Mountain Waves

Mountain waves are stationary waves occurring when stable air flows over an obstacle (a mountain).
The ideal case of a mountain wave is as follows:

Scheme of mountain waves formation
Mountain waves provide a possibility for gliders to gain altitude or fly long distances when soaring, but they also may be very harmful for other small aircrafts such as balloons, hang gliders and para gliders. They can even be a hazard for large aircrafts; there are some recorded cases of incidents caused by mountain waves and rotor clouds.
Mountain waves occur regularly in the parts of the world in which geographic and meteorological conditions for their occurrence exist. The strongest ones were recorded over Sierra Nevada, the Alps, the Patagonian Andes and the Southern Alps. In Croatia they may occur inland during the strong southeast (when coming from the Dinaric Alps) and southwest winds (when coming from the Alps). In the Adriatic region they may occur simultaneously with the bora.

Mountain wave over Rijeka Airport