From Glossary of Meteorology
A parameter used to define the local stability of an inviscid water column to double-diffusive convection.
It is defined as
where tan−1 is the four-quadrant arctangent; α is the coefficient of thermal expansion; β is the equivalent coefficient for the addition of salinity, sometimes referred to as the "coefficient of saline contraction;" T is temperature; and S is salinity. If −45° < Tu < 45°, the column is statically stable. If −90° > Tu or Tu > 90°, the column is statically unstable to Rayleigh–Taylor instability. If −90° < Tu < −45°, the column is unstable to diffusive convection. If 45° < Tu < 90°, the column is unstable to salt fingering.
Term edited 8 March 2021