Air waves in the lee of a mountain barrier, where a stable layer of air, after displacement by movement over the barrier, returns to its original level. This process results in a series of stationary (‘standing’) waves extending downwind on the lee side of the barrier. Clouds often form along the wave crests in lenticular form: they may appear stationary, due to condensation of water vapour at the upward side, caused by the upward air movement, and evaporation on the downward side of the wave. The wavelength can be up to 40 km and the wave amplitude is most pronounced in the intermediate levels of the airstream. Circular air motion (in the vertical plane) beneath the wave crests may reverse wind direction locally within the general air flow. This phenomenon is termed a ‘rotor’. In addition to stable air at an intermediate level, lee-wave formation requires a constant wind of at least 8 m/s. Well-known wave clouds on the lee side of barriers include the Sierra wave of the Sierra Nevada, California, the helm wave of Cumbria, England, and the moazagotl of Silesia.