Windspeed must be 20kt or more at the crown of the ridge, getting stronger with height. Just above the crown of the ridge there must be a stable layer of a few thousand feet in depth, with less stable layers of air above and below it. The amplitude and the length of the waves will determine the extent of the problem posed to pilots. The height of the ridge governs the amplitude. Usually, the strength of the wind dictates the wavelength, which extends normally from 3 to 30 nms.
Light wind means short wavelength.
The waves increase in amplitude into the middle troposphere. Very often the waves will persist up to the tropopause, and will decrease slowly above it. Where the wave meets the laminar flow of air below the level of the ridge on the lee side, rotor zones occur inside the wave crests, see Figure 1 . Flight is much smoother in the waved flow of air above the rotor zone. But maintenance of height and speed can still be a problem because significant vertical air speed may still be encountered.