Stationary front

Stationary front symbol: solid line of alternating blue spikes pointing to the warmer air mass and red domes pointing to the colder air mass

A stationary front (or quasi-stationary front) is a weather front or transition zone between two air masses when each air mass is advancing into the other at speeds less than 5 knots (about 6 miles per hour or about 9 kilometers per hour) at the ground surface. On weather maps, it is illustrated as a solid line of alternating blue spikes pointing to the warmer air mass and red domes facing the colder air mass.

Development

A stationary front may form when a cold or warm front slows down or grows 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 opposite directions along either side of the stationary front. A stationary front usually remains in the same area for hours to days and may undulate as atmospheric waves move eastward along the front.

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 waves aloft are fiercer, cold or warm air masses advance fast enough into other air masses at the surface.  For instance, when a cold air mass traverses sufficiently quick into a warm air mass, the stationary front changes into a cold front.

Because a stationary front marks the boundary between two air masses, there are often differences in air temperature and wind on opposite sides of it. The weather is often cloudy along a stationary front, and rain or snow often falls, especially if the front is in an area of low atmospheric pressure.

Characteristics

Although the stationary front's position may not move, there is air motion as warm air rises up and over the cold air, responsive to the geostrophic 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 are recurring, with storm trains or mesocyclone systems. When the warmer air mass is very moist, heavy or extreme rain or snow 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. That 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 temperature. That sometimes also provides enough heat energy and moisture to form subtropical storms and tropical cyclones at the surface.

Warm front

Warm fronts are boundaries of slow-moving air masses that replace masses of colder air ahead of them. Warm fronts typically travel between 10 and 25 miles per hour and contain warm, humid air. As the warm air is lifted, the temperature drops and condensation occurs, forming clouds.

Cold front

Cold fronts, on the other hand, travel more rapidly than warm fronts, at a rate of 25 to 30 miles per hour (potentially up to 60 miles per hour). Cold fronts are dense masses of air that remain close to the ground and displace warmer air by sliding under it. The ascending air rapidly decreases in temperature, forming clouds.

External links

• Quasi-Stationary Front (also known as Stationary Front).
• Stationary Front.