Pressure perturbations (colour scale) and wind vectors (arrows, panels b and d only) for the case of a planet with the same radius and rotation rate as the Earth but an isothermal temperature pressure profile set at 500 K. Panels a and c: latitude against height at a longitude of 200°; panels b and d: longitude against latitude at a height of 25 000 m. We report the values of ϕ₀ and Bu as defined in Wang & Mitchell (2014) and the growth rate σ_growth: (a) and (b): ϕ₀ = 50°, Bu ∼ 0.2 and , and (c) and (d): ϕ₀ = 35°, Bu ∼ 0.05 and . These results indicate that the growth rate of the most unstable mode is dependent on the characteristic height of the wave as found in Wang & Mitchell (2014).