trough

An isobaric configuration, analogous to a surface low-pressure furrow or hollow, that corresponds to a region of the atmosphere where the pressure is relatively low, not being associated with a closed circulation. Is a sunken area or valley, in the case at hand, the pressure field takes on a similarly elongated shape. On an upper-air map it presents a characteristic V shape, with the concavity directed toward a low. Its axis is the imaginary line joining its convex part (the vertex of the V) with the center of the depression, and it tends to be tilted relative to the direction of movement of the trough itself. Conceptually, troughs can be considered as large meanders of cold air that descend from the polar regions to the subtropical realm, forming part - as do ridges - of the great atmospheric waves that surround each of the Earth's hemispheres (Rossby wave). In the leading part of the trough (the area where cold air exits) cyclogenesis is favored, being a zone of marked atmospheric instability, where convection tends to develop, producing storm activity.