cyclogenesis

(meteorology) Any development or strengthening of cyclonic circulation in the atmosphere.

The formation of cyclones, especially mid-latitude depressions (also known as frontal wave depressions). Cyclogenesis occurs in specific areas, such as the western North Atlantic, western North Pacific, and the Mediterranean Sea, and is favoured where thermal contrasts between air masses are greatest.

Cyclogenesis is primarily the result of convergence of air masses. But cyclones are areas of low pressure. How does this fit with the occurrence of air masses piling up together? Quite simply, because cyclogenesis occurs when divergence in the upper troposphere removes air more quickly than it can be replaced by convergence at ground level. The net result is low pressure.

The significance of upper-air movements in cyclogenesis is also indicated by the link with Rossby waves. Surface depressions develop below the downstream, or eastern, limbs of Rossby waves, where the airflow is divergent. Furthermore, the routes of mid-latitude cyclones, known as depression tracks, closely parallel the movements of the upper-air jet stream.