What occurs when cold air undercuts warm air?

When cold air undercuts warm air, it creates a situation that can lead to the development of thunderstorms. This phenomenon occurs due to the differences in temperature and density between the cold and warm air masses. The cold air, being denser, forces the warmer air to rise rapidly. As the warm air ascends, it cools and may reach its dew point, which can lead to condensation and cloud formation. If this upward motion is strong enough and the conditions are right, it can result in the formation of cumulonimbus clouds, which are associated with thunderstorms.

In contrast, the other options do not align with the dynamics at play. The formation of fog typically requires a more stable air mass and usually occurs when warm, moist air cools near the ground rather than when cold air pushes beneath warm air. The act of cold air undercutting warm air destabilizes the atmosphere rather than stabilizing it. Consequently, such conditions are less likely to promote clear weather, as the lifting of warm air tends to encourage cloud development and can lead to precipitation events, including thunderstorms.