The production of small wavelength mountain waves near a mountaintop under conditions of very strong static stability. When the air is very stable and wind speeds are slow, the natural wavelength of air is often much shorter than the width of the mountain, as indicated by a very small Froude number. For this situation, the buoyant restoring force in the air is so strong that the air resists vertical displacement to get over the mountaintop, and instead most of the air flows around the sides of the mountain. The shallow layer of air near the mountaintop that is able to be displaced upward over the mountain will continue in vertical oscillation as it blows downstream, or separates, from the mountain. Compare lenticular cloud, downslope windstorm.