A warm front is formed when a warm air mass _____ a cold air mass.

A warm front is specifically characterized by a warm air mass rising over a cold air mass. This process occurs due to the differences in temperature and density between the two air masses. Warm air is less dense than cold air, which causes it to rise as it encounters the cold air. As the warm air ascends, it cools and condenses, often leading to cloud formation and, in many instances, precipitation.

This phenomenon is an important aspect of weather patterns because the rising of warm air over cold air is a primary driver of the development of various weather systems associated with warm fronts, such as gentle rain and overcast skies. Understanding this process is crucial for interpreting weather maps and forecasts.

The other options do not accurately describe the interaction between warm and cold air masses in the context of a warm front. For instance, sinking or dispersing does not reflect the dynamic that leads to the formation of a warm front and its associated weather phenomena.