Atmospheric water, in its liquid form, is deposited on surfaces mainly through wet deposition processes driven by gravity (rain, snow, or hail). However, at some locations, or during specific time periods, the dominant wet deposition process is the deposition of fog droplets.Unlike the relatively simple methodology for quantifying precipitation deposition, the deposition rate of fog droplets depends on the properties of the surface as well as the micrometeorological conditions present, such as the wind speed, fog liquid-water content, and droplet size distribution. Therefore, there is no standard method or simple equipment that is universally applicable for measuring fog deposition rates under all conditions. The methodology for quantifying fog deposition in diverse ecosystems has been developed over many decades. One straightforward method is the direct quantification of the water mass that is captured by plant surfaces under foggy conditions. When rainfall, canopy interception, throughfall, and stemflow are measured simultaneously, the amount of fog water deposited can be quantified by simple algebraic calculations. The downward flux of fog water from the atmosphere to the canopy can be determined via the eddy-covariance method using 3-D sonic anemometers and fog droplet spectrometers. Under some atmospheric conditions, the deposition of fog droplets is predominantly driven by gravitation, so the deposition of fog water can be calculated using droplet size classes and the respective deposition velocities. In this chapter, these methods are reviewed and the operational processes are summarized.Fog droplet diameters range from 1 to 40 μm, with the droplet size distribution varying considerably depending on the type of fog and meteorological conditions present. A typical mean volume diameter would be 10 μm. A droplet of this size has a terminal velocity of ≈ 0.3 cm s−1. This is much smaller than typical windspeeds near the surface, so the resulting fog flux is nearly horizontal and parallel to the surface. For this reason, fog flux measurements and fog water collectors are often made with specially constructed vertical surfaces. These and additional measurement techniques and instrumentation will be discussed in this chapter.

The collection of fog water is a simple and sustainable technology to obtain fresh water for afforestation, gardening, and as a drinking water source for human and animal consumption. In regions where fresh water is sparse and fog frequently occurs, it is feasible to set up a passive mesh system for fog water collection. The mesh is directly exposed to the atmosphere, and the foggy air is pushed through the mesh by the wind. Fog droplets are deposited on the mesh, combine to form larger droplets, and run down passing into a storage tank. Fog water collection rates vary dramatically from site to site but yearly averages from 3 to 10 l m(-2) of mesh per day are typical of operational projects. The scope of this article is to review fog collection projects worldwide, to analyze factors of success, and to evaluate the prospects of this technology.

The microphysical and dynamical characteristics of 156 natural summer cumulus clouds have been documented for three locations in North America: Yellowknife, Northwest Territories; Thunder Bay, Ontario; and Miles City, Montana. The measurements (469 aircraft penetrations) were made in six consecutive years from 1975 to 1980 using state-of-the-art cloud physics instrumentation. All measurements discussed were obtained near −7°C. Yellowknife clouds had low liquid water contents (0.3 g m−3) and high large (>70 μm) particle concentrations (0.9 L−1). Thunder Bay clouds had higher liquid water contents (1 g m−3) and low large particle concentrations (0.04 L−1). Miles City clouds, which were similar in dimensions to those near Yellowknife, had low liquid water contents (0.3 g m−3) and low large particle concentrations (0.1 L−1). Yellowknife and Thunder Bay clouds produced precipitation through the warm and cold rain processes but the observed Miles City clouds did not precipitate naturally. Measurements of cloud top lifetime appear to be useful in explaining the differences between locations. Cloud top lifetime is defined in this paper in terms of the persistence of cloud liquid water at the penetration altitude near −7°C. Lifetime was found to increase with cloud width in each location but did not appear closely related to initial LWC, cloud depth, cloud base temperature, inside-outside cloud temperature difference, environmental humidity, turbulent energy dissipation rate, energy flux, heat flux nor wind shear.

During 1999 a fog collection system was constructed to give water to the Sanctuary Padre Hurtado. It is located in km 275 of the Panamerican Highway in the Canela Commune. This system was financed by the Baehr Family, of British Columbia, Canada. The system has 10 large fog collectors of 400 m2 of Raschel mesh to collect an average of 2.000 liters of fog water a day. This amount of water is used for bathrooms, garden irrigation, domestic uses and there is a remaining volume that will be used in farming activities such as cattle and vegetables in greenhouses. The idea is to disseminate the technology by a training program.

This paper represents a stage within a larger project to estimate acid ion deposition from cloud impacting on high-elevation forests. Acid ion deposition depends principally on three factors: the liquid water content (LWC), the ion concentration(s) in fog or cloud water, and the efficiency of the deposition process, in the present paper, the objective is to estimate LWC on Roundtop Mountain in southern Quebec from routine meteorological measurements at the Sherbrooke weather station.After describing preliminary efforts, the methodology that was found to work best is presented. This scheme was a hybrid of applications of two statistical nonlinear regression schemes. First, the classification and regression trees (CART) algorithm was applied to predict the occurrence or nonoccurrence of fog at Roundtop. The algorithm produced by this application permitted the elimination of a large proportion of the data records for which fog was very unlikely to occur at Roundtop. The remaining data were then processed by a second application of CART to determine the predictors that are important for estimating LWC at Roundtop. Finally, these same remaining data were processed by the neuro-fuzzy inference systems (NFIS) algorithm to derive the final prediction algorithm. This hybrid method (CART-CART-NFIS) achieved a correlation coefficient of 0.810, with accuracies of 0.962 and 0.664 for the no-fog and fog events, respectively. (Corresponding threat scores were 0.916 and 0.530, respectively.) These measures of skill were significantly better than those obtained from initial estimates or from schemes that used CART alone.Although optical cloud detector and LWC data are necessary for derivation of the fog-occurrence and LWC prediction algorithms, in the end those algorithms are applied to only the predictor data. Fog-occurrence and LWC data are not required, except for verification purposes. The algorithms and list of predictors still need to be tested to determine how widely applicable they are.

This study documents the ion concentrations and ion enrichment relative to sea water, in Namib Desert fog water, with the purpose of establishing its suitability for future fogwater collection schemes, while also examining claims that Namib Desert fog water carries exceptionally high concentrations of sulphate, which may be responsible for the formation of gypsum deposits in the desert. The work suggests that Namibian fog water is at least as clean as has been reported from other coastal deserts in South America and Arabia, and provides a source of very clean water for the coastal desert region of south-western Africa. It does not appear that fog is an efficient sulphur source for the formation of the gypsum deposits, unless rare events with high concentrations of marine sulphur compounds occur.

A methodology for obtaining estimates of the spatial distribution of fog water volume collected by a tree canopy in complex terrain is described. The method includes assumptions about the shape and spacing of the trees, their fog water collection efficiency, the fog frequency, and the vertical rate of change of the liquid water content (LWC) within ground-based clouds.The method was applied to a 655-km(2) area surrounding Roundtop Mountain, Quebec, Canada, during a carefully selected sample period from the summer of 1993. Field measurements of fog water volume were used to estimate the cloud-base height and the Tate of change of the LWC with height. Topographic data were used both as a forcing function in the wind flow model and as a means of defining the three-dimensional geometry for deposition calculations. The goal is the development of a simple model that can be used over large geographic areas.Results of the application are presented over various domains ranging from 2 to 164 km(2) in size. Spatial variations in the wind velocity field just above the canopy were found to be related to the main terrain features (summits, ridges, and valleys). The fog water deposition rate was specified as a Linear function both of terrain height above cloud base and of wind speed Near the summit of Roundtop Mountain, variations in terrain height were more pronounced than those of treetop wind speeds. Spatial patterns of fog water deposition, therefore, strongly reflected the pattern of topographic contours, with some modifications being apparent due to spatial variations in wind speed. Calculated deposition values ranged up to 0.69 mm h(-1) and were found to be typical of measured values in the literature.

Results of the seasonal analysis of the Chemistry of High Elevation Fog (CHEF) project data for the years 1985–1991 are presented. The mountain sites are located in southern Quebec, Canada, and range in altitude from 250 to 970 m. Fog, precipitation, and mixed fog and precipitation chemistry data are given for major inorganic ions for all seasons of the year. This paper documents the occurrence of highly acidic fog events; it points out the potential importance of high elevation fog as a major pathway for acidic wet deposition in southern Quebec; and it provides results paralleling those from the American Mountain Cloud Chemistry Project (MCCP), thus supporting the relevance of investigations of high elevation fog to at least latitude 47°N. Data are from 8034 analyses of event-length samples, of which 1140 are mountain fog samples collected as duplicate pairs, making this one of the largest fog/cloud water databases available. The 1985–1991 fog sample mean pH at Roundtop was 3.90 and at Mont Tremblant 3.72. The mean fog pH for all sites and seasons was 3.79. The minimum fog pH was 2.80 and the maximum 7.20. The mean precipitation pH on Roundtop was 4.22 and at Mont Tremblant 4.19. Mixed sample pH values are intermediate between fog and precipitation. There is a small tendency (< 0.1 pH units) for both rain and snow to be more acidic at low elevations, with snow having a higher pH range. The more northerly Mont Tremblant site had higher ion concentrations and higher acidity than did the Roundtop Mountain site. The dominant anion was SO42− followed by NO3−. The dominant cation was NH4+. The NO3−SO42− equivalents ratio showed a strong wintertime peak from year to year with values sometimes exceeding 1.0. Over the 6 yr data set, the average concentrations of the major ions in the mixed and precipitation samples showed small decreases with time in all seasons. In contrast, the average and median wintertime H+, SO42− and NO3− concentrations, in the high elevation fog samples, increased. The results show that the mountain areas of southern Quebec have a potential for total wet deposition that is distinct from low elevation sites in the region.