In tropical meteorology, a wave trough in the circumpolar westerlies having sufficient amplitude to reach the Tropics in the upper air. At the surface it is manifest as a trough in the tropical easterlies, but at moderate elevations it is characterized by westerly winds. It moves generally from west to east, accompanied by considerable cloudiness. Cumulus congestus and cumulonimbus clouds are usually found in and around the trough lines. Early- and late-season (June and October) hurricanes of the western Caribbean frequently form in polar troughs. See meridional front.