Schmidt Number

The Schmidt number Sc is the dimensionless ratio of kinematic viscosity to the molecular diffusivity of a dissolved gas, Sc = nu/D, the mass-transfer analog of the Prandtl number. It governs how a gas crosses the thin diffusive layer at the sea surface, so the gas exchange velocity scales as Sc^-0.5 for a wavy, wind-roughened surface. By convention parameterizations are normalized to Sc = 660, the value for CO2 in seawater at 20 C, which lets a single wind-speed relation be rescaled to any gas and temperature. Schmidt number falls sharply with warming because diffusivity rises and viscosity drops.