Air movements
Weather is produced in the air which overlies a place at a given
time. Three main kinds of air flow over the British
Isles—maritime tropical air from the southwest, maritime
polar air from the west or northwest, and continental polar air
from the European mainland in winter. Each kind arrives as an
airstream, having acquired its original characteristics in
the source region of an
air mass.
An air mass is a large body of air, perhaps
thousands of miles across. Stagnating over an expanse of land or of
sea, it is cooled, warmed, dried or moistened according to the
character of the underlying surface. In a very few days it can
reach equilibrium, in respect of temperature and humidity, with
that surface; and when it moves away from the source region as an
airstream, it retains its distinctive character for some
time.
Maritime air, whether maritime tropical (mT) or
maritime polar (mP) air is brought in by the westerlies. Maritime
tropical air is supplied to the British Isles by the Azores High;
although it is typically moist it is also usually stable, so that
it tends to give little precipitation. In summer it can produce
warm, settled weather, and in winter it gives unseasonable
warmth—as, for instance, during the winter of 1956/57.
Maritime polar (mP) air is very variable. Its source regions lie
over the North Atlantic and eastern Canada. When it comes from a
northerly quarter, and when it arrives during the summer, it is
liable to be unstable and to discharge much precipitation, as in
the summer of 1958, but it can also come in from the west after a
long traverse of the warm ocean, and may then be fairly stable and
bring little rain to lowland areas. Continental air sometimes
reaches the British Isles in summer as a warm, dry air- stream, but
it is more frequently experienced in winter when, as icy currents
of continental polar (cP) air, it crosses the North Sea and brings
bitter weather to eastern and inland districts of Great Britain. In
some winters, cP air overspreads the whole of the British Isles and
occasions severe and prolonged frost. Such a winter was that of
1946/47.
Travelling Lows
Airstreams of varying type are drawn, or blown, across the British
Isles by temporary pressure-systems, some of which are themselves
moving. Low-pressure systems tend to be both vigorous and mobile.
Most of the travelling lows which cross the British Isles move from
west to east or from southwest to northeast, in accordance with the
general flow of the westerlies. Many of them are frontal
depressions, in which the individual airstreams are separated from
one another by narrow zones of transition known as fronts. Lows
commonly originate on the polar front, the boundary between the
westerlies on the south and cold air blowing outwards from higher
latitudes on the north. A bulge on the polar front becomes enlarged
into a deep re-entrant (Fig. 17) which encloses a warm sector
surrounded by cold air. The whole system travels towards the east;
since it can cross the British Isles in the space of one or two
days, a given station can experience three distinct kinds of
weather in a short time. In the forepart of the low, in advance of
the warm front, polar air is drawn northwards; in winter it reaches
most of Great Britain from the mainland of Europe, and may well be
very cold. Cloud thickens and rain or snow falls as the warm front
approaches; but as it passes there is a rise in temperature, for
the warm sector is typically occupied by maritime tropical air.
Precipitation in the warm sector is not often heavy, although there
is much cloud; precipitation is renewed along the cold front, the
passage of which is marked by a drop in temperature. If the
maritime polar air behind the cold front is unstable, it is likely
to give heavy showers.
Lows are often partly or wholly occluded by the
time that they reach the British Isles, the cold front having
caught up the warm front, so that the air in the warm sector is
raised above the ground. The weather typical of the warm sector is
omitted from the sequence, and the two frontal rain-belts merge
into one.
In the heart of a travelling low, whether it is
occluded or not, general rain often occurs. This means that any low
which traverses the British Isles is likely to supply more
precipitation in the north than in the south, even when the effect
of high ground in the north is left aside. Moreover, highs tend to
form over the English lowlands in the latter part of winter,
fending off the moving lows, forcing them to take a north-easterly
path and to bring precipitation to the northwest rather than to the
southeast. In both these ways the contrast between the drier and
the wetter parts of Great Britain is increased.
Systems of high pressure
Systems of high pressure are simpler than those of low pressure.
Highs are stagnant systems of stable air which give little
precipitation. Some bring overcast skies; if there is little cloud,
highs result in warm sunny weather in summer and in sunny but
frosty weather in winter. Because they are stable and inert, highs
may persist for days on end.
Seasonal Weather
British weather is affected by several other types of
pressure-systems, in addition to simple travelling lows and
more-or-less stationary highs, all of which increase the general
variability of British weather. But the number, strength, and
frequency of travelling lows and stagnant highs are the chief
influences upon the character of seasonal weather.
The contrast between weather dominated by
travelling lows and that dominated by persistent highs has rarely
been better shown than in the two summers of 1955 and 1956. In the
first of these two seasons highs were dominant. They blocked the
approaching lows, which moved away to the northeast; prolonged
droughts occurred, even in the west of Ireland. By contrast, during
the summer of 1956 many lows passed across the British Isles.
Unusually heavy rain was recorded in all parts, for large
quantities of polar maritime air streamed in and frontal rain was
common. Many stations recorded more than twice the average rainfall
in August, and the difficulties of harvesting will long be
remembered. Nevertheless, the total precipitation for the whole
year of 1956 differed very little from the average, for the very
wet summer was compensated by slight precipitation both in spring
and in late autumn and early winter.
Local climates
Just as weather varies during short periods, so does climate vary
over short distances. Local climate in highland areas changes very
rapidly from place to place. The contrasts are not merely those of
height, although precipitation can increase with height as
strikingly as temperature falls. Nor are they to be explained
wholly by aspect, although contrasts in aspect are important
where—as in the Scottish Highlands—climate is so
generally severe that every advantageous circumstance becomes
significant. In highland areas especially, and on uneven ground
everywhere, cold-air drainage exerts a powerful influence on local
climate. Cold air can only drain downhill when the air as a whole
is still, or very nearly so, and the air can remain still only
during the calm weather brought by high-pressure systems. Although
calms in the British Isles are not very common, they occur often
enough to encourage the drainage of cold air into certain enclosed
valleys, which in consequence experience frequent fog and frost in
the colder months. The Lea Valley, north of London, is perhaps the
most notorious example, but there is no doubt that many Welsh,
Pennine, and Scottish valleys suffer in a similar way. In coastal
districts, air chilled during the night can flow out to sea; sea
breezes rise by day if the air is still, giving the coasts their
characteristically small daily range of temperature.
