A weather front in which warm air advances and replaces cold air is called a warm front.
Warm air is less dense than cold air, so it usually cannot shove the colder air mass aside at ground level. Instead, it gradually rises over the cold air along a gently sloping boundary. This overrunning motion cools the rising air and often produces broad layers of cloud and extended periods of precipitation ahead of the front.
Warm fronts generally move more slowly than cold fronts because the denser cold air is difficult to displace. Before passage, clouds may thicken from high cirrus and altostratus to lower stratus, with rain or snow becoming more likely. Fog can also develop. After the front passes, temperatures usually rise and skies may gradually clear.
On weather maps, a warm front is shown as a red line with semicircles pointing in the direction of movement. It differs from a cold front, where advancing cold air replaces warm air more abruptly. An occluded front forms when a cold front catches a warm front, while a stationary front has little movement.