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. Cap cloud may form on the windward side. Roll clouds may form in the rotor zones. Roll clouds should be avoided at all costs due not only to the extreme turbulence present, but also because of the icing at sub-zero temperatures. Lenticular cloud may form in the crests of the waves.