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Among the various physical properties of the ocean, Salinity is the most vital factor besides temperature and pressure. The Challenger expeditions reveal that there exists 47 different sea salts. Of these sodium chloride accounts for 77.8%. The presence of salts in ocean water causes salinity, which is known as the salinity of ocean water. Salinity is expressed by the amount of salts dissolved. The salinity is expressed in %.

If the various specimens of sea water of different parts of oceans and seas are considered, the ratio of the main salts is almost the same but there is a variation in the salinity in them. Isohalines are drawn on the sea surface or at any depth of water to express salinity. Isohaline is an imaginary line drawn to join places having equal salinity. Evaporation, precipitation, river water, temperature, atmospheric pressure, wind direction and the movements of sea water are responsible in controlling the salinity. Higher rate of evaporation as in the sub-tropical ocean is resulting in high salinity. The high precipitation along the equatorial region reduces the salinity, therefore low salinity occurs. Similarly large quantity of fresh water from melting of snow is added in the region of Baltic Sea, thus reducing the salinity. When trade winds carry saline water away, upwelling of cool water reduce the salinity. The tides the ocean current help in mixing of surface water thus controlling the salinity. Yearly periodic variations are strongly felt in the enclosed seas.

Distribution of Salinity. The latitudinal distribution shows that there is a low salinity at the equator ranging from 34 to 35.5% because of high rainfall and the large number of days with overcast sky. the region between 20-40 degrees N and 10-30 degrees S are the region of high salinity due to large number of cloudless days, which increase heat and promotes evaporation. Sargasso sea (37%) S.E.Brazil (37%) Western Australia (36%) and near Peru-Chile (36.5%) are the high salinity zones. After obtaining maximum in the lower middle latitudes salinity again decrease to 31% in the northern hemisphere and 33% in southern hemisphere that is (40 - 60 degrees N and S) Further poleward salinity decreases due to the melting of ice. The average salinity for northern hemisphere is 34% while in South it is 35%. This is attributed to the fact that in the south a comparative less of mixing and less addition of fresh water take place and there is absence of land.

There is an excess of fresh water at the mouths of rivers because rivers bring a lot of fresh water. This is the cause of low salinity at the mouths of the Congo, the Sindh, the Amazon, the Niger, etc. In enclosed seas where the rivers bring fresh water and where there is an outlet into open seas, the salinity is low. The reason is that the salts brought by rivers, get an opportunity to escape into wide sea. This reduces salinity. Where there is no outlet for the water to escape the salinity increases. Examples have already been given earlier. In open seas, the waters of different far off places do not get an opportunity to mix. The result is that there is a variation of salinity from one part of the sea to another. For example, salinity changes from 36% in the Mediterranean Sea to 32% towards the west.

Salinity either decreases or increases with depth according to that nature of water mass. Generally there is a decrease with increase of depth. At the equatorial region of Indian Ocean salinity increase with depth (34% to 35%) At the southern boundary of the Atlantic surface salinity is 33% increasing to 34.5% and still deeper it reaches up to 34.8%. Generally it can be said that in high latitude salinity increases with depth due to denser water mass found at the bottom.

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