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