weather in the west. Occasionally thundery (cA, cP).
Southerly
Warm and thundery in summer. In winter, it may be associated with a low in the Atlantic, giving
mild, damp weather especially in the southwest, or with a high over central Europe, in which case
it is cold and dry in winter (mT, or cT, summer; mT or cP, winter).
Cyclonic
Rainy, unsettled conditions, often accompanied by gales and thunderstorms. This type may refer
either to the rapid passage of depressions across the country or to the persistence of a deep
depression (mP, mPw, mT).
Anticyclonic
Warm and dry in summer, occasional thunderstorms (mT, cT). Cold and frosty in winter with fog,
especially in autumn (cP).
The monthly frequency of different air-mass
types over Great Britain was analyzed by J. Belasco
for 1938-1949. There is a clear predominance of
northwesterly to westerly polar maritime (mP and
mPw) air, which has a frequency of 30 percent or
more over southeast England in all months except
March. The maximum frequency of mP air at Kew
(London) is 33 percent (with a further 10 percent
mPw) in July. The proportion is even greater
in western coastal districts, with mP and mPw
occurring in the Hebrides, for example, on at least
38 percent of days throughout the year.
Air-mass types may also be used to describe
typical weather conditions. Northwesterly mP
airstreams produce cool, showery weather at all
seasons. The air is unstable, forming cumulus
clouds, although inland in winter and at night the
clouds disperse, giving low night temperatures.
Over the sea, heating of the lower air continues by
day and by night in winter months, so showers
and squalls can occur at any time, and these affect
windward coastal areas. The average daily mean
temperatures with mP air are within about ±1°C
of the seasonal means in winter and summer,
depending on the precise track of the air. More
extreme conditions occur with mA air, the
temperature departures at Kew being approxi-
mately -4°C in summer and winter. The visibility
in mA air is usually very good. The contribution
of mP and mA air masses to the mean annual
rainfall over a five-year period at three stations in
northern England and North Wales is given in
Table 10.2
, although it should be noted that both
air masses may also be involved in non-frontal
polar lows. Over much of southern England, and
in areas to the lee of high ground, northerly and
northwesterly airstreams usually give clear, sunny
weather with few showers. This is illustrated
in
Table 10.2
. At Rotherham, in the lee of the
Pennines, the percentage of the rainfall occurring
with mP air is much lower than on the West Coast
(Squires Gate).
Maritime tropical air commonly forms the
warm sector of depressions moving from between
west and south towards Britain. The weather is
unseasonably mild and damp with mT air in
winter. There is usually a complete cover of stratus
or stratocumulus cloud and drizzle or light rain
may occur, especially over high ground, where
low cloud produces hill fog. The clearance of
