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OG:'~<I) and longer duration up-canyon. flows in sUlDlller than in winter. The cir- cumstances most adverse for plume dispersion should Occur in winter when the highest potential for stagnation should exist. Observations taken during dominance of high barometric pressures in- dicate that the subsidence inversion and the ground-level inversion pro- duced by nocturnal radiation are clearly separate. Bases of the subsi- dence inversion are around 2,000 feet above the surface while the ground inversion extends from the surface to about 400 feet. On stagnant winter days, insolation can be expected to eliminate the ground level inversions during the morning hours, but the heating is generally insufficient to remove the upper inversions formed between 2,000 and 3,000 feet by the subsiding high-pressure airmasses. Such days are characterized by weak up-canyon flow during the midday period with the vertical mixing depth restricted to about 2,000 feet. Night and morning flows during such cases of limited mixing provide persistent horizontal ventilation with a pro- nounced low level jet at about 300 to 400 feet above the surface. The constant volume ballon observations taken on up-canyon flow in- dicated chimney effects in the narrow upper part of Huntington Canyon. The balloons were either lifted over the ridge northwest of the plantsite or continued up Huntington Canyon. One case of stagnation was observed. In winter cases of down-canyon flow the trajectories of constant volume balloons tended to turn from southeasterly headings. to the south on nearing the canyon mouth. This suggests that the centerline of the diffusing plume from the generating plant would pass south of the town of Huntington during stable winter episodes. 12