Orographic forcing

An airstream encountering a barrier to its passage is forced to go around or over the obstacle. The upward deflection of the airflow is sufficient to give rise to adiabatic cooling, and if the air is moist enough, the formation of clouds, precipitation etc. In addition, convergence of the flow on the windward side (due to a rapid decrease in velocity) when the air encounters a sharply graded barrier not only enhances the vertical motion, but also leads to a deformation of the flow which in turn alters the vorticity of the air particles. Thus, hill & mountain ranges are most important in a study of meteorology.