A warm front exists when warm air is rising over cold air. In vertical cross-section, the boundary takes the form of a gradual slope (roughly 1:100) and lifting is slow but persistent. As the air lifts into regions of lower pressure, it expands, cools and condenses water vapour as flat sheet cloud (altostratus), from which rain can start to fall once cloud has thickened to about 2,500 metres from the ground. Cloud continues to lower towards the boundary at ground level, known as the surface front. This lower level cloud is called stratus or nimbostratus, from which appreciable amounts of rain may fall. Sometimes, nimbostratus cloud may be only a few hundred feet above the ground, and can completely cover hilltops and mountains.
Because frontal systems have a velocity of there own, an observer on the ground will witness a succession of cloud types with cloud gradually thickening before rain arrives. These telltale signs can be used by the observer to predict the onset of bad weather within a few hours. When the surface warm front arrives, there may be a burst of rather heavier rain, and this offers a hopeful sign that a drier interlude is on the way. Clouds will break, rain cease, and there may be a noticeable rise in temperature as the warm air engulfs the observer.
On synoptic (weather) charts a warm front is represented by a solid line with semicircles pointing towards the colder air and in the direction of movement. On colored weather maps, a warm front is drawn with a solid red line.