<P> A warm front is a density discontinuity located at the leading edge of a homogeneous warm air mass, and is typically located on the equator - facing edge of an isotherm gradient . Warm fronts lie within broader troughs of low pressure than cold fronts, and move more slowly than the cold fronts which usually follow because cold air is denser and less easy to remove from the Earth's surface . This also forces temperature differences across warm fronts to be broader in scale . Clouds ahead of the warm front are mostly stratiform, and rainfall gradually increases as the front approaches . Fog can also occur preceding a warm frontal passage . Clearing and warming is usually rapid after frontal passage . If the warm air mass is unstable, thunderstorms may be embedded among the stratiform clouds ahead of the front, and after frontal passage thundershowers may continue . On weather maps, the surface location of a warm front is marked with a red line of semicircles pointing in the direction of travel . </P> <P> Air masses are large bodies of air with similar properties of temperature and humidity that form over source regions . The warm air mass behind a warm front is not only warmer, but often (but not always) also higher in humidity than the colder air preceding it . Because of a warm air mass's higher temperature and thus lesser density, mixing between the two air masses is unlikely . Being light, the warm air mass is unable to displace the cooler air mass and instead is forced upward along the upper boundary of the colder air in a process known as overrunning . The boundary between the two air masses has a gradual slope of 1: 200 and lifting is slow but persistent . </P>

Where do warm air fronts tend to come from