Standard fog collector and dual FM-120 comparisons

For the first time in the northern hemisphere, extended measurements of fog droplet sizes and numbers were measured both upstream from and on the leeward side of a standard fog collector (SFC) using FM-120 optical spectrometers. These measurements, coupled with the water recorded by the standard fog collectors and accompanying meteorological data, have yielded efficiency measurements for field-based fog collectors for the first time in the northern hemisphere and only for the third time ever that we are aware of. Of note, two of these efficiencies are the ‘collector efficiency’ and the ‘mesh efficiency’. The collector efficiency is the ratio between the water draining from the collector and the upstream liquid water content (LWC) flux. This value describes the efficiency of the fog collector in collecting fog LWC. Using multiple-regression analysis, the collector efficiency has a significantly positive relationship with the mean volume diameter (MVD) ( p = 0.0019), is negatively related with upstream LWC ( p = 0.0012) and positively related with leeward LWC given all other variables held constant. The adjusted R-square of this regression model is 0.431. If we account for the interaction between upstream LWC and leeward LWC, which is statistically significant ( p = 0.0219) given all other variables held constant, the adjusted R-square increases to 0.538. This interaction indicates that the positive relationship between collector efficiency and leeward LWC is significantly positive especially when the upstream LWC is low, but the positive relationship becomes weaker when the upstream LWC is high. The mesh efficiency describes the efficiency of the mesh in removing the fog LWC and it was calculated to have an average value of 35 % ± 19 %. It is equal to the difference between the LWC arriving at the surface of the SFC and what is measured behind the mesh all divided by the LWC arriving at the surface of the mesh. Like the collector efficiency, the mesh efficiency has a significantly positive relationship with the MVD ( p = 0.0016), is positively related with upstream LWC ( p = 0.0024) and is negatively related with leeward LWC ( p = 0.0009) given all other variables held constant. In addition, the mesh efficiency is significantly related to the wind direction ( p = 0.0003) in that it increases as the wind direction becomes more aligned with the SFC direction. This model has an adjusted R-square of 0.780. The interaction between the upwind and leeward LWC was not statistically significant for the mesh efficiency ( p = 0.6521). Measurements from both FM-120 units generally indicate a clear reduction in the droplet numbers, MVD and LWC from the unperturbed ‘free-stream” fog on the upwind side of the SFC to the fog on the leeward side of the SFC, as would be expected. While the numbers of ∼5 μm droplets are reduced by the SFC to in some cases 50 % of their counts in the open-air fog, the volume of fog water reduced by the SFCs experiences its greatest reduction by the loss of relatively fewer droplets but at larger diameters that peak at around 25 μm. The droplets observed at this semi-coastal, low-elevation California location tend to have higher average diameters than those fog droplets observed at higher elevations in other studies done in Chile. Furthermore, the droplet numbers tend to be much lower at these sites compared to the higher-elevation sites at which studies in Chile took place, often close to 100 droplets per centimeter as compared to 300–500 droplets per centimeter in the Chilean studies. Similarly, despite the larger droplet sizes in the current study, the LWC in the coastal fogs observed in this study is typically at least a factor of 2 lower than that associated with the orographic fog observed in the Chilean studies. • Upwind (free-stream) and leeward fog droplet spectra measurements from standard fog collectors. • Measurements of multiple efficiencies of field-based fog collectors. • Impact of environmental variables on fog collector efficiencies assessed. • First fog droplet spectra as part of fog collection deployment in Northern Hemisphere. • Comparison of results with those of previous studies in Chile.