For more than a decade the trace chemical analysis of snowfall has been used in cloud seeding research and operations to qualitatively, or semi-quantitatively, evaluate the effectiveness of the cloud seeding. With few exceptions the cloud seeding agent in wintertime snowfall enhancement projects in the Sierra Nevada and other mountainous regions of the western U. S. has been some form of silver iodide (AgI) compound. Segmented profiles of the snowpacks in seeding target areas have been analyzed for the presence of silver. Detection of silver at levels above what is considered “background” has been used to determine targeting effectiveness, both temporally and spatially. Other tracers, such as Indium (In) and Cesium (Cs), have been used to differentiate between seeding methods, or between the microphysical processes of scavenging and nucleation.

As part of the U.S. Bureau of Reclamation Weather Damage Modification Program (WDMP) a cooperative research effort in Nevada during the winter of 2003-04 collected snow samples in two basins that were the target of both ground and airborne seeding with silver iodide. Samples were collected at eight different sites on multiple occasions during the winter. The samples were analyzed for Ag, In and Cs using a state-of-the-art Inductively Coupled Plasma Mass Spectrometer (IC-PMS). In addition, samples were analyzed for other elements that are relatively abundant in the earth’s crust to determine if and when samples might have been contaminated by dust from upwind sources.

The results of the trace chemical analyses are presented and compared with results from prior studies that used less sensitive detection methods. Spatial and temporal patterns of the trace elements are related to specific storms and cloud seeding activities. In addition, because a proposed new method of determining snowfall increase by cloud seeding relies on accurate determination of snow density and its relationship to Ag concentration, the techniques for measuring density are reviewed. The relationships between density, Ag concentration, and other parameters (such as sample depth) for the 2003-04 data set are presented.