Deposition of pollutants by cloud water exceeds deposition by precipitation and dry deposition in high elevation settings.  The large loading of pollutants in such environments is due to a combination of factors such as high frequency of cloud immersion, high wind speeds, orographic enhancement of precipitation, and large leaf area of tree species typical in these environments. Fog impacted coastal ecosystems also experience higher pollutant loadings similar to cloud impacted high elevation sites. Therefore, development of meaningful critical load values and total nitrogen budgets for high elevation and fog impacted sites requires reliable cloud and fog water deposition estimates. However, the cost and labor intensity of cloud water sample collection have made it difficult to conduct long-term studies that would provide the data needed to develop accurate estimates. Current understanding of fog formation, transport, and the role of fog in hydrogeological and biogeochemical cycles is incomplete due, in part, to lack of a concerted interdisciplinary approach to the problem. Historically, these obstacles have limited collection of cloud and fog water samples.

Summary results from a small cloud water monitoring network that operated in the Appalachian range from the mid-nineties through 2011, as well as a qualitative review of other cloud and fog water studies conducted in the United States, Europe, South America/Pacific, and Asia will be presented. Current research findings and collection methods will also be reviewed. Recent scientific efforts by the National Atmospheric Deposition Program’s (NADP) Total Deposition Science Committee and NADP’s Critical Loads of Atmospheric Deposition Science Committee have identified occult deposition as a “need” in developing critical loads for ecosystems that experience significant cloud and fog impaction. To this end, remaining knowledge gaps and suggestions for future research to investigate these gaps will be explored.