The nature of the Baiu frontal zone (a stationary precipitation zone appearing in June ∼July around the Japan Islands) is examined in comparison with polar frontal zone in the northern hemisphere for 19∼26 June 1975.
The polar frontal zone defined as the maximum zone of time variation of potential temperature and vorticity and the maximum zone of potential temperature gradient is located in 50∼60°N. Such characteristics of the polar frontal zone are not found for the Baiu frontal zone located along ∼30°N in 110∼150°E. The Baiu frontal zone is characterized by a narrow steady precipitation zone, strong gradient of equivalent potential temperature, thick moist neutral layer, steady generation of convective instability. It is concluded that the Baiu frontal zone is not a polar frontal zone but a significant subtropical front in the East Asia.
It is infered that the southwestward extension of the Pacific anticyclone into the region of very moist tropical maritime air-mass is the primal condition for the formation of the Baiu front, because the frontogenesis in equivalent potential temperature field, generation of convective instability and the strong moisture convergence in the Baiu frontal zone are owing to the SW wind in the tropical maritime airmass in the western periphery zone of the westward protruding Pacific anticyclone.