Mountains and highlands are typically areas that provide considerable quantities of water. The basic reason for this is that higher land causes air to rise and subsequently cool; it then releases the humidity it contains in the form of precipitation. In addition, there is less evaporation owing to a lower rate of net radiation, lower temperatures, more frequent snow cover and a shorter period of vegetation. In the European Alps, as compared with the surrounding lowlands, these factors result in considerably higher annual discharge. In the summer months in particular, the lower catchments of the Rhine, the Rhone, the Po and the Danube profit from the highly dependable summer runoff from the Alps.