Atwood number ($\mathcal{A}=\frac{{\rho }_h-{\rho }_l}{{\rho }_h+{\rho }_l}$)

The dimensionless density contrast $({\rho }_h-{\rho }_l)/({\rho }_h+{\rho }_l)$, conventionally using heavy and light densities. In the inviscid, zero-surface-tension heavy-over-light Rayleigh-Taylor instability, the growth rate is $s=\sqrt{\mathcal{A}gk}$. State the signed ordering if labels are not heavy/light.