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 . West-moving waves can also form from the tail end of frontal zones in the subtropics and tropics and may be referred 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 .