Abstract: It is shown that the large-scale orographically produced perturbations in a frictionless, adiabatic and baroclinic atmosphere can be described by a three dimensional Helmholtz's differential equation.The perturbations of westerlies, due to the Tibetan-Plateau and Rocky Mountains are computed. The numerical results shows that the positions of troughs and ridges, in the westerlies are well in agreement with the observation, but there is discrepancy in intensity. Further that the perturbations are damped rapidly with increasing height.