Cold fronts are usually associated with depressions. A cold front is defined as the transition zone where a cold air mass is replacing a warmer air mass. At a cold front cold air following warm air undercuts the warm air, heaving it upwards with a more violent thrust compared to the steady rise of air at a warm front. The air associated with a cold front is usually unstable and conducive to cumulonimbus cloud formation. Because the upthrust is delivered along a boundary between the two air masses, the cumulonimbus form a well-defined line in contrast to the well-spaced clouds forming during thermal convection. Usually, rainfall associated with cold fronts is in the form of heavy deluge. More rain may fall in a few minutes as the cold front passes than during the whole passage of a warm front. As the cold front passes, the clouds roll by and the air temperature may become noticeably cooler, with temperatures dropping by 5C or more within the first hour.
On synoptic (weather) charts a cold front is represented by a solid line with triangles along the front pointing towards the warmer air and in the direction of movement. On colored weather maps, a cold front is drawn with a solid blue line.