In the terrestrial atmosphere described as an unbounded medium with variable temperature, the equations of small Mach number airflows are written in the framework of an asymptotic Boussinesq's approximation. An equation is obtained which is a Helmholtz equation with slowly varying coefficients. This equation is then used to describe the stationary linearized flow over an obstacle of small height. It is shown under realistic conditions of the repartition of Scorer's parameter that the flow over the obstacle is divided in two regions: a lee wave system builds up in the lower layer while a progressive wave emerges in the upper layer. The two flows appear to be uncoupled in a first approximation, so that the classical representation of lee waves by confined flows is justified.