Development of Tropical Cyclones in Relation to Circulation Patterns at the 200-Millibar Level*

The 200-mb. flow existing above low-level perturbations at the time of development into tropical storm or hurricane intensity was studied. On the basis of observations in a sample of 40 cases, it is concluded that poleward flow aloft, such as is found in the eastern side of troughs in the westerlies and tropical upper-level cold Lows or in the western side of anticyclones, is more favorable for development of low-level perturbations underneath than equatorward flow. It is also shown that flow aloft with anticyclonic vorticity is more favorable than flow with cyclonic vorticity.