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# -! ". .,.. ABSTRACT II ,--- - II The procedures and results of a five-year program of cloud seeding to in- crease orographic precipitation in the Park Range of Colorado are presented. Winter precipitation systems were seeded with silver iodide from ground generators and aircraft. Measurements in the seeding target area included: total snowfall, snowfall rate, freezing nuclei counts, type of snow crystals, and concentration of silver in snow, as well as radar and rawinsonde obser- vations. As part of the investigation, diffusion tests were conducted to study the transport and dispersion of seeding agent, and a numerical model was developed to predict location and rate of snowfall as functions of observed wind, temperature, and stability. , , ._ I . ~+ a- _..____._-- /-;.+ "3 ~ve, o~- "5eel\~i - tvo -~w""/tL'~7' yJ.... During the ~!years\,of the program, the experimental design called for pulsed seeding at 1 cph frequency. Power spectra of snowfall rate were computed in an effort to show significant differences in the time histories of snowfall rate for pulse-seeded and natural snow days. Though seeding effects were evident in some observations from individual cases, the approach was not successful, largely due to the natural variability of snowfall rate and of seeding agent dispersion in the atmosphere. ~ ''''.....+ <.:l-\- f.:, "S1c"Q.,:;,o,.-s ov-lj For the latt~!'_Eor.'!j.on ofihl:~u?:}Cp~Ei~entY" experimental days were divIded into randomly selected three-hour seeded and unseeded blocks. This design produced statistically significant conclusions on the effect of seeding. It is concluded that seeding produced snowfall rate increases of 100% or more when cloud top temperature was warmer than -20oC, and snowfall rate de- creases for cloud top temperature colder than -240C. Supporting measure- ments show conclusively that: silver concentration in snow was higher for seeded than for unseeded periods, hexagonal plate snow crystals were much more numerous in seeded snow, AgI particles acted as nuclei for snow cry- stals, and seeding did frequently raise the surface air nuclei count in the target area. . On the basis of climatological analysis and the results of this program it is concluded that a combination of ground and airborne seeding, utilized when- ever forecast cloud temperatures are appropriate, could, on an operational basis, produce cumulative seasonal snowfall increases of approximately 25% in the Park Range. . ~,.__._.__.,... .. !l i ..oIIIiIlI"'; ......r--