Aircraft measurements at low-and mid-cloud levels near two isolated and persistent Great Plains thunderstorms concurrently scanned by radar are analyzed to determine the kinematic properties of the near-cloud air flow, the moisture budget, and the dynamical interactions between the cloud and its surroundings. Time variation in velocity divergence, relative vorticity, and moisture flux convergence in the subcloud layer relate well to changes in storm development and translation. Aircraft winds and radar chaff trajectories substantiate the premise in some mo.dels that mature thunderstorms, moving more slowly than ambient winds, divert and distort mid-tropospheric air motion in a manner similar to solid obstacles in relative streaming flow. Observed ingestion of mid-tropospheric air motion tracers demonstrates that, at the same time, internal circulations aloft are not entirely insulated from the environment. A graphical synthesis of three-dimensional air flow within and around a typical Great Plains cumulonimbus is presented, accommodating concepts in earlier models with the resolved circulation features. The involvement of cool, dry middle-level air in the internal circulation and its role in maintaining downdrafts are discussed. I n considering factors influencing storm movement, it is concluded that, in addition to propagative mechanisms, hydrodynamical drag and deflection forces acting a t cloud boundaries may play a significant role in determining a thunderstorm's preferred path with respect to the mean winds. 'The National Center for Atmospheric Research is sponsored by the National Science Foundation. f Project structure, program objectives, and the scope of the data collected are contained in the NSSP-NSSL technical report series, nos. 1, 2, 6. 14, 20, and 23 (available through NOAA, US. Department of Commerce, Washington, D.C.). rawinsondes, and surface recordings, a generalized interpretation of the three-dimensional circulation within and around an isolated Great Plains cumulonimbus is construed. Factors influencing storm 'development, structure, and persistence are discussed, with emphasis on propagative mechanisms and hydrodynamical forces acting at cloud boundaries. 9. CASE I, JUNE 1, 1965 SYNOPTIC SITUATION AND STORM HISTORY A feature recognized as favorable to thunderstorm development is the confluence of surface winds along the boundary between dry continental air over the southwest United States and the northward flow of maritime tropical air from the Gulf of Mexico. Figure 1 shows that such a zone existed in the Texas Panhandle on the afternoon of June 1, 1965. McGuire (1962) notes that these so-called "dry lines" may remain quasi-stationary for days in this region without becoming active; but under certain conditions, they appear to act as mechanisms aiding the formation of large thunderstorms and organized squall 171 172 MONTHLY WEATHER REVIEW Vol. 99, No. 3 (1964), who labeled them "SR storms" (S for severe, for right-moving). March 1971 James C. Fankhauser