A cold front is the leading edge of an advancing colder, denser air mass that displaces warmer air ahead of it. On a surface map it is drawn as a blue line with triangles pointing toward the warm side, marking the direction the front is moving.
How it works
Cold air is denser than warm air at the same pressure, so it wedges under the warmer air and forces that lighter air upward. The slope of the wedge is steep near the ground and can be nearly vertical in strong systems, which is why cold frontal ascent tends to be abrupt rather than gentle.
The rising warm air cools, condenses, and often produces a narrow band of showers and thunderstorms along or just ahead of the boundary. Behind the front, cooler and drier air arrives with a wind shift, typically from a southerly quadrant to a westerly or northerly one in the Plains.
How forecasters use it
A cold front is a first-order forecast anchor for the day. Its projected position at peak heating locates where convergence is strongest, where surface-based instability is likely to be deepest just ahead of it, and where the wind shift will trigger a change in storm mode.
Where the front interacts with other boundaries, such as a dryline extending south of a low or a warm front bending east of it, storm coverage and organization tend to concentrate. The intersection often provides more low-level turning than either boundary alone.
Important limits
Not every cold front produces storms. A front moving into a stable, dry, or strongly capped air mass may pass with only a wind shift and a temperature drop. The front provides lift, but the environment ahead of it decides whether the lift produces convection.
Cold fronts can also lag well behind the pre-frontal trough of low pressure where storms actually fire. Chasing the blue line on a surface map instead of the pre-frontal environment is a common way to end up behind the action rather than ahead of it.