The formation of atmospheric waves requires three basic ingredients:

1. Mountains or other obstacles
   In order to trigger oscillations and therefore waves, there must be obstacles for the air to flow over. These obstacles are often mountains but can sometimes also be a series of cumulus clouds that likewise present an obstacle to air flow due to their strong vertical updraughts. 
2. Wind
   The winds should be blowing as perpendicular as possible to the mountains at the level of the ridge and above. In addition, the flow speed must be at least 25–40 km/h. It is also beneficial if wind speed increases with altitude. 
3. Stability of the atmosphere
   Another key requirement is relatively stable stratification of the atmosphere at the height of the ridge. This means that there is only a slight decrease in temperature with altitude, or that temperature even increases with altitude – something that is referred to as an inversion.

The shape of the mountains and the exact wind and temperature profile of the atmosphere then determine the shape, propagation, length and height of the waves. Possible breaking of the waves also depends on these parameters.

If sufficient moisture is present in the atmosphere, clouds form in the updraughts of the mountain waves, making the waves visible. In this case, it is often possible to observe altocumulus lenticularis clouds. One weather situation in which lee and mountain waves appear quite often is the foehn. In this case, the aforementioned lentil-shaped clouds are known as “foehn fish”.

What is the difference between mountain waves and lee waves?

Why mountain and lee waves are important in meteorology

Waves can therefore result in more or less extensive cloud cover and thereby affect sunshine duration and temperature. Furthermore, atmospheric waves also play a role in downdraughts on leeward slopes – that is, in foehn events.

Lee and mountain waves are particularly relevant in aviation. They are very welcome in gliding, as they allow long-distance flight thanks to strong and stationary updraughts. For the remainder of aviation, however, the impact of waves is largely negative. For example, the strong, vertical air flow associated with the waves can mean that aircraft are no longer able to maintain their altitude. Moreover, mountain waves can break at the tropopause, which sometimes results in strong turbulence.