Many north-west Europeans consider the climate of Italy to be one of its principal assets. There is undoubtedly some justification for this, but simply to consider that it is uniform throughout the country or always a good one for agriculture or a desirable one for tourists is misleading. Before discussing its principal features a number of points should be noted about the situation of the country. In the first place Italy occupies a mid-latitude position. The river Po runs close to 45U+B00N, while south-east Sicily is only 37U+B00N. This means that although Italy is well outside the tropics it is near enough in the summer months to have very hot conditions; not having the sun overhead is offset by longer days. Secondly, the position of the country in a maritime setting should be noted. Though virtually a closed sea, and therefore not influenced by currents from other latitudes, the Mediterranean is large enough to exert a considerable influence on the lands along its shores. Few parts of the Peninsula of Italy are more than 50 miles from the sea and only part of the Alps is more than 100. The presence of the sea is one reason why the annual range of temperature in the Peninsula and Islands is not very great. Thirdly, weather conditions are profoundly affected and greatly complicated in Italy by the presence of mountain areas and by the arrangement of these in relation to the movement of air masses.
Temperature, of course, diminishes with increase of altitude and a map showing sea-level isotherms is misleading. In addition, mountains shut off valleys and basins from maritime influences and in places--the most striking instance being between the Riviera in Liguria and the Po valley to the interior--remarkable climatic contrasts occur over short distances. Indeed the Alps, which are extensive enough and high enough to form a climatic region of their own, are also a major climatic divide between north-west and central Europe on the one hand and northern Italy on the other. The alternation of land and sea and of mountains and plains have contributed to give Italy a great variety of climatic conditions.
For much of the year Italian weather is only a little less changeable than that of north-west Europe. At the same time there is considerable variability from year to year, winters sometimes being much colder than average, sometimes much milder, and rainfall varying greatly in quantity and occurrence from year to year. The typical Mediterranean climate, with a hot summer and little precipitation, and a mild winter with considerable precipitation, does not occur widely in Italy; it is confined to the southern part of the Peninsula and to the Islands.
Italy's weather is caused by the movement of air masses from various directions. From the Atlantic, mainly in winter, comes humid air, bringing widespread cyclonic rain; depressions tend to follow certain courses, altering their general west-east direction to pass south-east along either side of the mountains of the Peninsula. The depressions affect the South and Islands most profoundly in winter; in spring and autumn the North is more affected. Much of the rain comes from this source, but usually it occurs in short, heavy downpours.
During the winter the North comes under the influence of the margin of the continental anticyclone for considerable periods, with cold, dry, air extending from the interior of Eurasia into central Europe. It is this relatively cold air, coming into contact with the relatively warm, low-pressure area of the Mediterranean, that gives strong cold winds, the maestrale (mistral) in the west via the Rhone valley, and the bora at the head of the Adriatic. These are especially strong when depressions are passing. In general the Northern Lowland and the Alps themselves are unaffected by these winds and have long spells of still, cold, dry weather and clear skies. Mists are frequent in the lowland, especially in the vicinity of irrigated areas and the main rivers, and along the lagoon coast.
In the summer relatively stable conditions are established over the cool, high pressure of the Mediterranean Sea and the humid Atlantic air masses rarely penetrate. Winds tend to come from the south-east and south, and the scirocco africano at times makes the summer in Sicily extremely hot. Very little rains falls in the summer in the South and Islands but summer rain increases northwards, and north of Rome is considerable, often coming in the form of short heavy thunderstorms. North of the river Po summer rain actually exceeds winter rain, a feature that by definition excludes a considerable part of Italy from the typical Mediterranean climate.
To these general movements of air and features of rainfall regime must be added many more local features. Mountain and valley winds occur in the Alps, onshore and offshore winds along the coasts. The alignment of mountain ranges and valleys, the presence of lakes, and so on, also produce local climatic effects. It is not surprising that any division of Italy on a climatic basis distinguishes several regions.
In January there is a striking contrast between the Northern Lowland (around 0C, 32F) and much of the South and Islands (generally over 10C, 50F, along the coasts); the difference in winter of 8C, 14F, between the Riviera and the Po valley over a matter of 80 km is remarkable. In contrast, in July the difference between northern and southern Italy is slight (23-26C, 73-79F), though the highest temperatures are still of course in the South and in Sicily. Clearly therefore the annual range is much greater in the Northern Lowland (mostly over 22C, 40F) than in the South (less than 12C, 22F in places). This contrast is the most marked feature brought out in a comparison of the two temperature maps and reveals a very hot summer everywhere except at considerable altitudes, but a much milder winter in most of the Peninsula and Islands than in the North, which is clearly more 'continental' in nature. Other features to note are the somewhat colder conditions (a matter of 1-2C, 2-4F) in winter on the Adriatic side of the Peninsula than on the Tyrrhenian side at the same latitude; the greater range of temperature in the interior of the Peninsula than along its coasts; and the greater range in the North Italian Lowland than in the adjoining mountain areas, a feature not easily discernible on the maps on account of complications of relief.
Turning from averages to extremes, it will be appreciated that although not apparently differing greatly from the north in summer it is the extreme south that has the highest temperatures recorded: nearly 50C, 122F, in Sicily, which of course is about 600 miles from the Sahara Desert. The lowest temperatures not artificially induced by high altitude, naturally, occur in the Northern Lowland, especially in the western part, -18C, OF, having been recorded at Alessandria. Temperatures below 0C, 32F, do however occur in the Peninsula (lowest in Rome -8C, 18F, in San Remo only -4C, 25F) but do not generally last long.
Annual precipitation tends to diminish from North to South. More locally, the Tyrrhenian side of the Peninsula is wetter than the Adriatic side due to its more frequent exposure to depressions from a general westerly direction. On the other hand, in the Alps and Northern Lowland, precipitation is greater in the east near the head of the Adriatic than in the west. Particularly heavy rainfall is recorded along the foot of the Alps and again in Liguria, but here more on the Riviera di Levante, which faces south-west, than on the Riviera di Ponente, facing south-east. In the mountains many valleys receive only a small fraction of the amount of rain falling on adjoining mountain summits: for example the floor of the Valle d'Aosta has only 400 mm (16 inches) while nearby mountain areas have 3,000 mm (about 120 inches). The wettest parts of Italy are found in the Alps and Northern Apennines, where over 2,000 mm (about 80 inches) is common, and in the mountains of the Peninsula, with over 1,000 mm (about 40 inches). The driest places are in Apulia, Sicily and Sardinia, below 400 mm (16 inches) being common; the driest place of all is San Pancrazio Salentino in the tip of the 'heel'--virtually desert--200 mm (8 inches).
In the Alps and Lowlands north of the Po rain falls throughout the year, but more falls in the summer half. In most places here there are in reality autumn and spring maxima. Between the Po and roughly the latitude of Rome, the autumn and spring maxima still occur, but summer rain diminishes and winter rain increases. Only south of Naples and in the Islands is the winter maximum very marked; here the three summer months only have a negligible amount. In reality, autumn is the period in which Italy as a whole receives most rain; this follows a relatively wet summer in the North but a very dry summer in the South.
Such marked differences in occurrence of rainfall in different parts naturally affect river regimes, which are very irregular in most areas, and differ from place to place. Perhaps the most striking feature of Italian rivers is the contrast between those originating in the Alps, which have a considerable, though variable, flow throughout the year, and those originating south of the Po, which, except for the largest, the Tiber, dry up completely or almost completely for anything from a few weeks to a few months. The rivers originating in the high Alps have a maximum flow in the summer, being fed both by the heavy rains and melting ice. Tributaries from the Pre-Alps that join these have maxima in spring and autumn. On the south side of the Po the rivers from the Northern Apennines carry very little water in the summer. The Po, which receives water from all these sources with their different regimes, has a more constant flow than any single set of tributaries. The discrepancy between a winter or early spring maximum and a summer minimum grows southwards, and in some parts of the South and Islands as much as 70-80% of the total comes in the first four months of the year. In the limestone areas drainage is of course different, and underground channels and caverns are widespread in both the limestone Alps and parts of the Apennines.