Weather Front
A weather front is the boundary between two air masses of different temperature and humidity, where clouds, precipitation, and wind shifts commonly form.
Definition
A weather front is the boundary between two air masses of different temperature and humidity, and fronts usually bring clouds, rain, or storms along with a change in temperature once they pass. A cold front forms where advancing cold air wedges sharply under warm air, producing narrow bands of heavy showers and thunderstorms, while a warm front forms where advancing warm air slides gently up and over cold air, producing widespread layered clouds and steady rain well ahead of it; a stationary front does not move, and an occluded front forms once a cold front overtakes a warm one. These are zones of strong horizontal temperature contrast, typically tens of kilometers wide, sloping over the colder air at about $1$:$50$ to $1$:$150$ for cold fronts and more gently for warm fronts, and the Norwegian cyclone model describes how such fronts rotate around a developing low before occluding.
Example
On a weather map, a blue line with triangles marks a cold front, and when one passes Charlotte it often brings a thunderstorm followed by cooler, clearer air, sometimes dropping the temperature $10^\circ C$ in an hour and shifting the wind from south to northwest. The Piedmont's cold-air damming pins a stationary or backdoor front against the Appalachians, letting warm, moist air override shallow cold air to produce prolonged freezing rain, while a dry line on the Great Plains, a moisture front rather than a temperature front, is a favored trigger for supercell thunderstorms.
Key Insight
The word "front" comes from World War I battle lines, borrowed because two air masses collide much like opposing armies, and warm fronts give a day's warning through high, wispy clouds while cold fronts can arrive with little notice, a difference that comes down to the slope of the boundary. Fronts exist because the atmosphere cannot mix warm and cold air fast enough to erase their contrast, and the energy stored in that temperature difference is exactly what mid-latitude cyclones release when a front sharpens and finally breaks.