Salinity is the measure of how much salt is in water. Ocean water has a high salinity and fresh water has a very low salinity.
Sometimes, salinity is measured in parts per million (ppm). (See below, my brackets) Ocean water has a salinity that is about 35,000 ppm. That's the same as saying ocean water is about 3.5% salt. Sometimes, salinity is measured in different units. Ocean salinity is also about 35 psu (practical salinity units).
Ocean water has a lot of salt in it. Most of it is normal table salt (made up of chlorine and sodium). Chlorine, sodium and the other major dissolved salts of the ocean are listed in the following table:
The saltiest water (40 o/oo ) occurs in the Red Sea and the Persian Gulf, where rates of evaporation are very high. Of the major oceans, the North Atlantic is the saltiest; its salinity averages about 37.9 o/oo. Within the North Atlantic, the saltiest part is the Sargasso Sea, an area of about 2 million square miles, located about 2,000 miles west of the Canary Islands. The Sargasso Sea is set apart from the open ocean by floating brown seaweed "sargassum" from which the sea gets its name. The saltiness of this sea is due in part to the high water temperature (up to 83º F) causing a high rate of evaporation and in part to its remoteness from land; because it is so far from land, it receives no fresh-water inflow.
Low salinities occur in polar seas where the salt water is diluted by melting ice and continued precipitation. Partly landlocked seas or coastal inlets that receive substantial runoff from precipitation falling on the land also may have low salinities. The Baltic Sea ranges in salinity from about 5 to 15 o/oo. The salinity of the Black Sea is less than 20 o/oo. Water of the Puget Sound in the Tacoma, Wash., area ranges in salt content from 21 to about 27 o/oo. This area is drained by a number of fresh-water streams which discharge an average of about 4.1 billion gallons of water per day into Puget Sound. Salinity of sea water along the coastal areas of the conterminous United States varies with the month of the year as well as with geographic location. For example, the salinity of the ocean water off Miami Beach, Fla., varies from about 34.8 o/oo in October to 36.4 o/oo in May and June, while diagonally across the country, off the coast of Astoria, Oregon, the salinity of sea water varies from 0.3 o/oo in April and May to 2.6 o/oo in October. The water off the coast of Miami Beach has a high salt content because it is undiluted sea water. Off the coast of Astoria, however, the sea water is less saline because it is mixed with the fresh water of the mighty Columbia.
Sometimes river water travels far from shore before it mixes with sea water. This is shown by data gathered from a study of the Columbia River, which, in an average year, carries to the ocean enough water to cover an area of 1 million acres to a depth of 197 feet. Using a radio- active tracer, scientists at Oregon State University have followed the river's water from its mouth near Astoria to a point southwest of Coos Bay, 217 miles away.
The salt content of the open oceans, free from land influences, is rarely less than 33 o/oo and seldom more than 38 o/oo. Throughout the world, the salinity of sea water averages about 35 o/oo. This average salinity was obtained by William Dittmar in 1884 from chemical analyses of 77 sea water samples collected from many parts of the world during the scientific expedition of the British corvette, H.M.S. Challenger. The Challenger expedition, organized by the British Government at the suggestion of the Royal Society, set out to study the biology of the sea, examine the chemical and physical properties of the water, sample deposits on the ocean floor, and measure water temperatures. The voyage began in 1872 and ended almost 4 years later after covering 68,890 nautical miles. This expedition remains today the longest continuous scientific investigation of the ocean basins. Dittmar's 77 samples are
Supplement from 02/08/2009 01:23pm:
For some reason I'm unable to paste long answers so have had to do it in two parts.
This expedition remains today the longest continuous scientific investigation of the ocean basins. Dittmar's 77 samples are still the only worldwide set of samples of sea water for which complete data (each principal constituent) on chemical composition are available. More recent data, reflecting improvements in analytical and sampling techniques, show slight deviations from Dittmar's results, but these changes do not affect the overall usefulness of his work. The average composition of the 77 samples is as shown on the following table.
Salinity is now sometimes measured in Practical salinity Units.
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