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. |
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