Geography has everything to do with the weather in this very mountainous country, especially across the Alps that dominate the country's southern half. The north slopes of the Alps are prone to the same cold waves that put Germany on ice. Meanwhile, south slopes are exposed to Mediterranean flow that's often balmy but laden with moisture. As a result, the northern side of the Alps tends to get steadier rain or snow and less sunshine (a tendency accentuated by their orientation away from the sun). The southern side gets more sun and shorter bursts of more intense rain.
It's no wonder the Alps are a world mecca for cold-season sports. Winter storms that parade across the Mediterranean, or blow in from the Atlantic, can drop huge amounts of snow across the higher Alps. However, when it comes to cold, the Swiss champion isn't the Alps at all. La Brevine (nicknamed Swiss Siberia) is tucked into one of the many valleys gouged into the Jura Mountains, which stretch across northwest Switzerland. Cold air collects with ease in the Jura's steep-sided valleys, which allowed La Brevine to drop to a national record of –42°C/–44°F in January 1987. The same principle applies in somewhat diluted fashion to the much broader Mittelland, between the Jura and Alps, that holds the largest Swiss cities. Especially in mid-winter, Siberian air can funnel into this valley on strong northeast bise winds and stay for weeks below about 1000–1500m/3300–4900ft. Above this layer of biting cold – which is often accompanied by fog and light snow – skiers may be basking in sunshine and temperatures warmer by 10°C/18°F or more. Smaller valleys within the Alps, protected from the bise, may also be sunny and dry, though quite cold at night. Every few days, a round of fohn winds will sweep northward, bringing rapid warm-ups and extremely dry conditions. Relative humidities can drop below 10 percent, raising the risk of fire. Fohn winds may gust above 140kph/80mph in some north–south valleys immediately along the Alps. The wind usually drops to a more tolerable breeze by the time it reaches the population centres. Fohn conditions are a bit more likely in autumn and especially in spring, as warm air makes a run for the continent. The quick switches from showers to sunshine in mid-spring are called “April weather”.
As in many other high-altitude regions, summer is thunderstorm season across Switzerland. The Alps and Jura see showers or storms by mid-afternoon almost every day. Most valley cities will get wet every 2 to 4 days on average, with the rains heavier and more frequent as you move toward Zurich. Sunshine is plentiful, though – some central Alpine valleys tend to stay quite dry – and you may even encounter a bona fide heat wave. If you're planning a hike, you can expect a temperature drop in the neighbourhood of5°C/9°F per 1000m/3300ft.