Popular Science: Frequency of fog in the Czech Republic. What is the trend?
Fog is a meteorological phenomenon, which causes reduced visibility to less than 1 km. Fog consists of tiny water droplets or ice crystals suspended in the air. Fog formation occurs when the air temperature drops below the dew-point temperature, often supported by fog condensation nuclei (FCN). The most common FCN are SO42−, NH4+ and NO3−, which are produced by different types of transportation, local heating plants or industry, which are also some of the reasons for the higher fog occurrence in cities in the winter half-year.
Although fog is regarded as salvation and a highly appreciated phenomenon in some areas of the world, it is unwelcome elsewhere. In Central Europe, especially in relation to pollution (and the interaction of pollutants and water), it can have a serious negative impact on human health and nature (e.g. forest dieback). Fog is a meteorological phenomenon, which causes reduced visibility to less than 1 km. Fog consists of tiny water droplets or ice crystals suspended in the air. Fog formation occurs when the air temperature drops below the dew-point temperature, often supported by fog condensation nuclei (FCN). The most common FCN are SO42−, NH4+ and NO3−, which are produced by different types of transportation, local heating plants or industry, which are also some of the reasons for the higher fog occurrence in cities in the winter half-year.
With increasing time series of fog occurrence metering around the world, it is possible to observe long-term and seasonal trends. In most cases, the occurrence of fog has decreased over the last several decades, which is associated with decreasing ambient air pollution, as well as climate change and rising temperatures. The increasing trend in fog frequency is mainly associated with increasing ambient air pollution, particularly in Southeast Asia. So what is happening in the Czech Republic?
Iva Hůnová, together with her team, thoroughly analyzed data from 12 different selected places in the Czech Republic with long-term observations since the 1960s. It was a selection of stations representing different environments (urban, rural, mountain) in different geographical areas and altitudes (253–1,322 m above sea level).
A modern approach was used for the analysis - a semiparametric model based on the generalized additive model (GAM), which allowed for determining the probability of fog occurrence. A foggy day was defined as a day when fog in liquid state reaching more than two metres above the ground was recorded at least once.
As trained meteorologists were replaced by automated instruments to some extent after 2007, this change had to be considered in statistical analyzes as well. At about one-half of the studied sites, it was confirmed that the instrument measurements would capture more foggy days than human observations.
It was possible to observe a long-term decreasing trend in fog occurrence at all monitored stations, while the trend differed during the monitored period. The most ‘foggy’ period occurred in the 1970s and 1980s, which can be linked mainly to the increased level of air pollution. Since then, it has been possible to observe a decline, while a slight increase was detected at some mountain (Lysá hora, Milešovka) and urban (Cheb, Přibyslav) sites at the beginning of the 21st century.
Fog occurrence is significantly affected by the season, which was shown in the second half of the study. The occurrence of fog is related to three basic factors, which are the water vapor pressure, the dew-point temperature and the state of saturation and supersaturation. Another factor influenced their formation is the presence of fog condensation nuclei. Because of these factors, it was not surprising that the highest maxima could be observed especially in autumn and winter. The biggest difference between fog occurrence in summer and winter was observed at the Kuchařovice rural site. By contrast, differences in fog occurrence between the winter and the summer were not as pronounced at in mountain sites where, due to the local conditions, fog occurrence is generally the most frequent. When comparing the individual decades, a decrease in fog occurrence was mostly recorded, while for Lysá hora there was an increase in the probability of fog occurrence in the winter during the last decade (2005–2015).
In addition to the measurement method, the authors also point out the important influence of the length of the analyzed time series. An earlier study spanning 27 years (1989–2015) at three Czech sites representing different environments showed an increase in fog occurrence. However, the current study, which comprised an analysis of a 58-year time series (1961-2018) including periods with significant air pollution (especially the 1970s and 1980s), observed a decrease at all 12 monitored stations.
Fog is often associated with negative effects. However, it is not fog itself that poses the danger, but the polluting particles that often support its occurrence. Fog itself is an important segment of the water cycle and brings a lot of positive effects, such as moisture and a cleaner atmosphere. Until recently, the study of its occurrence and causes did not receive much attention. However, detailed analyzes provide important insights for a closer understanding of ongoing climate and environmental change from different perspectives and in different natural conditions.
Hůnová, I., Brabec, M., Malý, M., Valeriánová, A. (2020): Long-term trends in fog occurrence in the Czech Republic, Central Europe. Science of the Total Environment 711. https://doi.org/10.1016/j.scitotenv.2019.135018