Schematic of thunderstorms. Although best described in terms of its wavelike characteristics in the wind field, it also consists of a weak trough of low pressure. The presence of a disturbance like this indicates atmospheric instability and is often associated with tropical cyclone development. Easterly waves (dashed lines) propagating westward from the West African coast towards the Caribbean Sea in the trade winds on the southern side of the subtropical ridge. The subtropical ridge is one of the main sources of environmental wind that steers both easterly waves and tropical cyclones westward across the tropical North Atlantic