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cooperation with cloud seeding operations conducted by the North Dakota Atmospheric Resource Board. (Detwiler et al, 2005) Daily observations of cloud condensation nuclei (CCN) concentrations active at 0.4% supersaturation were conducted at the surface near mid-day for 3 weeks in August, 2004, at Bowman, ND . Average concentrations of these -3 CCN over ~30 minute-long sampling periods varied day-to-day from ~30 to ~200 cm. Preliminary analysis of weather conditions and 3-day back-trajectories associated with these measurements suggests that air reaching Bowman after recent association (within the prior two days) with precipitating clouds has generally lower CCN concentrations, while air that has not been recently associated with precipitation systems is more likely to have relatively higher CCN concentrations. Air from the west typically had the highest concentrations, while air from the north typically was the cleanest, other factors being equal. These conclusions are based on just 14 days of sampling. Additional sampling was conducted in 2005 to better characterize CCN variability. Instrumentation: The results reported here were obtained using a parallel-plate CCN counter similar in concept to the instrument used in 2004, but capable of more sophisticated computer- controlled operation. The instrument was provided for this project by the Research Applications Laboratory at the National Center for Atmospheric Research (NCAR). The instrument was designed and built at the University of Wyoming. It is similar in principle of operation to the balloon-borne instrument described in Delene and Deshler (2000). The counter automatically samples sequentially through a range of 4 supersaturations by varying the temperature difference between the upper and lower cloud chamber plates. Roughly 1 min. is required to complete a sample at each supersaturation. Unlike the instrument used in 2004, for which the data consisted of digital images of droplets activated in the diffusion chamber, the Wyoming counter utilizes a bulk light-scattering measurement that can be converted to a CCN concentration using an empirical calibration. The counter was installed in the upper floor of the radar operations building at Stanley at the beginning of June. See Figure 1. The samples were drawn in from outside through tubing made of material that did not readily acquire electrostatic charge. Each day around mid-morning new wetted absorbent discs of paper were applied to the inner surfaces of the cloud chamber plates inside the instrument. The instrument then was operated for ~3 hrs. The plates did not require re-wetting over this period of time. Data from the instrument were automatically logged on a small computer as text files. The principal data consisted of a 1 Hz time series of the voltage from the photodetector during each cloud formation cycle. Additional housekeeping data were recorded so instrument performance could be monitored and evaluated. 3