Number of foggy days - tendency to increase or decrease?

Fog – the old friend of winter

Many people know it all too well: While you are stuck in the gray soup in the lowlands, the sun laughs from a steel blue sky at high altitude. Especially during the winter months, the sunshine hours are unfairly distributed. In climatology, various weather phenomena are examined for trends, i.e. whether a phenomenon has occurred more frequently or less frequently in recent decades, whether the phenomenon has been more intense or whether it has occurred in a weakened form. Concerning fog and high fog there are hardly any climatological analyses for Switzerland, here some findings shall be collected.

The challenge of trend analysis of fog

In this blog we speak of a fog day as soon as fog can be observed during at least half a day. As Christof Appenzeller and Simon Scherrer explain in their 2014 study (Scherrer, S.C., and C. Appenzeller, 2014: Fog and low stratus over the Swiss Plateau - a climatological study, International Journal of Climatology, 34, 678-686, https://doi.org/10.1002/joc.3714), a trend analysis for fog days is very challenging because the observational data used for such an analysis are not always consistent – which is an absolute necessity for a trend analysis. For example, the observation data where a person notes whether there was fog on a day depends on the person himself and his subjective assessment. In addition, there has been a change in the time of measurement in the past, which makes it impossible to standardize these data. An alternative possibility for fog observations are satellite data, but these have only been available for a few years. In the mentioned study (C. Appenzeller & S. Scherrer, 2014), therefore, measurements of relative sunshine duration on the Säntis (2500 m a.s.l.) and in Zürich-Fluntern (550 m a.s.l.) were used. If sunshine was recorded all day on Säntis on one day, but only to a limited extent in Zurich Fluntern, this is an indication of a foggy day. The results of this study are explained below.

Are foggy days becoming more frequent or rarer?

The time period examined in the study mentioned above ranges from 1901 to 2012 and includes the days between September and March in each case. The results show that the number of days when there was fog or high fog for at least a few hours varies greatly between individual years as well as between decades (Fig. 1)! The lines of Figure 1 should be read as follows: The green line shows the number of days on which a person observed fog. This line is only for comparison for the results of the study. The blue line shows the number of days on which there was fog during at least half a day. The red line shows the number of days when there was fog all day. The black line shows the number of days when the fog dissipated during the afternoon. One possible reason for this variation is that the formation of fog also depends on the prevailing weather conditions. Thus, it may be that in one winter a high-pressure situation determined the weather particularly often (which strongly favors fog formation), while in other years, for example, there were more westerly weather conditions.

The number of days with fog all day at the station Zurich-Fluntern varies between 4 and 31 days per year (on average 17 days), and the number of days with fog for at least half a day ranges between 10 and 49 days per year (on average 28 days). Between 1984 and 1993 there were the most foggy days, but from 1999 to 2008 there were almost half as many days. What may seem at first as if the number of foggy days is decreasing is deceptive. In fact, it has increased again in recent years.

No change in the long term

On a time scale of a few years or decades, changes in the frequency of foggy days can be seen; the period from 1984 to 2008 discussed above is particularly striking. Looking at the long-term development, i.e. the entire period of investigation from 1901 to 2012, no increase or decrease in the number of foggy days can be determined, since any trends over time are always balanced out again.