Equation of piezotropy

From Glossary of Meteorology

equation of piezotropy

(Also called physical equation.) An equation relating the thermodynamic variables in processes of a piezotropic fluid.

In its general form it expresses the density ρ as a function of the pressure p:
The derivative dρ/dp is called the coefficient of piezotropy. The most familiar such equation is that for polytropic changes of state in an ideal gas:
where λ is the modulus of the polytropic process. Prior to the discovery of the first law of thermodynamics in the midnineteenth century, the equation of piezotropy was used to complete the hydrodynamic equation set consisting of the equations of motion and the conservation of mass.