When does a warm front typically occur?

A warm front typically occurs when warm air moves over cold air. This phenomenon happens because warm air is less dense than cold air, allowing it to rise above the colder air mass. As the warm air ascends, it cools and condenses, which leads to cloud formation and often results in precipitation. This process is vital in understanding weather patterns associated with warm fronts, as it generally brings lighter and continuous precipitation, which can occur over a wider area compared to the abrupt weather changes often associated with cold fronts.

In context, the other options do not accurately describe the dynamics of a warm front. A warm front does not occur when air masses are stationary, nor does it happen when warm air retreats. Additionally, a cold front is characterized by cold air pushing under warm air, which is distinctly different from the mechanism of a warm front. Understanding these concepts is crucial for forecasting weather and for safe flight operations, as pilots must be aware of the weather patterns associated with different types of fronts.