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OCTOBER 1988 HOLROYD, McPARTLAND AND SUPER 1141 " From each track the time of the seeding run pass through the parcel of air, which was later examined, was determined. An ice particle plume spreading rate was calculated for each subsequent pass by dividing the width of the plume by the elapsed time since the seeding pass. Spreading rates determined from the first encounters with the plumes tended to be small. The aircraft may not have penetrated the cores and widest parts of the early plumes and sometimes missed the plume entirely. An 0.1 mm threshold was used for ac- cepting an ice particle, so that many young particles tended to be rejected, possibly producing narrower ice particle plume width estimates. The upwind position of the cloud edge varied with each experiment, some- times delaying the time the IN could be activated to initiate ice particles in the fresh liquid water and dis- persed by cloud turbulence. Over the mesa, the wind shear and the acceleration of the winds sometimes en- hanced the spreading rates (Super and Boe 1988). On the downwind side of the mesa crest, subsidence some- times truncated the horizontal extent of the plume at the sampling level, yielding shorter widths than would be calculated without such truncation. In order to assemble a consistent set of ice spreading rates, data from early passes and from later truncated passes had to be excluded. The data were therefore restricted to a range from about 12 km upwind of the Snow Lab (site B in Fig. 1) to about 7 km downwind (or less if subsidence truncation obviously occurred closer). This set, measured nearly over the crest of the mesa, is averaged and listed in Table 4 under the "Crest" columns. The column of ice plume spreading rates is also from this "Crest" set. These data were then compared with other variables. The number of passes with ice particles included in this set was usually less than the number of AgI plume intercepts listed in Table 4. The range of acceptable times, with respect to the seeding time, is listed in Table 4. When averaged over the 28 passes (from 9 different experiments) allowed in the restricted dataset, the average spreading rate was 2.3, with a standard deviation of 1.8 m S-I over a range of 0.1 to 6.2 m S-I. These spreading rates can also be summarized as within a factor of 3 of about 2 m s -I . The average rate is slightly faster than the 1.7 m S-1 of Karacostas (1981) but may have been stretched by the shear and accelerated flow over the crest of the mesa. The measurements for AgI plumes, however, were more involved and more susceptible to error. Spreading rates were calculated for all appropriate pairs of recip- rocal passes. In clear air the actual spreading rates were so small that measurement problems associated with the use of an acoustical ice nucleus counter produced numerous negative widths. In cloudy air, apparent scavenging of the IN by a factor of over 7 (see below; see also Super and Boe 1988) usually made the AgI plume edges appear to be interior to the ice particle plume edges. The resulting AgI plume spreading rates were always less than, and on average, were half of, the ice particle plume spreading rates. This is about the same ratio as that between the ice particle plume spreading rates of Karacostas (1981) and the AgI plume spreading rates of Hill (1980), which suggests that Hill's measurements probably suffered from the same prob- lems as ours, although they were obviously carried out in different clouds. The details of these inferior mea- surements, therefore, are not presented here. Convection, turbulence and wind shear should all be expected to contribute to the spreading rate of the IN and ice particle plumes. No direct measurement of any of these factors was made by the aircraft, but by piecing together temperature and wind measurement data from several observation systems, we could get indicators of the conditions of the atmosphere during the experiments. However, due to the indirect nature of such observations and because of the low statistical significance of the results of the comparisons, which need to be repeated with better measurements, the de- tails are omitted. In the investigations involving the ice particle plume spreading rates, the restricted dataset was used for comparison. The clear air experiments were presumably carried out in flows with much less turbulence than the ex- periments in clouds. It was therefore expected that the AgI plume spreading rates in cloudy air would exceed those in clear air, as they did, by about a factor of three. AgI plume spreading rates determined with an acous- tical counter are of an inferior quality, but they are usually the only rates available for clear air. As shown in Fig. 3, there can be a large temperature variation at any flight level. In some cases, the size of that variation can be an indicator of the potential for convective turbulence. Horizontally adjacent air par- cels of differing potential temperature will experience buoyancy forces leading to vertical accelerations. The standard deviation in temperature data, sampled at a rate of 1 Hz, was calculated over each entire pass and its relationship to the ice particle spreading rate ex- amined. There was a suggestion of the expected cor- relation between the possible indicator of turbulence and the plume spreading rate in cloudy air, but it was not statistically significant. It is likely that some of the observed horizontal temperature variation on a pass resulted from the vertical transport of stably stratified air in response to terrain variations. Turbulence and vertical wind measurements were not available to help separate the cases. The vertical wind profile over the mesa was contin- uously monitored at 15 min resolution by an acoustic sounder. It is known that the speeds at that point are usually greater than upwind or downwind and that the directions can also be different. Even though these winds may not be fully representative, the speed shear was calculated for the 350 m layer below the aircraft flight level during the experiments with cloudy atmo- spheres. When the spreading rates were compared to the wind shear, there was no relationship, as indicated