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

The phenomenon primarily caused by warm air rising over mountains is mountain waves. When air encounters mountains, it is forced to ascend. As the warm air rises, it expands and cools, which can lead to the formation of specific cloud patterns and turbulence known as mountain waves. These waves can create oscillations in the air that continue downwind, potentially affecting weather conditions at significant distances from the mountains themselves.

Mountain waves can lead to various weather phenomena, including lenticular clouds and rotor clouds, which form on the leeward side of the mountains. Understanding this dynamic is crucial for pilots and meteorologists, as it can impact air travel and weather forecasting.

Other options like thunderstorms and fog formation do involve upward movement of air but are typically associated with different mechanisms. Thunderstorms result from a combination of factors including moisture, instability, and lift, while fog forms under conditions of cooling air near the surface, often related to high humidity rather than the specific dynamics of mountain airflows. Ice storms are generally caused by freezing rain conditions, which are unrelated to the rising of warm air over mountains.