The circumstances which cause the salinity of a body of water to differ include evaporation from its surface, precipitation on its surface, the freezing or melting of sea ice, the inflow of fresh river water, wind and wave movement that increase evaporation and the mixing of bodies of water of different salinities. The Baltic Sea for example is in a cool climatic area with low evaporation, has many rivers flowing into it, and on-and-off refilling from the open ocean. The occasional inflow of water from the North Sea creates a cold, dense under layer that hardly mixes with the surface layers. The topmost layer may have a salinity of 10 to 15ppt, with even lower levels in the river. The Red Sea experiences high atmospheric temperatures causing high evaporation but little precipitation, few rivers flow into it and the Bab-el-Mandeb joining it to the Gulf of Aden is narrow. Its salinity is high and averages 40ppt.