FIG. 17: Idealized schematic of an Atlantic easterly wave disturbance moving westward from Africa into the central and western Atlantic Ocean near 15N latitude during late summer. In the top diagram (a) only weak surface pressure, warm SSTs and weak subsidence drying conditions exist. A weak tradewind inversion is present. The top of the moisture level is high enough such that the wave's upward vertical motion is able to overcome the subsidence drying and a tropical cyclone is able to form. In the bottom diagram (b) surface pressure and subsidence are stronger than normal, the SSTs are cooler than normal, the height of the moist level is lower than normal and the tradewind inversion is stronger. The wave's upward vertical motion cannot overcome these adverse influences. The westward moving easterly wave disturbance of diagram (b) is not able to transform itself into a tropical cyclone and continues to move to the west as a cloud cluster.