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I I I I I I I I I I I I I I I I I I I layer and an extension of fallout with subsequent surges in the pre- cipitation. It appears that the buoyancy generated by the active seeding is sUfficient to accelerate the test cloud updrafts beyond the stable layer. Case V had a stable layer near cloud base and only weak vertical cell growth. Seeding response in this case was slow and showed up slightly in the precipitation fallout, but indicated no evidence of dynamical response. Case VI occurred with an intense mid-level stable layer. In this case glaciation appeared to be insufficient in creating enough buoyancy to push the test cells beyond the stable layer. In retrospect, we would have to question the seedability on this day due to the intense and relatively warm stable layer at mid-le11els. 6.3 General Characteristics of Showers In all cases cloud bases were at the 4, 000-6, 000 ft levels and temperatures at +15 Oc to +180C. Clearly, with on.ly 8, 000 to 12, 000 feet of cloud depth (tops 12, 000 to 18, 000 ft), these clouds have a good probability of initiating rain via the warm rain process. This is evident in most of the time-height diagrams which show first echoes tending to initiate at these altitudes. Liquid water contents measured at the middle levels (15, 000 to 20, 000 ft) appear to be similar to those measured under similar conditions elsewhere and are sufficiently large (1-3 g/m3) to give significant buoyancy effects when properly seeded. Mid-level stable layers (16, 000 to 20, 000 ft with temperatures -5 to -100C)appear to be a frequent feature of the atmospheric soundings. These stable layers exert significant influ- ence of the vertical growth of the natural convective cells and to a '. , ~, '.... 117