Since warm air is less dense and flows more freely, it rises over the cold air north of the front in a wedge shape, along a shallow incline. Modest instability and lifting generated. First evidence is cirrus and cirrostratus clouds, then altostratus, stratus, and finally nimbostratus as the front draws near. Cold air is denser, tends to barrel into warm air like a bulldozer. This produces a steep frontal boundary and considerable instability and lifting.
First evidence is cirrus clouds.
Sometimes, this will be followed by a squall line , a zone of convergence in advance of the cold front where cumulonimbus clouds congregate. Other times, no squall line forms, and cumulonimbus clouds hug the cold front boundary. Heavy precipitation can occur, brief rain showers that can be severe, occur in several episodes. Because of large temperature contrast and great instability, cold fronts and their associated squall lines can spawn tornadoes. 3. periods of brief, heavy rain gradually pass, replaced by clearer skies.