a measure of the saltiness of sea water. On average each litre (0.22 gals.) of seawater contains 35.5 grams (1.25 oz) of salt dissolved in it (i.e. has a salinity of 35.5). In the open ocean, salinity ranges from about 32 to 40. The highest values occur in the Red Sea where evaporation is very high. Salinity is important because together with the water temperature, it determines the density of the sea water. In tropical regions where rainfall is heavy, the rain dilutes the sea water and makes it more buoyant, but at the more arid latitudes around 30°, the low rainfall combined with strong solar radiation makes the surface water denser so that it tends to sink. As the sea freezes in winter in the polar oceans, the water left unfrozen is saltier and very much colder, so it tends to sink. Conversely, during summer when the ice is melting, the surface water is diluted and becomes more buoyant.
Once a body of water leaves the ocean's surface, its characteristics of temperature and salinity are conserved, and are only changed by turbulent mixing between the water body and its surrounding waters. The waters sinking into the ocean's interior in different regions have predictable characteristic temperature and salinity properties, and are known as water masses. The distribution of these water masses is used to follow the flow of deep currents and map the features of their large-scale circulation.