MOUNTAIN WAVES

(also known as standing, or lee waves..often abbreviated to MTW) -- Under certain conditions, which must include a stable layer around/just above a mountain/hill range, air flowing across the range will be found to oscillate vertically in a standing wave configuration with well defined crests/troughs in the wavetrain. The horizontal flow must be reasonably brisk and within roughly 30 degrees of the crest of the hill/mountain range, with speed increasing with height, and directional shear must be small. Airflow is often smooth, but the vertical velocities in the upward/downward legs of the standing waves can be strong, and may break down to give local severe turbulence. Surface winds downstream of the ridge may be notably gusty as well, but paradoxically, they may also be extremely light, or even reverse direction. The presence of MTW activity may be betrayed in visible satellite imagery, if the flow is moist enough, as a series of near-parallel bars of cloud downwind of the ridge causing the airflow displacement.