lee waves

A stationary gravity wave created by the forcing of air over high mountains of width 1 to 20 kilometers (0.6 to 12 miles). This lee wave appears downwind of the mountain and occurs due to the oscillation of the air in a stable atmosphere; the air flow streamlines, tracing out the wave. Lee waves are seen near the Rocky Mountains, Sierras, Cascades, and Appalachians. The properties of the wave depend on the atmospheric stability, the width of the mountain range, and the wind shear. These waves are most favored when the angle between the winds and mountain ridge is perpendicular and when the winds increase and the stability decreases with height. Lenticular or rotor clouds can form in the crests of the waves.