It is often the case that the terms quadratic in u and v, which represent the effect of bulk advection, are small compared to the other terms. This is called geostrophic balance, and is equivalent to saying that the Rossby number is small. Assuming also that the wave height is very small compared to the mean height (h ≪ H), we have (without lateral viscous forces): ∂ ∂ h ∂ ∂ t + H ( ∂ ∂ u ∂ ∂ x + ∂ ∂ v ∂ ∂ y ) = 0 , ∂ ∂ u ∂ ∂ t − − f v = − − g ∂ ∂ h ∂ ∂ x − − k u , ∂ ∂ v ∂ ∂ t + f u = − − g ∂ ∂ h ∂ ∂ y − − k v .