The formation of cyclones, especially mid-latitude 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 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; see Molinari et al. (forthcoming) J. Atmos. Sci. Surface depressions develop below the downstream, or eastern, limbs of Rossby waves, where the airflow is divergent. The routes of mid-latitude cyclones (depression tracks) closely parallel the movements of the upper-air jet stream.