<P> 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 . </P> <P> 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> An occluded front is formed when a cold front overtakes a warm front, and usually forms around mature low - pressure areas . The cold and warm fronts curve naturally poleward into the point of occlusion, which is also known as the triple point . It lies within a sharp trough, but the air mass behind the boundary can be either warm or cold . In a cold occlusion, the air mass overtaking the warm front is cooler than the cool air ahead of the warm front and plows under both air masses . In a warm occlusion, the air mass overtaking the warm front is warmer than the cold air ahead of the warm front and rides over the colder air mass while lifting the warm air . </P> <P> A wide variety of weather can be found along an occluded front, with thunderstorms possible, but usually their passage is associated with a drying of the air mass . Within the occlusion of the front, a circulation of air brings warm air upward and sends drafts of cold air downward, or vice versa depending on the occlusion the front is experiencing . Precipitations and clouds are associated with the trowal, the projection on the Earth's surface of the tongue of warm air aloft formed during the occlusion process of the depression . </P>

What type of front has cold air moving into warm air