Weather and climate in middle and high latitudes - Atmosphere, Weather and Climate - page 268

formation of these Genoa-type depressions whenever
conditionally unstable mP air invades the region. These
depressions are exceptional in that the instability of
the air in the warm sector gives unusually intense
precipitation along the warm front. The unstable mP air
produces heavy showers and thunderstorm rainfall
to the rear of the cold front, especially between 5 and
25°E. This warming of mP air produces air designated
as Mediterranean . The mean boundary between this
Mediterranean airmass and cT air flowing northeast-
ward from the Sahara is referred to as the Mediterranean
front (see Figure 10.26). There may be a temperature
discontinuity as great as 12 to 16°C across it in late
winter. Saharan depressions and those from the western
Mediterranean move eastward, forming a belt of low
pressure associated with this frontal zone and frequently
drawing cT northward ahead of the cold front as the
warm, dust-laden scirocco (especially in spring and
autumn when Saharan air may spread into Europe). The

movement of Mediterranean depressions is modified
both by relief effects and by their regeneration in
the eastern Mediterranean through fresh cP air from
Russia or southeast Europe. Although many lows pass
eastward into Asia, there is a strong tendency for others
to move northeastward over the Black Sea and Balkans,
especially as spring advances. Winter weather in
the Mediterranean is quite variable as the subtropical
westerly jet stream is highly mobile and may occa-
sionally coalesce with the southward-displaced polar
front jet stream.
With high index zonal circulation over the Atlantic
and Europe, depressions may pass far enough to the
north that their cold-sector air does not reach the
Mediterranean, and then the weather there is generally
settled and fine. Between October and April, anti-
cyclones are the dominant circulation type for at least 25
per cent of the time over the whole Mediterranean area
and in the western basin for 48 per cent of the time. This

Figure 10.27 Tracks of Mediterranean depressions, showing average annual frequencies, together with airmass sources.
Source : After Weather in the Mediterranean (HMSO, 1962) (Crown Copyright Reserved).