front

A zone of separation between two air masses, of markedly dynamic character, in which a cloud structure, elongated and narrow, habitually forms, usually leaving precipitation in its wake. Fronts are associated with the low-pressure systems that travel through mid-latitudes. On isobaric maps where they are plotted, each thick line representing one of them indicates the zone of intersection of a given frontal surface with the Earth's surface. Although strictly speaking a front is not the same as the aforementioned frontal surface, it is common to treat both concepts as one. As a general rule, a moving front takes the name corresponding to the air mass being displaced: cold front and warm front. The theory explaining their formation and the life cycle of the extratropical cyclones that generate them was developed in the second decade of the 20th century by Jacob Bjerknes (1897-1975), in collaboration with his father Vilhelm [Bjerknes] (1862-1951). It took the name "front" from military language, on relating its appearance to that of the front lines of soldiers in the battles of the First World War.