Away from this cirrus shield, some low-level spiral cloud lines can also be clearly seen. Most of the heavy rain occurs near the storm centre and along these spiral rainbands. The rainbands rotate in the same sense as the storm circulation and tend to sweep through an area one after another. At a given location, heavy precipitation is usually pulsing at intervals of a few hours. Squalls and gusts increase during the approach and passage of rainbands.
Rain becomes persistent and winds violent as the centre of the storm draws near.
A tropical cyclone is akin to a travelling heat engine. It feeds on an incessant supply of latent heat released from condensation in ascending moist air. The reliance on readily available moisture also explains why tropical cyclones can only survive over the warm oceans and invariably weaken once over land. However, not all cloud clusters that hover over the tropical oceans succeed in developing into tropical cyclones. Otherwise, further development and intensification will be inhibited.