ATPL Meteorology Practice Test

The primary step in cyclone formation involves the sinking and divergence of air. In the initial stages, for a cyclone to develop, there must be an area of low pressure that allows for rising air. However, before that can happen, sinking air (subsidence) plays a crucial role. As air sinks, it leads to a divergence of airflow at the surface. This divergence contributes to the creation of a vacuum effect where lower pressure develops beneath the descending air column.

This phenomenon allows surrounding air to rush in, contributing to the low-pressure system necessary for cyclone development. Additionally, when conditions are right, this setup enables the formation of cumulonimbus clouds and further intensification of cyclonic activity.

While the other choices contain elements associated with cyclones, they do not represent the primary step in the formation process. High pressure at the bottom and low pressure at the top, for example, describes a condition found during the mature stage of a cyclone rather than its formation. Evaporation of moisture is necessary for providing energy through latent heat as the cyclone matures but is not the initial step. Lastly, low cloud formation is a reaction of the cyclone's development rather than a step in the process itself.