A squall line is a linear organisation of severe thunderstorms extending over hundreds of kilometres that moves as a unit, generating a narrow but prolonged band of strong winds, heavy rain, hail, and occasionally tornadoes. It typically forms along or ahead of an active cold front, where surface convergence forces vigorous uplift. The typical structure includes an active convective line (the zone of most intense storms), a zone of downdrafts behind, and a gust front advancing ahead, causing the abrupt wind shift that heralds the line's arrival. The most destructive winds concentrate in a bow echo—a forward-bowing curvature of the line where winds can exceed 120 km/h. An extreme case of a persistent, destructive squall line is the derecho. Squall lines are frequent during cold air incursions in spring and autumn, especially when crossing landmasses associated with deep depressions. On radar, they are identified as a continuous line of intense echoes that can stretch 200–500 km in length.