Sigma-t

Sigma-t is a compact density variable, the in-situ seawater density at atmospheric pressure computed from measured temperature and salinity, minus 1,000 kg per cubic meter. A reading like 25.5 means a density of 1025.5 kg per cubic meter. It ignores the pressure effect on density, so deep comparisons use potential-density variants sigma-theta and the sigma-0 to sigma-4 series referenced to set pressures. On a temperature-salinity diagram sigma-t contours let you read density directly and identify water masses. Under TEOS-10 density is computed from Absolute Salinity and Conservative Temperature, but the sigma notation remains in wide use.