The theoretical researches on the problem of the disturbance of an atmospheric current flowing over a mountain range, carried on during the last ten years, prove that most of the characteristic observed features can be explained, to a very large extent, by the hydrodynamical theory of internal, small adiabatic perturbations in a stratified rotating atmosphere. In the case of a uniform current of velocity u, in which the coefficient of vertical stability is also uniform (θ is the potential temperature, g the gravity, z the elevation), the perturbation pattern corresponding to a smooth, low, gently-sloping mountain range varies widely according to the half-width a of this mountain range: i) If a is comparable to the critical value (Ls/2π) = (u/s) ≈ 1 km, there is a system of short stationary lee waves, or gravity waves, with the wave length Ls. ii) If a is comparable to (Lf/2π) = (u/f) ≈ 100 km, where f is the Coriolis parameter, there is a complex system of gravity-inertia lee waves, with the horizontal wa... The theoretical researches on the problem of the disturbance of an atmospheric current flowing over a mountain range, carried on during the last ten years, prove that most of the characteristic observed features can be explained, to a very large extent, by the hydrodynamical theory of internal, small adiabatic perturbations in a stratified rotating atmosphere. In the case of a uniform current of velocity u, in which the coefficient of vertical stability is also uniform (θ is the potential temperature, g the gravity, z the elevation), the perturbation pattern corresponding to a smooth, low, gently-sloping mountain range varies widely according to the half-width a of this mountain range: i) If a is comparable to the critical value (Ls/2π) = (u/s) ≈ 1 km, there is a system of short stationary lee waves, or gravity waves, with the wave length Ls. ii) If a is comparable to (Lf/2π) = (u/f) ≈ 100 km, where f is the Coriolis parameter, there is a complex system of gravity-inertia lee waves, with the horizontal wa...