Grashoff number can be interpreted ratio of buoyancy force and viscous force $Gr_L=\frac{g\beta (T_s-T_{\infty})L^3}{\nu^2}$ where $\beta$ is volumetric thermal expansion coefficient (volume expansivity in [[REFPROP]]) i.e. fractional change in density to a change in temperature at constant pressure $\beta = -\frac{1}{\rho}\left(\frac{\partial \rho }{\partial T}\right)_p$ for ideal gas $\rho=p/RT$ and the above equation reduces to $\beta=\frac{1}{T}$