The amount and origin of the moisture that is transported to Switzerland depends on large-scale atmospheric circulation and the resulting winds. As shown in Figure below, about 40% of the precipitation that falls in Switzerland is evaporated from the North Atlantic. Another 25% comes from the Mediterranean, 20% from the land surface of Central Europe, and 15% from the North and Baltic Seas. However, these figures can vary significantly depending on the season. In winter, the North Atlantic contributes an even greater proportion of moisture, while in summer the Central European land area plays a more important role than is shown in the illustration. In addition, the Alps act as a weather divide, often preventing the transport of moisture from one side of the mountains to the other. Thus, southern Switzerland receives most of its moisture from the Mediterranean while northern Switzerland gets it from the northern seas. In general, the source regions for moisture change significantly throughout the year. This doesn’t necessarily affect the total annual rainfall in an area, however.