A cold front is the boundary between an advancing mass of cold air and the warmer air ahead of it. Because cold air is denser, it undercuts the warm air like a wedge, forcing it to rise rapidly. This violent uplift produces a narrow band of intense weather: heavy showers, thunderstorms, gusty winds, and sharp temperature drops of 5–10 °C in under an hour.
On weather maps, cold fronts are drawn as blue lines with triangles pointing in the direction of movement. They typically trail south-west from the centre of a depression and move at 30–50 km/h — faster than warm fronts — which explains their steeper slope (roughly 1:50 to 1:100). The passage of a cold front brings a wind shift (usually veering from south-west to north-west in Europe), a pressure jump, and clearing skies as the cold air mass settles in.
Behind the front, visibility improves markedly and cumulus clouds with blue skies between them replace the earlier grey overcast. In summer, cold fronts can trigger severe supercell thunderstorms ahead of the line. In winter, they bring snow to mountains and sometimes to lower elevations. Cold fronts crossing the Mediterranean can regenerate over warm waters, producing heavy orographic rainfall on coastal mountains.