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Robert S. Schemenauer and Pilar Cereceda

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Pilar Cereceda and Robert S. Schemenauer

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The topography in Chile is extremely complex and many types of fog are found; both factors complicate the presentation of the data. Despite this, measurements from standard meteorological stations suggest a latitudinal maximum in fog frequency between 35° and 40°S for coastal stations. This is supported by data from inland stations in Chile and the available observations from Argentina on the Atlantic coast of South America. Along the Chilean coast the average number of days with fog ranges from 3 to 59 per year. The variation in fog frequencies is related to persistent synoptic-scale circulation patterns and to ocean currents.

Specialized fog observations wore made at three remote locations in northern Chile to determine fog frequencies on the coastal mountains. The sites were in a very add region (26°–28°S) near a large-scale fog-water collection project. Fog frequencies as high as 189 days per year with another 84 days of patchy fog were reported at an altitude of 860 m. These are 3–15 times higher than at low-elevation coastal locations at similar latitudes. Clearly, observations from standard meteorological stations are not suitable for estimating higher-elevation fog frequencies.

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Robert S. Schemenauer and Pilar Cereceda

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The collection of fog droplets by vegetation is an important wet deposition process. It can, in fact, dominate the chemical and hydrological input to certain high elevation watersheds. However, measurements of fog deposition are rarely made and, where they do exist, comparisons of deposition rates in different locations have been hampered by the use of innumerable types of collection devices. A simple, inexpensive, 1-m2 fog collector that can produce measurements of the deposition of fog water to a vertical surface is described here. The collector has been used successfully in five countries to investigate the variation of fog deposition in complex terrain and to estimate the deposition to trees and to much larger fog collectors. It is proposed that it be employed widely as a standard to quantify the importance of fog deposition to forested high elevation areas and to measure the potential collection rates in denuded or desert mountain ranges.

The standard fog collector costs about the same as a rain gauge ($100 U.S.) to construct and can be used with a variety of recording devices. It is a flat panel made of a durable polypropylene mesh and mounted with its base 2 m above ground. Fog collection rates are typically 1–10 L m−2 of vertical collecting surface per day but can reach values of 30–40 L m−2 day−1. The presence of drizzle or light rain with the fog, coupled with 10 m s−1 winds, has produced collection rates as high as 300 L m−2 day−1. If a standard fog collector is installed at a site with wind speed measurements and a conventional rain gauge, a reasonable estimate can be made of the proportions of fog and rain being deposited on the vertical mesh panel. This information is fundamental to the understanding of acidic wet deposition at higher elevations and to comprehensive hydrological calculations in watersheds.

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Robert S. Schemenauer and Pilar Cereceda

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One exciting new application of meteorology is the prospect of using high-elevation fogs as an and land's water resource. This has now become reality in northern Chile where a pilot project has used 50 fog collectors to generate an average of 7200 1 of water per day during three drought years. The chemical composition of the fog water is of primary importance and is examined in this paper.

A small, carefully cleaned fog-water collector was used at the site (elevation 780 m) to study the incoming fog (cloud). The ion and trace-element concentrations met Chilean and the World Health Organization's (WHO) drinking-water standards. The pH values, however, were at times extremely low. Samples from 1987 and 1988 were consistent with those from the larger dataset in 1989. The lowest observed pH was 3.46. The acidity was associated with high concentrations (89%) of excess sulfate in the 15 fog-water samples (based on Cl as the seawater tracer element). The NO3 /SO4 equivalents ratio for the fog samples was 0.18, showing the dominance of SO4 in determining the acidity of the fog samples. The relative abundances of ions and trace elements in the dry deposition are very similar to those in the fog water, suggesting that the aerosols originate primarily from evaporated cloud droplets over the ocean. Based on enrichment-factor calculations (with Cl as the indicator element for seawater and A1 for the earth's crust), sea salts were the main source of Na&plus, Mg++, and Cl in the fog water; soil dust was the main source of Fe, Al and Ti; and other sources provided Ca++, K+, NH4 +, Br SO4 NO3 As,Cd,Pb,V,Mn,Ni,Cu,SrSb,and Ba in the fog water.The use of enrichment factors based on the relative abundances in soil extracts suggests that As, V, Cu, and Sr may be available from wetted soil dust.

The output from the large (48 m2) fog collectors was also acceptable, except for several of the 24 trace elements, which exceeded the maximum allowable values in the first flush of water after a dry period of a few days. The pH values were again near 4 and would have to undergo a simple treatment to raise them to a value of 6 or more to meet the drinking-water standard. The output from a 2000-1 fog-water storage tank was completely acceptable and that from a 25 000-1 storage tank completely acceptable, except for a low pH. In contrast, both the water presently being used in a nearby village and local spring water were unacceptable. It is concluded that fog water is an attractive alternative as a water supply even after collection on the large meshes at this site.

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Robert S. Schemenauer, Pilar Cereceda, and Nazareno Carvajal

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Robert S. Schemenauer, Humberto Fuenzalida, and Pilar Cereceda

Many parts of the world are currently suffering water shortages. Few areas, however, have as little precipitation and groundwater available to alleviate the problem as does the northern coast of Chile. The historical background of the attempts to collect water directly from the coastal stratocumulus decks is reviewed in this paper as are the meteorological and geographical considerations important to the collection of the cloud water. Calculations of water availability and cost indicate that this may well be an important source of water for some coastal regions. A combined research and applied project to study the properties of high-elevation fogs and their use as a water supply will be conducted by Chilean and Canadian agencies from late 1987 to the end of 1988.

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