<P> A wide variety of weather can be found along a stationary front, but usually clouds and prolonged precipitation are found there . Stationary fronts either dissipate after several days or devolve into shear lines, but they can transform into a cold or warm front if conditions aloft change . Stationary fronts are marked on weather maps with alternating red half - circles and blue spikes pointing in opposite directions, indicating no significant movement . </P> <P> When stationary fronts become smaller in scale, degenerating to a narrow zone where wind direction changes significantly over a relatively short distance, they become known as shearlines . A shearline is depicted as a line of red dots and dashes . Stationary fronts may bring snow or rain for a long period of time . </P> <P> A similar phenomenon to a weather front is the dry line, which is the boundary between air masses with significant moisture differences . When the westerlies increase on the north side of surface highs, areas of lowered pressure will form downwind of north--south oriented mountain chains, leading to the formation of a lee trough . Near the surface during daylight hours, warm moist air is denser than dry air of greater temperature, and thus the warm moist air wedges under the drier air like a cold front . At higher altitudes, the warm moist air is less dense than the dry air and the boundary slope reverses . In the vicinity of the reversal aloft, severe weather is possible, especially when a triple point is formed with a cold front . A weaker form of the dry line seen more commonly is the lee trough, which displays weaker differences in moisture . When moisture pools along the boundary during the warm season, it can be the focus of diurnal thunderstorms . </P> <P> The dry line may occur anywhere on earth in regions intermediate between desert areas and warm seas . The southern plains west of the Mississippi River in the United States are a particularly favored location . The dry line normally moves eastward during the day and westward at night . A dry line is depicted on National Weather Service (NWS) surface analyses as an orange line with scallops facing into the moist sector . Dry lines are one of the few surface fronts where the pips indicated do not necessarily reflect the direction of motion . </P>

The only type of front involves three air masses is