A warm front marks the boundary between cold and warm air masses, whereby the warm air pushes the relatively cool air towards the ground. Since the warm air is lighter than the cold air, it slides over the cold air in front of it and is forced to rise. As a warm front passes over, the wind from the southwest or west freshens and becomes stronger, while the temperature gradually rises and the air pressure falls. Weather maps depicting the weather close to the ground show a warm front as a red line with semicircles.
The upward movement created by the warm front forms clouds that often produce precipitation. Typically, a warm front consists of nimbostratus and altostratus clouds. The arrival of a warm front can be identified by a gradual increase in cloud cover with cirrus and then cirrostratus clouds. When subtropical warm air is unstable, cumulonimbus clouds can also form that can lead to thunderstorms. However, these are less common and less intense than thunderstorms that accompany a cold front.