Lee wave

The lee waves always occur in groups are gravity waves in the air flow on the leeward side ( Lee) of Bergen. They often occur together with an air vortex, which is referred to as a rotor or vortex roll.

Identifying

Both the lee waves and the rotor, the vortex axis is horizontal, are often accompanied by characteristic stationary cloud formations:

• (also called " Föhnfisch " ) lens-shaped lenticular in the crest of the wave
• Cumulus in the ascending part of the rotor ( rare )

Storage clouds on the windward side of the mountain is indeed frequently observed, in contrast to the two aforementioned cloud shapes but not a reliable indication of lee waves.

Importance for aviation

The parts of the wave, which represent updrafts are a way for gliders to gain height. For other aircraft, such as balloons, hang gliders and paragliders the lee wave rather constitutes a danger. The rotors of the lee waves are severe turbulence and thus dangerous for airliners, see BOAC Flight 911

When passing the lee waves ( in particular the rotor) in an aircraft, the occupants can perceive lee waves than air hole.

Formation regions

Lee waves typically occur in areas where strong winds often occur transversely to mountain ranges, such as in North and especially South America ( Andes). Therefore, this special test and measurement flights have already been successfully carried out in Argentina.