Stationary Front

Description

A stationary front is a weather front or transition zone between two air masses (cold and warm), when neither air mass is advancing into the other at a speed exceeding 5 knots at the ground surface. In terms of meteorological analysis, the front must be in roughly the same position between standard observations times of three or six hours. It is technically referred to as a quasi-stationary front since in the real world there is always some movement or undulation. The temperature difference across the front can be small or great depending on the nature of the two air masses involved.

Formation

A stationary front may form when a cold or warm front slows down, or it may grow over time from underlying surface temperature differences when there is little air mass movement, like a coastal front. Winds on the cold air and warm air sides often flow nearly parallel to the stationary front, and often in nearly opposite directions along either side of the front. At upper levels, the front is typically parallel to the prevailing flow. 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.

Stationary 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 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 temperature and density contrast across the front vanish, 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. If the water temperatures are warm enough, the clouds and rainfall associated with the decaying stationary front sometimes also provides enough heat energy and moisture to form subtropical cyclones or tropical cyclones at the surface.

Stationary fronts may also change into a cold or warm front, particularly in response to cyclone development due to atmospheric short waves aloft. Then, the cyclonic circulation superimposed on the front will develop the cold and warm fronts characteristic of low pressure areas. See the separate article on Extratropical Cyclone Models.

Depiction on weather charts

The stationary front symbol on a chart is a solid line of alternating blue spikes pointing to the warmer air mass and red domes pointing to the colder air mass.

Chart symbol for a stationary/static front

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