A front is a boundary separating two masses of air of different densities.- the principal cause of meteorological phenomena. Cold fronts = the leading edge of a cooler and drier mass of air.- may feature narrow bands of thunderstorms and severe weather, - may on occasion be preceded by squall lines or dry lines. - move up to twice as fast and produce sharper changes in weather than warm frontsWarm fronts are usually preceded by stratiformprecipitation and fog. The weather usually clears quickly after a front's passage. Because of the greater density of air in their wake, cold fronts and cold occlusions move faster than warm fronts and warm occlusions. Mountains and warm bodies of water can slow the movement of fronts. When a front becomes stationary, and the density contrast across the frontal boundary vanishes, the front can degenerate into a line which separates regions of differing wind velocity, known as a shearline. This is most common over the open ocean.