Which phenomenon is primarily caused by warm air rising over mountains?

The phenomenon predominantly caused by warm air rising over mountains is mountain waves. When warm air encounters mountainous terrain, it is forced to rise, creating a wave pattern in the atmosphere. As the air ascends, it cools and can lead to the formation of clouds and potentially turbulence during the descent on the leeward side. This process is critical because it affects flight operations by creating areas of turbulence that pilots need to navigate.

In the context of the other phenomena listed: thunderstorms are typically associated with significant atmospheric instability, which involves more complex interactions than just rising warm air over mountains. Fog formation generally occurs in stable, cool conditions and is related to moisture in the air rather than the direct effect of warm air over mountains. Ice storms are a result of freezing rain conditions and specific temperature profiles, not the direct outcome of warm air rising. Thus, mountain waves are the most closely linked phenomenon resulting from the interaction of warm air with mountainous regions.