The Apennine screen reduces the influence of the sea on this zone. So the North Italian Plain has long warm summers and short but raw and often foggy winters. Conditions are most severe when cold stagnant air from the surrounding mountains sinks down to the plain (see Inversion, Temperature). The long, warm growing period, together with an adequate rainfall of 750 to 1,150 mm, favors many kinds of crops. Except for a few very mild districts such as the shores of Lake Garda, however, winter temperatures are too low for delicate crops like olives and citrus fruit.
Temperatures in Milan (Milano) average 2°C in January and 25°C in July.
In the Alps summers are predictably shorter and cooler, and winters cold and snowy. Conditions vary from place to place and according to altitude and aspect. The deeper valleys are much drier than the surrounding mountains and can be stiflingly hot in summer, but extremely cold in winter when temperature inversion occurs. On the higher slopes air temperatures are always lower, but this is offset by the intensity of insolation in the pure atmosphere. In winter the mountains often have brilliant sunshine while the valleys are drowned in mist, and there is a strong contrast between north-facing and southfacing slopes. Occasionally a warm dry wind-the Fohn down the valleys.
Peninsular and Insular Italy
Peninsular and insular Italy has climatic variations on a Mediterranean theme. Summers, if sunnier, are only marginally hotter than those of the Northern Plain; the chief contrast is in the mildness of the winters. Much of Mediterranean Italy is mountainous, however; the Apennines, for example, often remain snow-capped until May.
The cyclones or depressions which rarely leave continental Italy entirely are infrequent in summer over the peninsula; so summer drought, increasingly prolonged toward the south, is a characteristic feature. The problem to the farmer is aggravated by intense evaporation, the unreliability of the rainfall, and its incidence in brief and often destructive showers. Precipitation and relief are correlated; the higher Apennines receive over 1,525 mm, while Apulia receives less than 500 mm.
In winter deep cyclones over the Ligurian Sea generate strong winds. Sardinia is occasionally swept by the northerly maestrale, and the Ligurian and Tuscan coasts by the westerly libeccio. In spring the sirocco, blowing from North Africa, brings hazy, stifling conditions to southern Italy.
Except where the Swiss canton of Ticino intrudes almost to the Lombardy Plain, the Italian frontier coincides with the Alpine divide or watershed for most of its length.
The highest sectors are the Graian Alps (13,323 feet); the Mont Blanc massif, which in Italy rises to more than 15,500 feet; the Pennine Alps, contammg Monte Rosa (15,203 feet) and Cervino, or the Matterhorn (14,691 feet); and the Rhaetian Alps (13,284 feet). The commonest rocks are granites, gneisses and schists, all of them very resistant. But east ofLake Maggiore the southern Alpine belt (the Pre-Alps) is dominated by limestones. Magnesian limestone is chiefly responsible for the distinctive and spectacular scenery of the Dolomites (Marmolada, 3,342 meters), a part of Italy much favored by tourists, climbers and winter sports enthusiasts. During the Ice Age typical Alpine landforms such as pyramidal peaks, cirques (steep-sided, bowl-shaped hollows) and aretes (sharp ridges) were produced by glacial erosion. The valleys were overdeepened and in places choked with debris (moraines) as the glaciers melted. Lakes Maggiore, Como, Iseo and Garda were all formed in this way.
A number of deep valleys penetrate the mountain rampart leading to passes over the divide. The Dora Riparia, for example, leads up to the Mont Cenis pass; the Dora Baltea to the Great and Little St Bernard passes and the Mont Blanc tunnel; the Ticino valley to the San Gottardo, and the Adige valley to the Brenner. Most of these passes are closed in winter, but allweather road tunnels have been constructed-for example under Mont Blanc and under the Great St Bernard and San Bernardino passes.
Railroad traffic is concentrated on the Mont Cenis, Simplon, St Gotthard and Brenner routes.
The North Italian Plain
The North Italian Plain, more than 200 miles long and up to 125 miles wide, occupies a vast depression filled in by material eroded from the Alps and the Apennines. The deeper deposits are marine, but those nearer the surface were laid down by rivers that were specially active while the Ice Age glaciers were melting. The Alpine margin is fringed in places by moraines, but towards the Po River the coarser gravels of the upper plain shade off into the finer alluvia of the lower plain.
The Po's Alpine tributaries have cut the upper plain into a number of separate platforms where the permeability of the gravels reduces the effectiveness of irrigation. In the lower plain the chief problem is maintaining efficient drainage. The reclamation of the marshy zone along the Po, which widens downstream into the deltas, lagoons and sandspits along the Adriatic, is mainly the work of the last 100 years. Even today the Po delta is flooded occasionally, a problem aggravated by the slow subsidence of the area.
Agriculture has prospered on the plain not because of any natural fertility but more from the success of many generations in controlling and exploiting its waters. Irrigation supplies from the rivers, especially those of the Alps, are supplemented by thousands of reliable springs welling up along the junction of the upper and lower plains. The upper Emilian plain is formed of alluvium deposited in recent geological times; it has had a flourishing agriculture ever since it was first settled by the Romans in the 2nd century BC.
The main Apennine ridge swings across from Liguria to the Adriatic before continuing down through the Abruzzi and Campania to Calabria. It rises to 9,500 feet in the Gran Sasso d'Italia, but mostly varies between 2,500 to 6,000 feet). In the north undulating ridges mark the divide, while in the center and south massive blocks penetrated by intermontane basins are more common.
Geological composition changes from north to south. In the Tuscan-Emilian Apennines resistant sandstones alternate with erodible and unstable clays and shales; in the Abruzzi, limestones predominate; in Calabria the high relief, including the La Sila, is underlaid with granite and other crystalline rocks. There are many trans-Apennine routes, but all are long and tortuous.
The outer flank, the Adriatic Sub-Apennines, is cut by scores of rivers into a comb-like succession of ridges and valleys. The coast is straight and the coastal plain narrow.
In Apulia the Tavoliere plain and the Bradano River trench separate the Apennines from the waterless limestone platforms of Gargano and Le Murge.
West of the Apennines, in Tuscany, Umbria, Lazio and Campania, lies the complex hill and plateau country of the AntiApennines, penetrated by many rivers, some (such as the Arno, Tiber, Liri and Volturno) with well developed longitudinal courses.
Some areas are mountainous, such as the Apuan Alps, source of the famed Carrara marble. The uplands between Monte Amiata and the Alban Hills are of volcanic origin; lakes Bolsena, Vico, Bracciano, Albano and Nemi occupy the craters of extinct volcanoes.
Volcanic, too, are the Phlegraean Fields, the islands of Ischia, Procida and Panza. Vesuvius (4,190 feet), Italy's most famous volcano, dominates the Bay of Naples.
The Ligurian coast is steep and rocky, but has some good natural harbors such as Genoa (Genova) and La Spezia. Farther south, rocky headlands alternate with shallow bays backed by alluvial plains. For centuries these plains were waterlogged in winter and malarial in summer; one of the few positive achievements of Mussolini was the reclamation of the Pontine Marshes and Tiber delta in the 1930s. Since 1950 water control has enabled the Maremma, between Pisa and Civitavecchia, and the lower Volturno and Sele plains to be resettled.
Similar projects have been carried out in the Tavoliere (Apulia) and along the Metaponto coast (Basilicata) and lower Crati valley (Calabria).
Sicily, like peninsular Italy, is mostly hill and mountain country, and the development of its largest plain (the plain of Catania) has been impeded by unruly rivers. The Apennines continue along the northern coast of the island with the same geological diversity as their mainland ranges. Limestones dominate in the west, clays and sandstones in the center (Monti Nebrodi), and crystalline rocks in the east (Monti Peloritani). Volcanic activity is spectacularly represented by Mt Etna (11,122 feet).
Tertiary limestones, sands and clays underlie the dreary hills and plateaus of the interior. The clays are unstable and fertility is poor.
Sardinia is an island apart. Its surface is largely composed of crystalline rocks formed during a much earlier mountain-building phase than that which raised the rest of Italy. One huge block occupies the eastern two-thirds of the island and is separated from the smaller Iglesiente mass by the Campidano lowland . Though the island is not impressively high (Gennargentu, 6,018 feet is its highest point), it has some of the most rugged terrain in Europe. River control is a major problem that has largely been overcome by taming the Tirso and Flumendosa rivers for power and for irrigation of the Campidano lowland.