| Summary: | Malta’s climate can be described in the context of the topography of the Mediterranean basin where the flow of the lower atmosphere into the basin occurs mainly through mountain gaps, except over the southern shores east of Tunisia. The seasonal features of the Mediterranean can be traced from the motion and development of the pressure systems over the Atlantic, Eurasia and Africa. While the Mediterranean spring is often a period of indecisive weather, summer is characterised by the intensification of the Azores High which tends to stably extend up to the Central Mediterranean, giving general weather conditions consisting of light surface winds ranging from the northwest to northeast. Autumn is relatively short and leads to wintry conditions in a fairly decisive and quick way. During this season Atlantic depressions move eastwards across northern Europe into the Mediterranean bringing with them waves of cold air. In its path, this cold air comes into contact with warm moist air causing vertical instability, the development of vigorous depressions, rainfall and frequent gales. From time to time the eastward march of travelling depressions is interrupted by cold air coming from the Arctic via the Norwegian Sea or Russia. This great thermal contrast leads to very active depressions. In the Central Mediterranean region both Sicily and the Tunisian peninsula may play an important part on the local weather. Under certain prevailing conditions Sicily can act as a barrier against strong low-level northerly winds. This Italian island can also create local instabilities due to land heating effects or heat lows which may be advected towards the Maltese Islands depending on the prevailing winds. Transient North African low pressure systems have the potential to produce strong winds over the Central Mediterranean. When for example North African lows occur south of the Atlas Mountains, strong easterly to southeasterly winds are likely over the Central Mediterranean resulting in high seas. The presence of the surrounding water mass shapes significantly the climate of the Maltese Islands. The general weather is often cooler and more humid than what is experienced in inland areas of larger land masses. The high thermal capacity of the sea also reduces large fluctuations in the ambient temperature of the islands. But the presence of surrounding warm waters during the end of the summer season is a source of major weather instability when colder air migrates into the Central Mediterranean, thus creating areas with heavy thunderstorms and intense precipitation. Advances of continental tropical air into the Central Mediterranean after a cold spell can give rise to active warm fronts, sometimes producing very active cumulonimbus clouds, copious rainfall and thunderstorm over the Maltese Islands. A period of sirocco in the Central Mediterranean, sometimes lasting for many days, often follows the first autumnal invasion of cold air. peer-reviewed
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