Despite its geographical position at the centre of the temperate zone, Italy has rather variable climatic characteristics. This is due to the presence of the Mediterranean, whose warm waters mitigate thermal extremes, and the Alpine arc, which forms a barrier against the cold north winds. Furthermore, Italy is subject to both wet and moderate atmospheric currents from the Atlantic Ocean and dry and cold ones from eastern Europe. The Apennine chain too, confronting the wet winds from the Tyrrhenian, causes considerable climatic differences between the opposite sides of the peninsula. The differences in temperature between the winter and summer months are more marked in the northern regions than in the south and along the coasts. The mean temperatures for the month of January in the Po Plain fluctuate around zero, while in the Alpine valleys the thermometer can drop to -20º and snow can remain on the ground for many weeks. In the southern regions, instead, the mean temperatures for January remain around 10º, with the exception of the inland mountainous zones. Mean summer temperatures throughout all Italy rise to 24º-25º for July, only being lower in the highest zones. Rainfall distribution also varies considerably, due to the influence of both mountains and prevailing winds. The highest quantities are registered in the Alpine arc (over 3,000 mm pa in the Lepontine and Julian Alps) and on the Apennines (over 3,000 mm pa in the Apuan Alps). The plains, however, including that of the Po, receive scarce precipitation. Generally it is less than 800-900 mm pa but in the southern regions (Tavoliere and southern Sicily) it falls below 600 mm pa. The great internal Alpine valleys and the coastal plains of the Tyrrhenian (Maremma) and Sardinia also receive little rain. Altogether, six large climatic regions can be distinguished, mainly characterized by mountain influence. 1) An Alpine region, strongly influenced by altitude, with long cold winters and short cool summers having an elevated day-time temperature range; precipitation is more intense in the summer months, especially in the pre-Alpine belt. 2) A Po region, with continental conditions, consisting of cold and often snowy winters and warm and sultry summers; precipitation is greatest in the spring and autumn months; the climate becomes milder, however, around the pre-Alpine lakes; fog is frequent, due to the wetness of the land. 3) An Adriatic region, whose sea has lit tle influence due to the inability of its shallow waters to trap the summer heat; consequently the climate has a continental character, with its winters being dominated by cold north-east winds (bora). 4) An Apennine region, also with continental tendencies and cold snowy winters; precipitation is more intense on the Tyrrhenian slopes and is abundant in all seasons apart from the summer. 5) A Ligurian-Tyrrhenian region, with a maritime climate and heavy and frequent precipitation, which is less in the summer and distributed irregularly; the winters are cool and the annual temperature range narrow. 6) A Mediterranean region, also with a limited annual temperature range; precipitation is frequent, especially in winter, and the summers are hot and dry. The interior and mountain zones of the islands and Calabria also have an Apennine type climate due to the altitude.
The characteristics of the Italian water network are closely associated with morphological and climatic conditions. There are only a few tens of watercourses longer than 100 km, though the Po, which is also the longest of them all (652 km) has a rainwater basin almost equal to a fourth of the national territory (74,970 sq km). Other important rivers are the Adige and Piave, descending from the Alps and flowing from the north into the Po, and the Arno and Tiber, flowing through central Italy into the Tyrrhenian. The other main tributaries of the Po are the Ticino, Adda and Oglio, arising in the Alps, the Tanaro, from the Apennines, and the Reno too, though it has its mouth to the south of the Po delta. The rivers running down the Tyrrhenian slopes of the peninsula are usually longe than those of the Adriatic, because of the Apennine watershed being further to the east. The Italian waterways are little used for transport due to their rather limited and variable flow. In fact the Alpine rivers have a cycle conditioned by the winter snow cover, being high in the summer and low in the winter; while the pre-Alpine and northern Apennine source rivers are mainly rain-fed and are only full in spring and autumn. Consequently, the cycle of the Po River is the most regular and therefore best suited to navigation. The other rivers of the peninsula and islands are heavily influenced by climatic conditions, being full in winter and empty in summer. In the latter case it is not unusual for the bed to remain completely dry, as in the case of the typical fiumare in Calabria and Sicily. Italy is fairly well supplied with lakes, having several thousand natural and artificial basins of different sizes and origins. The largest and deepest occupy the bottom of the great pre-Alpine valleys at their junction with the Po Plain (from Lake Orta to Lake Garda, which is the largest of all, while Lake Como is the deepest) and they were all excavated by Pleistocene glaciers. Also along the Apennine spine there are fairly frequent large lakes, such as Trasimeno the remains of an older lake that together with others occupied the bottom of the internal basins of the peninsula. The numerous small lakes scattered inside the spent craters of Latium and Campania are volcanic in origin. The coastal plains of the Tyrrhenian, Adriatic and large islands contain basins that are sometimes extensive and derived from lagoons. Furthermore, the Italian Alpine slopes, above 2,800 m., contain about a thousand glaciers. Some of these are of a considerable size, such as the Miage Glacier, which is some 10 km long and descends the southern slope of Mont Blanc in Valle d'Aosta. The glaciers are especially important for their function as water reserves, providing as they do a constant supply for the Alpine rivers. The central Apennines also have a small glacier, under the northern walls of the Corno Grande (Gran Sasso). Finally, Italy's water system is completed by the many underground water bearing strata of the numerous limestone karst massifs in the pre-Alps and Apennines. These produce springs bearing a considerable volume (as that of the Peschiera in Latium or the Sele in Campania, etc.). In addition, there are those reaching to varying depths under the Po Plain and the other alluvial plains.
With its extension from southern Europe towards Africa, the Italian peninsula almost divides the Mediterranean in two separate basins. Leaving aside the Strait of Messina, the shortest distance between Sicily and Africa (NE Tunisia) is circa 140 km, reduced to 70 km if it is measured from the island of Pantelleria. In this part of the sea (Channel of Sicily) the depth does not exceed 500 m. Furthermore, the eastern Mediterranean section, known as the Sea of Sicily and from which emerge the Maltese Islands, the Pelagian and Pantelleria, rarely exceeds a depth of 1,500 m. Considerably deeper, on the other hand, is the Ionian Sea. This extends eastwards from Sicily and Calabria and southwards from the Salentina Peninsula, touching on the 4,000 m isobath. Equally deep is the Tyrrhenian Sea, within the triangle formed by Corsica and Sardinia, Sicily and the Italian peninsula. At its centre it often exceeds a depth of 3,500 m. A narrow channel (the Canale di Corsica) separates it, to the north, from the Ligurian Sea. This latter exceeds a depth of 2,000 m in its western section corresponding to the Riviera di Ponente. The shallowest of the Italian seas is the Adriatic, which up to the level of Ancona does not exceed 80 m and only at Pescara does it decend below 200 m; off the coast of Puglia, however, it exceeds a depth of 1,200 m. Finally, in the area of the Strait of Otranto the two shores of the Adriatic draw close together and here the Italian and Albanian coasts are only 75 km apart. As for the rest of the Mediterranean, the surface temperature of the Italian seas is on average rather high. In the northern Tyrrhenian, the Sea of Sicily, Ionian and southern Adriatic it is circa 13º; in the Ligurian Sea circa 12º; in the southern Tyrrhenian circa 14º; but in the northern Adriatic, because of the shallowness of the waters, it drops to 9º. The quality of the water is also rather elevated, reaching over 38 per mille in the southernmost zones and the Sea of Sardinia, while being slightly less (33 per mille) in the northern Adriatic. The Adriatic is also subject to tides (which can range over about a meter) and these can sometimes create problems, such as the high waters in Venice and the lagoon.
Man's intense exploitation over many thousands of years has greatly altered the original condition of the vegetation cover, this is true also for the high mountain zones of the Alps and Apennines, which were subject to systematic deforestation until the end of the last century. Despite massive attempts to protect the mountains from the beginning of this century, many of Italy's mountain regions still remain without tree cover and are therefore susceptible to hydrogeological disaster, especially in the zones with particularly unstable rock types. At the present time little more than a fifth (21.2%) of Italy is covered by trees, which altogether occupy an area of circa 64 sq km. In the strictly floral category, the region of Italy unites Mediterranean and central European species. When these are combined with morphological and altimetrical influences a varied floral landscape results that is more dependant on climatic conditions than soil types. Thus it is possible to identify at least four principal floral regions. 1) An Alpine region, divided into bands according to height, with oaks and other broad-leaved trees prevailing in the lower areas and valley bottoms, followed up to circa 1,000 m by chestnuts and then beeches followed still higher up, but not beyond 2,000 m., by a mixture of needle-leaved trees (firs, larches and Scotch pines); the summit areas are dominated by meadows and pastures together with shrub vegetation (rhododendrons and dwarf pines) or, on the margins of permanent snow (circa 2,400-2,800 m.), by Alpine tundra with mosses and lichens. 2) An Apennine region, similar in character and sequence to the Alpine but with the presence of temperate species in the valley bottoms and a lesser spread of conifers in the upper levels. 3) A Po region, dominated by broad-leaved trees (willows, alders, poplars and oaks), which still form small woods but only along the river banks, while on the upper plain survive extensive stretches of the original heath with American acacias, heathers and brooms. 4) A Mediterranean region, covering the Ligurian and Tyrrhenian coasts as well as those of the central and southern Adriatic and the islands, dominated by a mixture of maritime pine and evergreen macchie (with olives, cypresses, corks, etc.) derived from the spoiliation of the original ilex groves.