"Mountain Wave" or "Lee Wave" is a type of atmospheric internal gravity wave that occurs when stable air is vertically displaced, then oscillates around the equilibrium level. Atmospheric stability, advection (horizontal wind flow) and sloping terrain are required for waves to form. As stable air is forced up the slope of rising terrain, it cools adiabatically below the temperature of the surrounding air and becomes negatively buoyant. As the air then encounters down-sloping terrain, it accelerates downwards towards the equilibrium level. Air has mass, and therefore momentum; this momentum causes the descending air to "overshoot" the equilibrium level, and begin warming adiabatically above the temperature of the surrounding air. The air begins accelerating upwards, again overshooting the equilibrium level, setting off a series of oscillations, or waves. ![[gravity wave oscillation.png]] Waves may take the form of [[Trapped Lee Waves]], [[Vertically Propagating Waves]], or some intermediate forms, depending on the [[Conditions Associated with Wave]].