There are two possibilities for air to flow past a mountain ridge. Either the air parcels go around the mountain or it is forced to rise over the mountain: Which of these cases occurs, is dependent on several parameters: The static stability, the height of the mountain or mountain ridge and the wind component perpendicular to the mountain. The term combining these parameters gives an idea whether there is a flow across the mountain or not: If U is small the mountain is difficult to cross; also if N increases the mountain will be more difficult to cross. But even in the case of air partially streaming around the mountain, those streamlines approaching a greater height will cross the mountains. When the wind has a component perpendicular to a mountain chain the air accumulation due to deceleration of wind speed by the obstacle on the upwind side creates high pressure. Part of the air is deflected upward giving rise to mountain waves. According to the theory of internal gravity waves, an air parcel within a stable stratified atmosphere will start to oscillate, as long as the waves are damped by friction. Waves can only occur in a stable atmosphere. An air parcel which is removed from its original place cannot oscillate if there is no returning force. Lee Cloudiness will form where there is a sufficient supply of humidity near the wave crests and where there is upward motion.
Filed under Keywords:
Conceptual Models, Synoptic Scale Meteorology, Orography