 Warm fronts are at the leading edge of a homogeneous warm air mass , which is located on the equatorward edge of the gradient in isotherms , and lie within broader troughs of low pressure than cold fronts . A warm front moves more slowly than the cold front which usually follows because cold air is denser and harder 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 . 
