| OCR Text |
Show An example of the detailed use of this technique as a special support tool in the analysis of seeding effect can be seen in a series of crystal photographs taken from a replica series collected during operations on 29 January 1969 (Figures 82a-m). Evidence of both scavenging and Agl nucleation are evident in the photos during the calculated seeding effect period (see Monthly Progress Report No. 43 for a detailed analysis of this case). When the entire replica series was analyzed, of the ice crystal replicas which grew microscopically visible Agl particles at their centers, 15% were large dendrites (as in Figures 82e and 82f), 35% were large hexagonal plates (Figures 82g, h and i) and 50% were small hexagonal plates (Figures 82j, k, and 1). Predominant crystal type changed from large dendrites prior to the arrival of seeded crystals to mostly small hexagonal plates during the main seeded period, with increasing numbers of dendrites again after the main seeding effect had ended. In . summary, this simple, inexpensive analysis technique for the identification of Agl particles as ice nuclei has proven to be feasible for use in connection with large cloud seeding field projects such as the Park Range Project and it gave reasonable timing agreement with calculated seeding effect perio 4. 4 Silver Content of Snow Water The atomic absorption spectrophotometer method was used to analyze collect snow samples for silver during the 1967-68 and 1968-69 seasons. During the second season the change in operational routine allowed sufficient time between seeding periods to obtain a sizeable true unseeded set of snow samples, as compared to the previous year when 1 cph to 1 / 4 cph pulsed seeding very likely contaminated almost the entire precipitation analysis period in most cases. Employing the computer model orographic vertical velocity field, crystal terminal velocities, and vertical profiles of'temperature, humidity, and win! from Mt. Harris, estimates of arrival time of seeded precipitation at the snow sampling site were made for each of 44 six-hour analysis periods for the 1968-69 season. Snow samples collected during the three hours followinj earliest arrival time of seeded precipitation were considered as seeded sam ples, while those outside this three hour were designated unseeded. This yielded 14 6 uns e eded a nd 1 75 seeded samples for the season. The meal valu e of silver co n centration for the seeded sample was 0. 844 parts per billion compared to 0. 354 for the unseeded sample. This difference of sam· ple means is statistically significant at the 0. 1 % level. Thus, seeding mate' was deposited in precipitation over the target area in accordance with the cal culated period of seeding effect during many of the analysi s periods in 1968· 69. 134 |
