A brief review of the classical theory for convective instability is presented, with primary emphasis on the more recent theoretical and experimental findings that pertain to mesoscaie cellular convection in the atmosphere. Important physical and geometrical features of three-dimensional patterns of free convection commonly observed over the oceans are discussed, especially those features that point to distinct differences when compared to laboratory convection. Some suggestions for future study are offered with particular reference to the AMTEX program being planned by the Japanese Committee for GARP.

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Long-range 500 mb and 300 mb air trajectories were computed in an attempt to interpret the daily variation of the number of fallout particles in the ground-level air observed at Fayetteville (3eoN, 94"W), Arkansas, after a nuclear weapon was tested at Lop Nor (40"N, 9O"E), China, December 24, 1967. The dates of occurrence of peak concentrations of fallout particles generally coincided with (a) the arrival times of air masses at 500 mb and/or 300 mb after completing a cycle around the world, and (a) the passage of 500 mb troughs at Fayetteville. A significant deviation from the above rule occurred on January 20, 1968, when the highest concentration of fallout particles in air was observed a t Fayetteville. According to the computed air trajectories, such a maximum should have occurred 3 days earlier on January 17, 1968. The discrepancy was explained as due to local meteorological conditions: the occurrence of positive vorticity transport at 500 mb followed by the formation of slow-moving cutoff low during January 17 through January 19, 1968.

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