Weather Tidbits: The Lee Trough

This edition of Weather Tidbits will be discussing the lee trough. A lee trough is a strip of low pressure and surface convergence that forms on the leeward side of a mountain range. This can enhance the development of thunderstorms. Factors effecting lee trough development can be altered by thermodynamics. For simplicity’s sake, the video demonstration assumes no thermodynamic processes are in place and an air column presents no spin initially (relative vorticity = 0). When an air column flows toward a mountain, the height of the column decreases and its width increases. Essentially getting squished. When this happens, the relative vorticity becomes negative. You can compare this to stretching your arms out when spinning on an ice-skating rink. The further your arms extend, the slower you will spin. Since the air column assumes an initial relative vorticity of zero, this will lead to anticyclonic circulation at the summit.

When the air column moves toward the leeward side of the mountain, it’s height increases and width decreases. Essentially being stretched. When this happens, the relative vorticity becomes positive. This leads to cyclonic circulation and thus lower surface pressure. This trip of lower surface pressure along the leeward side of a mountain range is the lee trough. When you add ample thermodynamic support like increased heat and moisture content, the lee trough can be a focus point for enhanced thunderstorm development.