A cold front occurs where a large mass of cold air meets a mass of warmer air, and the cold air advances on the warmer air. The cold air undercuts the warm air pushing it upwards. Cumulonimbus clouds form a well-defined line along the boundary between the air masses. As the cold front passes, the clouds roll by and the air temperature may become noticeably cooler, with temperatures dropping by 5 °C or more within the first hour. Rain, gusty winds, and, sometimes, thunderstorms occur with the passage of the cold front.
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 coloured weather maps, a cold front is drawn in blue.