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Behind the fog as thick as pea soup and even further…

...we often find our country. This is because in the “Czech basin surrounded by mountains”, fog is a relatively frequent phenomenon, in part because of the geomorphology. We talk about fog when a cloud of suspension of water droplets reduces the visibility to less than one kilometer in at least one direction. If the visibility is from 1 to 2 km, it is called mist and from 2 to 5 kilometers, haze. The most intense fog in terms of visibility and density is found in the mountains, where they also have a significant impact on the water balance, so we can talk about a major influence on the mountain ecosystem. In the lowlands, fog does not occur as often. In urban agglomerations, however, the occurrence of condensation nuclei formed by aerosol particles with radii from 10-8 to 10-5 support its occurrence. At the same time, the high levels of pollutants present a great danger. Individual places in Czechia differ not only in intensity, range and type, but also in the fog frequency, from only tens of days to almost 300 days a year. Several studies from different parts of Europe show a decrease in fog frequency due to either climate change or an improvement of the ambient air quality. And how does the long-term trend look in our territory? A group of scientists under the leadership of Iva Hůnová from the Institute for Environmental Studies is dealing with the question.

Scientists analyzed the presence and condition of fog over a period of 27 years (1989-2015) in three different locations. The Churáňov mountain site, representing a relatively very clean environment, the Košetice site, representing a rural area and the Prague 4 – Libuš site, representing the urban background of the capital city of Prague. Using a logistic regression GAM model (generalized additive model), they have investigated the association between fog occurrence, air pollution and meteorology. The explanatory variables were relative humidity, SO2 and NOx concentration, air temperature and seasonality. 

Polluted fog poses a problem. Compared with rainfall, it has a greater impact on the deposition of pollutants, which causes deforestation.
Author: Kateřina Fraindová.

Fog frequency was highest at the mountain site (99 to 188 days a year); at rural and urban stations the number of days with fog was similar (around 20-60 days). The year-to-year variability in fog was significant at all sites surveyed. In recent years, the probability of fog occurrence has increased at all monitored stations, with the highest increase at the Churáňov mountain station.

Over the monitored period, the average annual temperature rose by about 1 ° C at all stations. Because the study period was after the period of heavy pollution due to the enormous burning of fossil fuels in the 1970s and 1980s, there was a significant decrease in SO2 concentrations and a less significant decrease in NOx, the fog condensation nuclei precursors. The highest NOx concentrations were recorded at the urban site, mainly because of the road transport, while at the Churáňov station (where the NOx concentration is generally the lowest), there was a slight increase of 2.2 μg.m-3 over 27 years.

Although there is an obvious decrease of fog occurrence across Europe in recent years, the results of the current study in Czechia show an increase in the probability of fog occurrence since the mid-2000s, especially in mountain areas, the cause of this increase being unknown at present. However, the logistic regression GAM model reveals the most important factors influencing the occurrence of fog. Although the air temperature and seasonality did not have such a significant impact, the relative humidity and SO2 and NOx concentrations were significant. Higher levels of air pollutants, which are potential condensation nuclei, have a major influence on the occurrence of fog. Due to long-distance transport, they can also cause harmful effects far from the source of pollution.

Fog can appear suddenly in a few minutes. Rokytka, Šumava 13.11.2018, 13:50 CET (left), 14:00 CET (right).
Author: Kateřina Fraindová.

Hůnová, I., Brabec, M., Malý, M., Valeriánová, A. (2018): Revisiting fog as an important constituent of the atmosphere. Science of the Total Environment. 636, s. 1490–1499.


Published: Mar 25, 2019 06:40 AM

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