On the theory of cyclogenesis in middle latitudes

The author gives a short introduction into theories of cyclogenesis which are based on instability. The polarfront and baroclinic instability theories are discussed along with the difference between linear and non-linear instability. The author stresses that one may consider cyclogenesis from the point of view of the production of cyclonic vorticity. The terms responsible for the production of vorticity become increasingly important as soon as the horizontal and vertical wind shear as expressed by the Rossby and Richardson numbers reach a critical combination. This critical combination can be reached more easily if heat of condensation is set free than with purely dry-adiabatic processes. Therefore, cyclogenesis is more easily possible in saturated air than in non-saturated air.