A linear stability analysis of the hyperbolic tangent profiles is made. A Boussinesq primitive equation model with high vertical resolution is used. Unstable modes of intermediate scales (Lx ≈ 1000 km) are generated when the curvature, d 2ū/dz 2, of the basic flow in the lower levels is negative. Even if the curvature in the lower levels is positive, intermediate-scale unstable modes appear for smaller static stability and shear (Richardson number not necessarily small) or for certain vertical distributions of diabatic heating due to the liberation of latent heat in the lower troposphere. The amplitude of the most unstable intermediate-scale wave is confined to the lower troposphere and its growth rate increases with the inclusion of diabatic heating.