The climate in Switzerland is strongly influenced by the Alps extending across the country and representing the most important meteorological divide in Central Europe. Thanks to the Alps, large climatic differences in Switzerland are to be found within a small geographic area. The largest contrasts exist between the northern side of the Alps with its temperatre climate and the southern side of the Alps characterized by mediterranean climate.
In the Swiss Midland, extending between Lake Geneva and Lake Constance as well as between Jura and the Pre-Alps, climatic conditions which are typical for Central Europe are to be found. The average annual temperature is just below 10 °C, with mean values around the freezing point in January and between 16 and 19 °C in July. The average annual precipitation amounts are just above 1000 mm. In winter often a so-called inversion layer results in a large-scale low stratus cloud coverage which partly persists for several days or even weeks. Thereby the cold wind from northeast (named Bise) is pressed between Jura and Pre-Alps and can thus reach gale-force on the western shores of Lake Geneva.
In the northwest the Jura chain edges the Midland. With average altitudes ranging from 1000 to 1500 metres it has slightly more humid and lower temperatures in comparison with the Midland. In summer, thunderstorms are more frequent. In winter, so-called cold air pools form inside of closed valleys during windless and cloudless nights, which are responsible for especially low temperatures. Therefore, the lowest temperature ever reached was -41.8 °C, recorded in La Brévine (January 12th 1987).
The Pre-Alps as a transition between Midland and alpine region are among the most diversified areas in Switzerland concerning the weather. Humid air masses coming from Northern Europe dam up along the Pre-Alps and frequently cause large amounts of precipitation – significantly larger ones than in the Alps. Here again, thunderstorms become more frequent, not seldom accompanied by intensive hail. During Foehn (a warm and dry wind coming from south) conditions, the temperatures are substantially higher than in the Midland and thanks to the dry air a spectacular visibility can often be enjoyed. But beware of its strength: During a Foehn storm sailors are recommended to avoid Lake of Uri, Lake Brienz or Lake Walen, for example!
The Alps accommodate a high complexity of microclimates commonly changing within a few kilometres depending on the slope or a valley’s orientation. So-called intra-alpine dry valleys as Valais (Rhone Valley) or Engadine are topographically shielded to the north as well as to the south. These regions not only get a lot of sun (the Kleines Matterhorn above Zermatt and the Diavolezza in the Upper Engadine with 2000 to 2500 sunshine hours per year) but also accommodate the driest place in Switzerland (Ackersand in the Matter Valley with about 500 mm per year). In addition, heavy thunderstorms occur less often than elsewhere in Switzerland.
The Simplon region, Ticino and the southern Grisons are counted the southern Alps, characterized a meditterranen type of climate. Average temperatures are higher (Locarno-Monti for instance has 11.5 °C on average) and dry periods are longer than elsewhere making it a popular holiday destination. However, if a low pressure area develops over the Gulf of Genoa, enormous amounts of precipitation can occur within a very short time. When a severe weather warning is issued keep away of creeks and rivers which might become riptides! Concerning the amount of precipitation, Camedo in Centovalli is record holder with 2400 mm per year.