stationary front

A cold or warm front that stops advancing and comes to a halt, remaining stationary for a long period of time, which can range from a few hours to several days. This circumstance occurs when the push of the trailing air mass is abruptly halted by the leading one. This situation sometimes occurs when a front approaches a mountain range. The mountain obstacle manages to slow the front's advance and it becomes anchored on the windward side, causing persistent precipitation in the area. A front can also become stationary when, in the zone of separation of the two air masses - where the front is located -, two air flows blowing in the same direction but in opposite senses become balanced. Sometimes, the balance of forces between the two air masses is not perfect and they move slightly; in such cases we have what is known as a semi-stationary front. On weather maps, stationary fronts are represented with a thick line alternating red semicircles and blue triangles, one and the other pointing in opposite directions.