Stationary fronts

A stationary front is a pair of air masses, neither of which is strong enough to replace the other. On a weather map, this is shown by an inter-playing series of blue spikes pointing one direction and red domes pointing the other. They tend to remain essentially in the same area for extended periods of time, and waves sometimes propagate along the frontal boundary.

A wide variety of weather can be found along a stationary front, but usually clouds, prolonged precipitation, and storm trains are found there. When there is a lot of water vapor in the warmer air mass, significant amounts of rain or freezing rain can occur.

Stationary fronts will either dissipate after several days or devolve into shear lines, but can change into a cold or warm front if conditions aloft change. Additionally, if one air mass enters the other, then it will change into a cold or warm front, and the stationary front will be reclassified as moving. For instance, if a warm air mass enters a cold air mass, since the advancing air mass is warm, the stationary front will change into a warm front.

A stationary front becomes a shear line when the density contrast across the frontal boundary vanishes, usually as a result of temperature equalization, while the narrow zone of wind-shift persists for a time. This is most common over the open ocean as the temperature of the ocean surface is usually the same on both sides of the frontal boundary and modifies the air masses on either side of it to correspond to its own temperature. Stationary fronts always stay still. When a warm or cold front stops moving, it becomes a stationary front.

Occasionally cold air and warm air may flow parallel to the front. The position of the front may not move. The two different air masses are then said to be separated by a stationary front. Although the two air masses are not moving, warm air may still rise over the cold air. The slope of a stationary front is as gentle as the slope of a warm front. Weather conditions, too, are similar to those of a warm front.

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Wikipedia