Low-season airfares are usually offered from November 1 to December 14 and from December 25 to March 31. Fares are slightly higher during shoulder season (during Apr and May, and from Sept 16 to the end of Oct). High-season fares apply the rest of the year (June-Sept 15), presumably when Switzerland and its landscapes are at their most hospitable and most beautiful.
Keep in mind that it's most expensive to visit Swiss ski resorts in winter, and slightly less so during the rest of the year. Conversely, it's cheaper to visit lakeside towns and the Ticino in winter. Cities such as Geneva, Zurich, and Bern don't depend on tourism as a major source of capital, so prices in these cities tend to remain the same all year.
The temperature range is about the same as in the northern United States, but without the extremes of hot and cold. Summer temperatures seldom rise above 80°F (26°C) in the cities, and humidity is low. Because of clear air and lack of wind in the high alpine regions, sunbathing is sometimes possible even in winter. In southern Switzerland, the temperature remains mild year-round, allowing subtropical vegetation to grow.