Mountain Waves

Source: www.pilotworkshop.com
By Scott Dennstaedt

When strong winds are nearly perpendicular to a mountain range, you can expect mountain waves, correct? Well not necessarily.

Mountain waves require a very specific atmospheric structure before waves become apparent. Reasonably strong and nearly perpendicular winds along the ridge line are just one ingredient.

Mountain waves can only exist when there's unstable air below the ridgeline capped by stable air above the ridge line. As the buoyant and unstable air is forced up the ridge into the stable layer on the windward side, it will be forced downward on the lee side of the mountain ridge.

The air descends back into the unstable layer below the ridgeline and once again becomes buoyant and begins to rise back into the stable layer. This oscillation produces the mountain wave activity.

If you see a lot of cumulus cloud activity along the mountain range, this implies the atmosphere is unstable above the ridgeline. This environment doesn't support mountain wave activity.