Salinity - Definitions

Definitions

Salinity in the ocean refers to the water's "saltiness". In oceanography, it has been traditional to express salinity not as percent, but as permille (parts per thousand) (‰), which is approximately grams of salt per kilogram of solution. Other disciplines use chemical analyses of solutions, and thus salinity is frequently reported in mg/L or ppm (parts per million). Prior to 1978, salinity or halinity was expressed as Cl ‰, usually based on comparison with IAPSO Standard Seawater ("Copenhagen water"), a natural sea water distributed to serve as a world standard. In 1978, oceanographers redefined salinity in the Practical Salinity Scale (PSS) as the conductivity ratio of a sea water sample to a standard KCl solution. Although PSS is a dimensionless quantity, its "unit" is usually called PSU. It is not the case that a salinity of 35 exactly equals 35 grams of salt per liter of solution.

These seemingly esoteric approaches to measuring and reporting salt concentrations may appear to obscure their practical use, but it must be remembered that salinity is the sum weight of many different elements within a given volume of water. It has always been the case that to get a precise salinity as a concentration and convert this to an amount of substance (sodium chloride, for instance) required knowing much more about the sample and the measurement than just the weight of the solids upon evaporation (one method of determining salinity). For example, volume is influenced by water temperature; and also the composition of the salts is not a constant (although generally very much the same throughout the world ocean). Saline waters from inland seas can have a composition that differs from that of the ocean. For the latter reason, these waters are termed saline as differentiated from ocean waters, where the term haline applies (although is not universally used).

Contour lines of constant salinity are called isohales.