Warm front

A warm front occurs when warmer air glides over colder air. This happens because warm air is generally lighter than colder air. As warm air rises, it cools and becomes saturated with moisture. Clouds form when the moisture in the warm air condenses.
Warm fronts are often accompanied by low nimbostratus clouds with rain or drizzle. Because of the rain, the colder air may become saturated and cooled, often resulting in fog formation.
If the warm air is stable, altostratus and cirrostratus cloud forms. Conversely if the warm air is unstable, cumulonimbus and altocumulus cloud forms. The latter clouds produce severe thunderstorms.