 A weather front is a boundary separating two masses of air of different densities , and is the principal cause of meteorological phenomena . In surface weather analyses , fronts are depicted using various colored triangles and half @-@ circles , depending on the type of front . The air masses separated by a front usually differ in temperature and humidity . Cold fronts may feature narrow bands of thunderstorms and severe weather , and may on occasion be preceded by squall lines or dry lines . Warm fronts are usually preceded by stratiform precipitation and fog . The weather usually clears quickly after a front 's passage . Some fronts produce no precipitation and little cloudiness , although there is invariably a wind shift . 
