Sigma-t is a quantity used in oceanography to measure the density of seawater at a given temperature. ÃÂ<sub>T</sub> is defined as ÃÂ(S,T)-1000 kg m<sup>âÂÂ3</sup>, where ÃÂ(S,T) is the density of a sample of seawater at temperature T and salinity S, measured in kg m<sup>âÂÂ3</sup>, at standard atmospheric pressure. For example, a water sample with a density of 1.027 g/cm<sup>3</sup> has a ÃÂ<sub>T</sub> value of 27.