A squall line is a non-frontal, narrow band of active thunderstorms. They often develop ahead of a cold front in moist, unstable air, but may also develop in unstable air far removed from any front. The line may be too long to easily detour around and too wide and severe to penetrate. Squall lines often contain severe steady-state thunderstorms and present the single most intense weather hazard to aircraft. They usually form rapidly, reaching a maximum intensity during the late afternoon and the first few hours of darkness.