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Education: Salinity Basics
How much salt is there? Probably more than you think. Some scientists estimate that the oceans contain as much as 50 quadrillion tons (50 million billion tons) of dissolved solids. If the salt in the sea could be removed and spread evenly over Earth's land surface it would form a layer more than 500 feet thick (152.4 meters), about the height of a 40-story building. Seawater is 220 times saltier than fresh lake water. Unrefined sea salt contains 98.0% sodium chloride and up to 2.0% other minerals (salts). Together there are over 100 minerals, composed of 80 chemical elements, in sea salt. The composition of a single crystal of ocean salt is so complicated that no laboratory in the world can produce it from its basic 80 chemical elements. (From "Why is the Ocean Salty" by Herbert Swenson, US Geological Survey)
Where does the salt come from? The answer may surprise you.
How much salt is there relative to water? Yes, there is a lot of salt in the ocean, but there is a lot more water. In ocean waters of average salt content (35 psu), the weight of water is over 28.5 times greater than the weight of salt it contains. Oceanographers use the measure of salinity to describe the relative amount of salt to water.
Is salinity uniform throughout the ocean? No. Although the amount of salt in the ocean is relatively constant on time scales of years to decades, sea surface salinity (SSS) varies because freshwater input & ouput – part of the global hydrologic or water cycle – varies from place-to-place. The large scale pattern of evaporation and precipitation is established by earth's atmospheric convection cells. In the simplest case – for example, if earth were covered by oceans and not spinning (shown at right) – the atmosphere would move heat between the hot tropics and cold poles in a very simple way. Hot air would rise along the equator, creating a band of low atmospheric pressure. Cold air would sink at the poles, creating regions of high atmospheric pressure.
The presence of continents and earth's rotation complicates the idealized "two cell" system. A fairly realistic model of earth's atmospheric circulation consists of six cells: three on each side of the equator (shown at left). This pattern has atmospheric pressure highs at the poles, 30°N and 30°S. It also has atmospheric lows along the equator, 60°N and 60°S. Away from the poles, cloudiness and precipitation (P) dominate bands of low pressure: these latitudes host the world's rainforests. Dryness and evaporation (E) dominate bands of high pressure: these latitudes are home to deserts.
In terms of SSS, lower salinity generally occurs where precipitation is greater than evaporation (P>E). Higher SSS generally occurs where evaporation is greater than precipitation (E>P). Compare this "six cell" model with the map of global average SSS (below). How well do SSS patterns mirror bands of high and low atmospheric pressure? In places where they do not match, what other types of freshwater inputs & outputs might be occurring?
To learn more about monthly SSS variations in our "saltiest" ocean, the North Atlantic, visit the "Salinity Data and Tools" section.
To learn more about how salinity influences ocean circulation – and earth's climate zones – visit the "Ocean Circulation & Climate" section.
Aquarius is NASA's first mission to measure ocean salinity. Our first installment of educational materials – "Salinity Patterns & the Water Cycle" – meet physical science content standards of the National Science Education Standards. Find out more about the importance of ocean salinity, we offer several classroom activities and supplemental ocean-atmosphere data investigation tools. And, learn more from Aquarius Project Scientist Dr. Yi Chao who explained the mission to students during an "Online Learning Community LiveRoom Chat".
Questions or comments? Contact Annette deCharon, Senior Science Educator and Aquarius EPO Manager.
atmospheric pressure: Force per unit area (pressure) exerted on a surface area, created by the weight of air above it.
climate: The prevailing or normal pattern of weather at a place, or in a region, averaged over a long period of time; in contrast to weather, which is the state of the atmosphere at a particular time.
conductivity: A measure of the ability of a material to conduct or transmit an electric charge.
convection: Vertical air circulation in which warm air rises and cool air sinks, resulting in vertical transport and mixing of atmospheric properties; the flow of heat by this circulation.
evaporation: The physical process of converting a liquid to a gas. Commonly considered to occur at a temperature below the boiling point of the liquid.
practical salinity unit (psu): Used to describe the concentration of dissolved salts in water, the UNESCO Practical Salinity Scale of 1978 (PSS78) defines salinity in terms of a conductivity ratio, so it is dimensionless. Salinity was formerly expressed in terms of parts per thousand (ppt) or by weight (parts per thousand or 0/00). That is, a salinity of 35 ppt meant 35 pounds of salt per 1,000 pounds of seawater. Open ocean salinity is generally in the range from 32 to 37.
precipitation: Water released from the atmosphere in the form of rain, snow, hail, or sleet onto Earth's surface.salinity: A measure of the quantity of dissolved solids in ocean water. In general, salinity reflects the total amount of dissolved solids in ocean water in parts per thousand by weight after all carbonate has been converted to oxide, the bromide and iodide to chloride, and all the organic matter oxidized. Salinity is now measured as practical salinity units (psu).
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