A stationary front (or quasi-stationary front) is a weather front or transition zone between two air masses, when neither air mass is advancing into the other at a speed exceeding 5 knots (about 6 miles per hour or about 9 kilometers per hour) at the ground surface. On weather maps, it is shown by a solid line of alternating blue spikes pointing to the warmer air mass and red domes pointing to the colder air mass.
A stationary front may form when a cold or warm front slows down, or it may grow over time from underlying surface temperature differences, like a coastal front. Winds on the cold air and warm air sides often flow nearly parallel to the stationary front, often in nearly opposite directions along either side of the front. A stationary front usually remains in the same area for hours to days, and may undulate as atmospheric short waves move eastward along the front.
Although the stationary front's position may not move, there is air motion as warm air rises up and over the cold air, as a response to the ageostrophy induced by frontogenesis. A wide variety of weather may occur along a stationary front. If one or both air masses are humid enough, cloudy skies and prolonged precipitation can occur, with storm trains or mesocyclone systems. When the warmer air mass is very humid, heavy or extreme precipitation can occur.
Stationary fronts may dissipate after several days or devolve into shear lines. A stationary front becomes a shear line when air density contrast across the front vanishes, usually because of temperature equalization, while the narrow wind shift zone persists for some time. This is most common over open oceans where the ocean surface temperature is similar on both sides of the front, and modifies both air masses to correspond to its own temperature. This sometimes also provides enough heat energy and moisture to form subtropical cyclones and tropical cyclones at the surface.
Stationary fronts may also change into a cold or warm front, and may form one or more extratropical or mid latitude cyclones at the surface, when atmospheric short waves aloft are stronger and air masses advance fast enough into other air masses at the surface. For example, when a cold air mass advances fast enough into a warm air mass, since the advancing air mass is cold, the stationary front changes into a cold front.