A tropical wave, also known as African easterly waves, or easterly wave are elongated areas of low pressure, north/south oriented, that originate from eastern Africa. They move from east to west across the tropics, causing areas of cloudiness and thunderstorm. Also, westward moving waves can form at the tail end of frontal zones in the subtropics as well as tropics, and those waves may be called easterly waves. However, these waves are not true tropical waves; rather they are a form of inverted trough that share many of the characteristics of a true tropical wave. All tropical waves form from the easterly flow associated with the southern side of the subtropical ridge (it can be thought of as a belt of high pressure that lies north and south of the Intertropical Convergence Zone (ITCZ). Finally, tropical waves move generally westward because of the prevailing easterly winds along the tropics and subtropics near the equator. In the Atlantic Ocean and the northeast Pacific Ocean, they can lead to tropical cyclone formation.