Boundary between dense, cold air and the warmer, less dense air ahead of it, which the cold air tends to undercut as it advances. The gradient of the cold front may be steep, e.g. 2°. Air is forced to rise along this steep front, commonly triggering the development of cumuliform cloud, often with precipitation. Weather changes occur with the passage of a cold front, sometimes including a pronounced temperature fall, a rise in pressure, and a change in wind direction. The passage of the front commonly brings clearer, brighter weather, but the unstable air in the cold sector can produce showers which may be heavy, with thunder and lightning.