Thermal expansion coefficient ($\alpha =-{\rho }^{-1}(\partial \rho /\partial T{)}_p$)

For a fluid at fixed pressure, the volumetric thermal expansion coefficient obeys $\rho (T_0+\theta )\simeq \rho (T_0)(1-\alpha \theta )$. Under the Boussinesq approximation it determines the buoyancy proportional to $g\alpha \theta$, while mass density is otherwise held constant.