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LeeWaves.png|left|thumb|300px|Generation of Lee Waves (schematic drawing)
1 = Mountain
2 = Wind
3 = Rotor
4 = Lee Wave
5 = typical cloud (lenticularis)
6 = typical cloud (cumulus)]]
In meteorology, Lee waves, also known as mountain waves, are periodic changes of pressure in a stream of air when the wind moves over mountains. This forces periodic changes in speed and direction of the air within this air stream. They always occur in groups on the lee side of mountains. Usually a horizontal turbulence is generated at the lee side of the first mountain (following the air stream), the so called rotor.
Both lee waves themselves as well as the rotor may be indicated by typical clouds which do not move with the wind as clouds usually do.
- At the vertex of the wave usually exists a cloud in shape of a lens (lenticularis).
- The rotor may generate a cumulus cloud in its raising part.
Lee waves provide a possibility for gliders to gain altitude when soaring. But especially the rotor may be harmfull for other small air crafts such as balloons, hang gliders and para gliders. They can even be a hazard for large aircraft; the phenomenon is believed responsible for the in-flight break up of a Boeing 707 near Mt. Fuji, Japan in 1966.
Varieties of Lee Waves
There are a variety of distinctive types of Lee waves which form under different atmospheric conditions. Some less commonly known types are as follows:
- Hydraulic Jump Induced Waves: this type of wave forms when there exists a lower layer of air which is dense, yet thin relative to the size of the mountain. After flowing over the mountain, a type of shock wave forms at the trough of the flow, and a sharp vertical discontinuity called the hydraulic jump forms which can be several times higher than the mountain. The hydraulic jump is similar to a rotor in that it is very turbulent, yet it is not as spatially localized as a rotor. The hydraulic jump itself acts as an obstruction for the stable layer of air moving above it, thereby triggering wave. Hydraulic jumps can distinguished by their towering roll clouds, and have been observed on the Sierra Nevada range * as well as mountain ranges in southern California.
- Hydrostatic Waves: hydrostatic waves are vertically propagating waves which form over spatially large obstructions. In hydrostatic equilibrium, the pressure of a fluid can depend only on altitude, not on horizontal displacement. Hydrostatic waves get their name from the fact that they approximately obey the laws of hydrostatics, i.e. pressure amplitudes vary primarily in the vertical direction instead of the horizontal. Whereas conventional, non-hydrostatic waves are characterized by horizontal undulations of lift and sink, largely independent of altitude, hydrostatic waves are characterized by undulations of lift and sink at different altitudes over the same ground position.
See also
"Lee waves"