Tropical waves, or easterly waves, also known as African easterly waves in the Atlantic Ocean, are a type of atmospheric trough, an extended area of moderately low air pressure, leaning north to south, which move from east to west across the tropics creating areas of cloudiness and thunderstorms. West-moving waves can also form from the tail end of frontal zones in the subtropics and tropics and may be called to as easterly waves, but these waves are not properly called tropical waves; they are a form of inverted trough sharing many characteristics with fully tropical waves. All tropical waves form in the easterly flow along the southern side of the subtropical ridge or belt of high pressure which is north and south of the Intertropical Convergence Zone (ITCZ). Tropical waves are normally carried westward by the prevailing easterly winds along the tropics and subtropics near the equator. They can lead to the formation of tropical cyclones in the north Atlantic and northeast Pacific Oceans. They can also lead to tropical storms which can also lead to hurricanes.