Precipitations

Rain and snow are the main sources of the water that flows into Swiss lakes, rivers and groundwater. From which region the humidity for our precipitation originates, depends on the general weather situation and on the time of year.

The dominant weather system at a particular point in time determines the origin, form and intensity of precipitation in Switzerland. In the summer, precipitation usually falls as rain and as a result of convective processes (thunderstorms). In winter, precipitation is more widespread (advective) and often falls as snow. Two weather patterns in particular, although rare, can lead to extreme conditions, as shown in Figures below. The pattern shown in the bottom left figure leads to drought; the pattern in the bottom right figure, to flooding. In the mountainous areas of Switzerland, the elevation above sea level plays an important role in the amount and form of precipitation: the higher the elevation, the more the precipitation, and the more frequently it falls as snow.

Two typical weather patterns that can lead to extremes in Switzerland: The “Omega block” (left) has a strong and stable anticyclone (H) over Europe that leads to droughts like the one in summer 2003. The “5b weather type” (right) is a characteristic low pattern (L) that is known to result in large amounts of rain. This pattern brings very moist and mild air into Switzerland, especially in the spring and autumn, causing the snowline to rise. If the soil is already saturated with water, this can lead to major flooding.