Geoscience Reference
In-Depth Information
is reflected in the high mean pressure over the latter area
in January (see Figure 10.26). Consequently, although
the winter half-year is the rainy period, there are rather
few rain-days. On average, rain falls on only six days per
month during winter in northern Libya and southeast
Spain; there are twelve rain-days per month in western
Italy, the western Balkan Peninsula and the Cyprus area.
The higher frequencies (and totals) are related to the
areas of cyclogenesis and to the windward sides of
peninsulas.
Regional winds are also related to the meteorological
and topographic factors. The familiar cold, northerly
winds of the Gulf of Lions (the
mistral
), which are
associated with northerly mP airflow, are best developed
when a depression is forming in the Gulf of Genoa east
of a high-pressure ridge from the Azores anticyclone.
Katabatic and funnelling effects strengthen the flow in
the Rhône valley and similar localities, so that violent
winds are sometimes recorded. The mistral may last for
several days until the outbreak of polar or continental air
ceases. The frequency of these winds depends on their
definition. The average frequency of strong mistrals in
the south of France is shown in Table 10.3 (based on
occurrence at one or more stations from Perpignan to
the Rhône in 1924 to 1927). Similar winds may occur
along the Catalan coast of Spain (the
tramontana
, see
Figure 10.28) and also in the northern Adriatic (the
bora
) and northern Aegean Seas when polar air flows
southward in the rear of an eastward-moving depression
and is forced over the mountains (cf. Chapter 6C.1).
In Spain, cold, dry northerly winds occur in several
different regions. Figure 10.28 shows the
galerna
of the
north coast and the
cierzo
of the Ebro valley.
The generally wet, windy and mild winter season in
the Mediterranean is succeeded by a long indecisive
spring lasting from March to May, with many false
starts of summer weather. The spring period, like
that of early autumn, is especially unpredictable. In
March 1966, a trough moving across the eastern
Mediterranean, preceded by a warm southerly
khamsin
and followed by a northerly airstream, brought up to 70
mm of rain in only four hours to an area of the southern
Negev Desert. Although April is normally a dry month
in the eastern Mediterranean, Cyprus having an average
of only three days with 1 mm of rainfall or more, high
rainfalls can occur, as in April 1971 when four depres-
sions affected the region. Two of these were Saharan
depressions moving eastward beneath the zone of
diffluence on the cold side of a westerly jet and the other
two were intensified in the lee of Cyprus. The rather
rapid collapse of the Eurasian high-pressure cell in
April, together with the discontinuous northward
and eastward extension of the Azores anticyclone,
encourages the northward displacement of depressions.
Even if higher latitude air does penetrate south to the
Mediterranean, the sea surface there is relatively cool
and the air is more stable than during the winter.
By mid-June, the Mediterranean basin is dominated
by the expanded Azores anticyclone to the west,
while to the south the mean pressure field shows a
low-pressure trough extending across the Sahara
from southern Asia (see Figure 10.26). The winds are
predominantly northerly (e.g. the
etesians
of the
Aegean) and represent an eastward continuation of the
northeasterly trades. Locally, sea breezes reinforce these
winds, but on the Levant Coast they cause surface
southwesterlies. Land and sea breezes, involving air
up to 1500-m deep, largely condition the day-to-day
weather of many parts of the North African coast.
Depressions are by no means absent in the summer
months, but they are usually weak. The anticyclonic
character of the large-scale circulation encourages
subsidence, and airmass contrasts are much reduced
compared with winter. Thermal lows form from time to
time over Iberia and Anatolia, although thundery out-
breaks are infrequent due to the low relative humidity.
The most important regional winds in summer are
of continental tropical origin. There are a variety of local
names for these usually hot, dry and dusty airstreams -
scirocco
(Algeria and the Levant),
lebeche
(southeast
Table 10.3
Number of days with a strong mistral in the south of France.
Speed
J
F
M
A
M
J
J
A
S
O
ND
Year
≥
11 m s
-1
(21 kt)
10
9
13
11
8
9
9
7
5
5
7
10
103
≥
17 m s
-1
(33 kt)
4
4
6
5
3
2
0.6
1
0.6
0
0
4
30
Source
: After
Weather in the Mediterranean
(HMSO, 1962).
