Geoscience Reference
In-Depth Information
the first half, cyclonic and other stormy spells
generally in October to November.
4
Early winter
(from about the third week in
November to mid-January). Long spells are
less frequent than in summer and autumn.
They are usually of westerly type, giving mild,
stormy weather.
5
Late winter and early spring
(from about the
third week in January to the end of March).
The long spells at this time of year can be of
very different types, so that in some years it is
midwinter weather, while in other years there
is an early spring from about late February.
60
60
ALL TYPES
40
40
20
20
LATE WINTER
EARLY SPRING
LATE WINTER
EARLY SPRING
SPRING / EARLY
SUMMER
SPRING / EARLY
SUMMER
EARLY
WINTER
EARLY
WINTER
HIGH SUMMER
HIGH SUMMER
AUTUMN
AUTUMN
0
0
20
20
CYCLONIC
0
0
40
40
WESTERLY
20
20
0
0
20
20
NORTHERLY
0
0
20
20
EASTERLY
0
0
J
FM
A
M
J
J
A
S
O
N
D
5 Synoptic anomalies
The mean climatic features of pressure, wind and
seasonal airflow regime provide only a partial
picture of climatic conditions. Some patterns
of circulation occur irregularly and yet, owing
to their tendency to persist for weeks or even
months, form an essential element of the climate.
Blocking patterns
are an important example. It
was noted in Chapter 7 that the zonal circulation
in mid-latitudes sometimes breaks down into a
cellular pattern. This is commonly associated with
a split of the jet stream into two branches over
higher and lower mid-latitudes and the formation
of a cut-off low (see Chapter 8H.4) south of a
high pressure cell. The latter is referred to as a
blocking anticyclone
since it prevents the normal
eastward motion of depressions in the zonal flow.
Figure 10.9
illustrates the frequency of blocking
for part of the Northern Hemisphere with five
major blocking centers shown (H). A major area
of blocking is Scandinavia, particularly in spring.
Cyclones are diverted northeastward towards the
Norwegian Sea or southeastward into southern
Europe. This pattern, with easterly flow around
the southern margins of the anticyclone, produces
severe winter weather over much of northern
Europe. In January to February 1947, for example,
easterly flow across Britain as a result of blocking
over Scandinavia led to extreme cold and frequent
snowfall. Winds were almost continuously from
Figure 10.8
The frequency of long spells (25 days
or more) of a given airflow type over Britain, 1898-1947.
The diagram showing all long spells also indicates a
division of the year into 'natural seasons'.
Source: After Lamb (1950). By permission of the Royal
Meteorological Society.
In addition to these singularities, major
seasonal trends are recognizable. For the British
Isles, Lamb identified five
natural seasons
on the
basis of spells of a particular type lasting for 25
days or more during the period 1898-1947 (
Figure
10.8
). These seasons are:
1
Spring to early summer
(the beginning of April
to mid-June). This is a period of variable
weather conditions during which long spells
are least likely. Northerly spells in the first
half of May are the most significant feature,
although there is a marked tendency for anti-
cyclones to occur in late May to early June.
2
High summer
(mid-June to early September).
Long spells of various types may occur in
different years. Westerly and northwesterly
types are the most common and they may be
combined with either cyclonic or anticyclonic
types. Persistent sequences of cyclonic type
occur more frequently than anticyclonic types.
3
Autumn
(the second week in September to
mid-November). Long spells are again present
in most years. Anticyclonic spells are mainly in
