A line of thunderstorms, near whose advancing edge squalls occur along an extensive front. The thundery region, 12–30 mi (20–50 km) wide and a few hundred to 1250 mi (2000 km) long, moves at a typical speed of 15 m/s (30 knots) for 6–12 h or more and sweeps a broad area. In the United States, severe squall lines are most common in spring and early summer when northward incursions of maritime tropical air east of the Rockies interact with polar front cyclones. Ranking next to hurricanes in casualties and damage caused, squall lines also supply most of the beneficial rainfall in some regions.