A
cold front is defined as the leading edge of a cooler mass of air, replacing at ground level a warmer mass of air, which lies within a fairly sharp surface
trough of
low pressure. It forms in the wake of an
extratropical cyclone, at the leading edge of its cold air advection pattern, which is also known as the
cyclone's dry conveyor belt circulation. Temperature changes across the boundary can exceed . When enough moisture is present, rain can occur along the boundary. If there is significant instability along the boundary, a narrow line of thunderstorms can form along the frontal zone. If instability is less, a broad shield of rain can move in behind the
front, which increases the temperature difference across the boundary. Cold fronts are stronger in the fall and spring transition seasons and weakest during the summer. When a cold front catches up with the preceding
warm front, the portion of the boundary that does so is then known as an
occluded front.