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Seedability in Oklahoma During the conduct of this program I got subjective impressions of natural and artificial seedability of Oklahoma clouds which deserve mention here. First and most important, in the typical summer convective cloud situations there appears to be a deficit of natural nuclei to serve as hydrometeor embryos, both giant condensation nuclei and ice nuclei. By deficit, I mean fewer than optimum for maximizing rain. Flying under the bases of convective clouds several miles in vertical dimensions, I was struck by the rarity of small raindrops. The clouds were big enough and of sufficient duration such that giant condensation nuclei would be expected to give rain. The lack of rain until the clouds showed glaciated tops therefore implied that natural giant nuclei were rare. These are what our hygroscopic seeding introduces artifically, and so evidently Oklahoma can benefit from such seeding. The only case of natural rain I noted from non-freezing clouds was October 1 when the clouds were associated with a diffuse frontal system. . By looking at a physical mechanism one may be able to explain the apparent deficit of giant hygroscopic particles in Oklahoma. Cloud bases are fairly low (less than 2 km above ground), and relative humidity is rather high even at the ground. Thus liquid droplets which form giant condensation nuclei will not get much smaller by evaporation as they descend from cloud base to near the ground. Also the average mixing depth is small, being only 2 km deep if we ignore the isolated clouds. The final factor is that vertical mixing is absent for about 16 hours from about 6 P. M. to 10 A. M. Droplets (or any sort of particles) which are big enough to serve as good raindrop embryos, say 50 p.m diameter particles, fall just fast enough in 16 hours to be removed at the ground. Ones which fall slower and are not removed are too small to be good initiators of raindrops. Thus there is not a daily carryover of such particles; those which are present must.come from more recent sources, such as from dust devils and other wind-created particulates. At Flagstaff, Arizona, MacCr.eady and Takeuchi (1968) noted that drizzle droplets from giant condensation nuclei appeared in the summer con- vective clouds about half the time. An hypothesis as to the rarity of such particles in Oklahoma and their commonness in central Arizona is as follows. In Arizona, the mixing depth is twice as great as in Oklahoma, and the relative humidity near the ground is much less. Hygroscopic particles, say put up to heights of 6 km or more by convection in the deserts of California or Arizona, can stay airborne overnight even if they are large. In summary, hygroscopic particles giving rain-embryo droplets in the 50 pm - 100 pm diameter range at cloudbase will fall out overnight in 33