A pressure trough formed on the lee side of a mountain range in situations where the wind is blowing with a substantial component across the mountain ridge; often seen on United States weather maps east of the Rocky Mountains, and sometimes east of the Appalachians, where it is less pronounced. Its formation may be explained thermodynamically by the warming due to adiabatic compression of the sinking air on the lee side of the mountain range, or dynamically by generation of cyclonic circulation (cyclogenesis) by the horizontal convergence associated with vertical stretching of air columns passing over the ridge and descending the lee slope. Alternatively, the latter viewpoint is often expressed as the conservation of potential vorticity, where the vertical stretching of the columns is compensated by an increase in their relative vorticity. See lee cyclogenesis.