The stable high-pressure weather conditions prevailing over central Europe and the Alps during autumn and winter create cold air masses that result in lowland fog, a climatic phenomenon with widely varying consequences. The mouths of the northern Alpine valleys, the basins of the Jura Mountains, and the villages and cities of the low areas of the Mittelland are blanketed for days and often for weeks on end, while towns located at higher altitudes enjoy warm, brilliant, high-pressure conditions and the view of the glistening sea of fog below them. Temperature inversions between mountain and valley locations in close proximity can be quite pronounced, with higher elevations having higher temperature readings. Frequent temperature inversion has made Switzerland’s high-altitude resorts healthful places even during winter and has helped the Alpine winter season gain popularity in Europe for sports; in addition, because of these inversions polluted air is much less common in areas of high elevation than in the lowlands. In fact, the temperature inversions that affect the Mittelland tend to trap polluted air for weeks when cyclonic activity stagnates.

Avalanches

With the increase in winter tourism, the study of avalanches has developed as a branch of Alpine climatology, and in wintertime the research station near Davos releases daily avalanche bulletins as a warning for villagers and tourists. The Alpine cantons have about 10,000 avalanches annually, with about four-fifths of them occurring in February, March, and April. For centuries, village communes have relied on forests on the mountain slopes for protection from these slides, because a 20- to 30-year-old forest can inhibit or stop small avalanches. Villages, highways, and Alpine paths are also protected by costly artificial structures such as metal barriers, earthen walls, and concrete wedges and enclosures. However, in the late 20th century, acid rain caused the illness and death of many trees in the mountain areas of Switzerland and posed a serious threat to their ability to act as barriers to avalanches. In the mountain forests, some two-fifths of the trees were classified as damaged, sick, or dying. Pollution-control legislation across Europe did much to reduce the harmful effects of acid rain in Switzerland, and a concerted effort was made by Swiss land managers to introduce healthy trees into unused Alpine pastureland and to increase the density of existing Alpine forests. By the early 21st century, avalanche-control forests had become healthier and denser, especially at higher elevations and in steeper terrain.

Vegetation

Vegetation in Switzerland is derived from that of the four European climatic regions that converge in the country and has been influenced by the varied relief. It includes the beeches and oaks of the maritime west; hornbeam and larch trees in the more continental east, predominantly in the Engadin and the dry Valais; extensive spruce forests in the northern subalpine region; and chestnut groves in the south. Differences in vegetation are evident in the Alpine valleys because of exposure to the sun. The vegetation boundaries are several hundred feet higher in the south of the country—for example, in Valais—than in the north because of the southern exposure. Alpine vegetation, similar to that of Arctic tundra, prevails above the tree line. It is very susceptible to erosion through skiing impacts and as a result of paths or four-wheel-drive trails cut into the slopes.