Popular Science: Type of water really matters
The fog is formed by droplets with a size of 5-50 μm. Compared to raindrops, which can reach a diameter of up to 6 mm, this is a very fine water dispersion in the air with a low vertical speed. However, its role in the ecosystem is irreplaceable and at the same time it is an essential part of the hydrological cycle.
Although fog plays an important role and in practice its occurrence directly affects our lives (e.g. in transport), the prediction of fog is still relatively problematic. Therefore, there is still a constant improvement of fog occurrence predictions and also a detailed study of the fog formation conditions.
The last study of the doc. Hůnová team continued previous work, in which they dealt with the influence of various factors on the fog occurrence, namely the role of meteorology, air pollution, the influence of terrain and long-time trends in Central Europe. Now, the researchers focused on the fresh- and ocean- water effect on fog occurrence in the immediate vicinity of the place of its observation, i.e. in the relatively close vicinity of the fog observation station.
For the research, they used long-term observations of the daily occurrence of fog from the period 1981–2017. This was measured at 56 professional meteorological stations spread throughout Romania. The individual stations were located in different environments and geographical areas with different distances from water bodies. Another data source was information about water areas derived from GIS (Geographic Information Systems), as well as two topographic indices, namely TWI (topographic wetness index) and TPI (topographic position index), in the vicinity of these stations. A semiparametric model based on the GAM (generalized additive model) model was used for the resulting analyses and modelling.
Water areas around the station were a significant factor influencing the fog occurrence, which the authors assumed. However, water bodies were considerably minor player when compared to altitude or seasonality. It turned out, that the radius of 9 km around the station appeared to be the most influential for fog formation. This is certainly surprising especially in comparison with the previous work dealing with the influence of the forest, when the highest fog occurrence was conditioned by a significantly smaller area in a radius of 3 km around the observation station. This may indicate, inter alia, that the forest is significantly more efficient at supplying water to the air (evapotranspiration) than the water itself (simply by evaporation), which is also in line with other professional sources.
A fundamental and interesting result was that the occurrence of fog, both in terms of seasonal profile and frequency, depended on the water type - whether it was fresh (watercourses and water bodies) or seawater. Fog occurrence peaks near freshwater areas were in winter, consistent with previous studies. By contrast, for seawater, the curve of fog occurrence changes was more balanced, with the increase in the spring with a maximum in May with a gradual decrease until mid-November, when there was a minimum lasting until January. The influence is related mainly to the air circulation and the influence of the temperature contrast between land and sea.
The incorporation of both simple topographic indices, which indicate the ability of the terrain to collect water above and below ground, and thus indicate wet spots in the landscape, has significantly improved the original model considering only visible water in the landscape.
The new knowledge gained in this research dealing with the influence of sea- or fresh-water on fog occurrence can be further used for a more accurate prediction of the fog occurrence. Especially in areas where there are no fog observation stations, these results are crucial.
Hůnová, I., Brabec, M., Geletič, J., Malý, M., Dumitrescu, A. (2021): Local fresh- and sea-water effects on fog occurrence. Science of the Total Environment 807, 150799. https://doi.org/10.1016/j.scitotenv.2021.150799