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about 5 NM. The points on Figure 3 with the crosses are copied from Figure 4, where the major SF6 detection points are marked. The comparison of the points on Figure 3 with the release point marked on the map, where distances are about the same scale suggests an apparent change in the drift direction from about 1100 T to 1600 T after crossing the peak. There is a strong correlation between altitude changes and SF6 signal which is evident from the plot. There is little doubt, however, that no SF6 was detected upstream from points 4 and 5 in Figure 4 on the higher run, at a location just before point K. On the following reverse leg there does seem to be some SF6 after point L. The aircraft altitude was reduced by about 500 ft to about 100 or 200 ft above terrain for the detection at points 4 and 5. Since SF6 was not found at one of the higher runs upstream this seems to indicate that the SF6 may tend to remain close to the ground even when there is appreciable convec- tion. Flights were conducted on the following days: 26 March, and 7, 9, 10, 11 and 22 April. The most satisfactory flight was the last one, which is discussed above. Thus the SF6 tracing program has proved to be a viable operating scheme and the initial results have been of interest in that on very convective days with little wind the SF6 was found only within about a mile of the release point. When wind is present the SF6 is found several hundred feet above the ground in convective conditions but in a comparatively narrow band. The SF6 flowed around the 1500 ft hill we investigated but experienced a clear change in direction after passing the hill, possibly due to the wind funneling down a valley upstream. 2. The Convection Over Snow Earlier work has indicated that over the ocean evaporation from the sea will provide water vapor which will drive convec- tion, because it is less dense than air. Over a snow field the same mechanism will apply, since snow also evaporates. Although the convection is slow it will operate in completely overcast conditions and may provide a mechanism whe~eby ground release of seeding material can find its way into clouds. The model based on the entity entrainment concept of plume structure has been reconfigured to describe these conditions, and the calculations indicate that such behavior will occur. Some new restrictions relate to the higher vapor pressure of the water which condenses at cloud base, relative to the surface ice, so that the cloud base must be high enough to be colder than the surface and hence have a lower mixing ratio. 3. Ice Formation Consideration of the continuing turbulent motion in cumulus clouds has led to the idea that as the cloud dilutes from the 6