Clouds that form north of warm fronts are often stratiform, according to the Department of Meteorology at Penn State University. A warm front is the leading edge of moving warm air that is advancing upon retreating cold air. The warm air presses over the cold air, creating widespread cloud sheets.
At the same pressure, cold air has a higher density than warm air. For warm air to advance forward, the cold dense air in its path must first retreat. The cold air retreats more slowly than the wind speeds on the warm side of the front. The warm air is forced to overrun the cold dense air near the Earth's surface, rising up an incline to colder atmospheric temperatures. As the warm air parcels cool, they stop ascending, and the moisture they carry condensates. The cloudy air parcels that are produced spread out horizontally in thin sheets of stratiform cloud layers.