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3. 2. 1 Ground Generator Releases -," 'c 3. 2 Diffusion Tests The diffusion characteristics of an aerosol released from the surface in mountainous terrain, such as the Park Range area, . are extremely compli- cated and variable. The terrain influence on the related wind and temper- ature structures complicates the diffusive characteristics over those of relatively flat terrain. A pivotal problem in evaluating the results of any ground based cloud nucleation program in mountainous terrain is the sis of where; and in what concentrations, the nucleating material was dis- tributed. The purpose of this series of tests was to describe what happens to ground released artificial nuclei in the Park Ralfge area. \ Fifty-two aircraft-tracked ground r~leases were made in the series. The diffusive behavior is sufficiently complicated that a relatively small does not lend itself to statistical treatment and the data can best be on a case study basis. 'The results of several cases will be presented to illustrate different categories of behavior typical of releases in the area. 3.2.1.1 Topography The major topographic feature in the experimental area (Figure 32) is the north-south oriented Park Range, forming a nearly unbroken barrier (10, oo~ ft. MSL) just east of Steamboat Springs. At the base of this barrier the Yampa River Valley at an elevation .of 7, 00 ft. MSL approaches Steamboat Springs from the south and thence turns westward in a broad valley opened through lower terrain to the west. West of Steamboat Springs this valley is joined by the Elk River Valley from the north. The release site for most these tests was 8, 300 ft. Emerald Mountain located two miles southwest of Steamboat Springs. 3. 2. 1. 2 Equipment U. S. Radium Corp. No. 2267 zinc-cadmium sulfide tracer material of 2 micron mean diameter and of approximately 2 x 1010 particles per gram ~ was aerosolized using a dry system. The real time sampling unit used on / the aircraft consisted of a vacuum source drawing particles and air into a chamber where they were irradiated by an ultraviolet lamp. The phosphor- escing tracer then passes through a light trap and past a sensitive photo- multiplier tube. The anode current of the phototube, which is proportional, t6 tracer concentration, is monitored on a recorder. This unit was describ by Nickola. et al (1967). Silver iodide relea'ses for tracer studies were rna with a modified Skyfire High-Output ground generator. An NCAR-Bollay iC,e nucleus counter was used in the aircraft; this equipment has been described by Langer, et a1 (1967). 60