Geoscience Reference
In-Depth Information
Figure 10.28
Areas affected by the
major regional winds in Spain as a
function of season.
Source
: From Tout and Kemp (1985), by
permission of the Royal Meteorological
Society.
Spain) and
khamsin
(Egypt) - which move northward
ahead of eastward-moving depressions. In the Negev,
the onset of an easterly
khamsin
may cause the relative
humidity to drop to less than 10 per cent and temper-
atures to rise to as much as 48°C. In southern Spain, the
easterly
solano
brings hot, humid weather to Andalucia
in the summer half-year, whereas the coastal
levante
-
which has a long fetch over the Mediterranean - is moist
and somewhat cooler (see Figure 10.28). Such regional
winds occur when the Azores high extends over western
Europe with a low-pressure system to the south.
Many stations in the Mediterranean receive only
a few millimetres of rainfall in at least one summer
month, yet the seasonal distribution does not conform to
the pattern of simple winter maximum over the whole
of the Mediterranean basin. Figure 10.29 shows that
this is found in the eastern and central Mediterranean,
whereas Spain, southern France, northern Italy and
the northern Balkans have more complicated profiles
with a maximum in autumn or peaks in both spring
and autumn. This double maximum may be interpreted
as a transition between the continental interior type
with summer maximum and the Mediterranean type
with winter maximum. A similar transition region
occurs in the southwestern United States (see Figure
10.20), but local topography in this intermontane zone
introduces irregularities into the regimes.
4 North Africa
The dominance of high-pressure conditions in the
Sahara is marked by the low average precipitation in
this region. Over most of the central Sahara, the mean
annual precipitation is less than 25 mm, although the
high plateaux of the Ahaggar and Tibesti receive over
100 mm. Parts of western Algeria have gone at least two
years without more than 0.1 mm of rain in any twenty-
four-hour period, and most of southwest Egypt as much
as five years. However, twenty-four-hour storm rainfalls
approaching 50 mm (more than 75 mm over the high
plateaux) may be expected in scattered localities.
During a thirty-five-year period, excessive short-period
