dynamic trough

(meteorology) A pressure trough formed on the lee side of a mountain range across which the wind is blowing almost at right angles. Also known as lee trough.

As a stable air column rises to cross over a ridge, it shrinks in the vertical, therefore diverging in the horizontal. This gives it a negative relative vorticity. Accordingly, in the Northern Hemisphere, the flow is deflected anticyclonically: to the right. Since low pressure always lies to the left of an airstream in this hemisphere (Buys Ballot's law), the pressure is lower on the lee side of the mountain than on the windward side. The lee trough to the east of the Rockies is a major factor precipitating the north-south meanders of the Rossby waves.