OCR Text |
Show in the low level, highly concentrated portion of the silver iodide plume) by the falling larger crystals. No conclusions relative to overseeding can be drawn from the observation of this phenomenon, since vertical diffusion of silver iodide may allow Agl to reach a significant depth of the precipitating cloud in properly dilute concentrations while the highly concentrated low level plume simultaneously overseeds the thin layer near the surface. Three hundred and fifty replica slides taken during operational periods in the 1968-69 season were stratified into seeded and unseeded categories according to calculated arrival times and duration of seeding effect. The frequency of occurrence of slides containing at least one hexagonal plate averaged 62. 2% for the seeded sample and 35. 6% for the unseeded sample. A non-parametric statistical rank test (Mielke, 196 7) indicates the difference in means to be significant at the 0. 1% levels. Thus, it is concluded that silver iodide particles produced significant quantities of hexagonal plate snow crystals during 1968- 69 operational periods. During pulsed seeding operations prior to the 1968-69 season, short period changes in vertical profiles of wind generally precluded reliable stratification of data into seeded and vnseeded categories. However, on a few occasions sufficiently steady wind regimes prevailed to allow detection of a pulsed seeding effect. One such case occurred on 10 February 1967 (see Section 4. 5 for a detailed case study of this event). During that analysis period, more than 50% of the crystals collected on the slides were the small hexagonal plates with inner stellar structures, and there was strong evidence of a positive seeding effect in the pulsed character of precipitation rates in the tar get area. Predominant crystal types observed for all years of collection were dendrites and hexagonal plates (as would be expected) plus a large number of irregulars, many of which were apparently rimed fragments of dendrites. In the 1968-69 season some dendrites were present on 98% of the slides. A total count of identifiable crystals for the 1968-69 collection yielded the percentage contribution to total identifiable crystal population shown in Figure 79. Needles occur only rarely at the elevation of the Park Range and when they do they are accompanied by properly warm cloud base temperature (~ -7°C). While time did not permit sizing of all crystals observed on the replicas, sizing of crystals from over 200 photographs from the 1967-68 season yielded the average maximum dimensions by crystal type listed in Table 13. An ana_lysis of riming indicated light to moderate riming to be present in most cases, with heavy riming occasionally occurring (see Figures 80a, b, c , and ·d for examples of riming intensity). Total absence of riming was rare. Rime droplets ranged in diameter from 12 µ to 100 µ with the pref erred size range from 30 µ to 50 µ. 129 |