Fog gage performance under conditions of fog and wind-driven rain
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Fog and wind-driven rain (WDR) are difficult to measure separately and reported measurements of “fog” often represent a combination of fog and WDR. In this chapter the term “horizontal precipitation” (HP) is used instead. Understanding of “typical” amounts of HP intercepted by different types of cloud forest is hampered by a lack of comparative information on local fog climatology. Usually some kind of “fog gage” is used to characterize fog occurrence and amounts. Collection efficiencies of three passive fog gages, viz. a wire harp, a modified cylindrical gage (Juvik-type), and a tunnel gage (Daube-type), were derived by comparing the volumes of water collected by the respective gages with cloud water fluxes derived from fog liquid water content (LWC) as measured by a cloud particle spectrometer during conditions of fog at a windward cloud forest site in northern Costa Rica. The collection efficiency of the three gages proved linearly related to the horizontal cloud water flux as measured by the gages themselves. Therefore, no additional information on wind speed, droplet size, and fog LWC was needed. During conditions of HP, relative collection efficiencies were derived by comparing the volumes collected by the respective gages. The modified Juvik gage had an efficiency close to 100%, independently of wind speed and direction. The efficiency of the wire harp depended critically on wind speed, whereas the tunnel gage collected additional precipitation at small precipitation angles and low wind speeds. […]
