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top or cloud top. Studies in the field and laboratory have indicated that in rugged mountainous terrain one can expect as fast a rise in plume top as in the Pasquill-Gifford neutral plume even though the air is stable (Cermak et. 03.1., 1969). In the unstable upslope mode dispersjon aloft is more rapid, and apparently highest mass concentration also skips aloft. In the upslope mode, particularly with a strong wind and a stable air mass, an individual plume may be stretched out longitudinally with its breadth narrowed so that the plumes from different generators may not overlap. How- ever, over a period of time their various horizontal meanderings should result in a rather uniform time averaged concentration of nucleant over the target area. Unfortunately, the time averaged seeding effect of an instantaneous meandering plume containing an overseeding concentration of nucleant may be the opposite of the calculated effect from the time averaged value of the same plume. The overall impression given by Figures 6 and 8 is that of overseed- ing concentrated in the Wolf Creek Pass area. A funneling of nuclei plumes could occur over the Wolf Creek Pass region in which plumes from different sources tend to converge, thus, enhancing the tendency for overseeding there. The sharp angle in the crest orientation at Wolf Creek Pass favors funneling, particularly of stable air. Anemometer records near Wolf Creek Pass are rather incomplete, presumably due to icing, so support for this idea cannot be found in them. In the extreme case of a stretched plume the plume top may not reach cloud top, thus having the opposite effect of limiting nucleating power to a marked degree. 4. SYNOPTIC CLIMATOLOGY The determination of the cloud top from a sounding can be made as follows. On proceeding upward along the sounding plot on a thermodynamic diagram to the level where the temperature and dew point curve, which had been together start to spread, one reaches a level which can be reasonably termed the top of the water cloud. At a still higher level the spreading curves may be just at ice saturation, and this level can be called the top of a mixing ice-water cloud. In some cases, the two curves hover near ice saturation on further ascen t, then begin to spread abruptly at some still higher level. This level can be called the top of the ice cloud. In still other cases, there is a rapid spreading of the curves aloft so there is only a water cloud and a thin mixed cloud present. In the evaluation a system closely fitting the mixed top has been used to represent effective cloud top from the nuclei supply viewpoint. In the case of multiple layers, the question of ice falling from the upper deck and seeding the lower deck arises. It has been assumed that two layers are microphysically connected in this sense if the distance between them is under 50 mb. With a larger separation complete evaporation of the falling ice particles is assumed to occur. A correction is applied to the cloud top to allow for its being lifted over the barrier. All preceding references to cloud top include this correct~on. ! / Figure 10 shows for unseeded cases thElaverage water top, mixed top, and ice top arranged by hours before and after/a system passage. System passage is here defined as a definite peak in clotid depth that is associated with a synoptic scale front or trough passage., For comparison purposes, the average height of the -27 C level is shown. Quite clearly at system passage there is a period of several hours during which the mixed top and ice top height exceed the height of the critical temperature (here assumed to be -27 C). There is 17