Negative impacts of fog
For many people living in the Swiss lowlands, fog is a considerable nuisance in the winter months, and impacts on their well-being. Fog also has implications for other aspects of daily life. The reduction in visibility is a major issue for traffic both on land and in the air. Many leisure and tourism services benefit if they are in an elevated and therefore less foggy position that has more hours of sunshine than the lowlands. Moreover, fog is closely associated with air-quality issues: In weather situations with fog, air exchange in the lower layers of the atmosphere is severely restricted. This means that pollutants such as fine particulate matter can accumulate and attain concentrations that pose a danger to health.
The development of fog conditions
Low stratus is a typical winter phenomenon that forms in high-pressure weather conditions, particularly when there is a Bise wind. This is when cold air blows into the “bathtub” of the Swiss plateau from the north-east underneath the light, warmer air, where it remains. This creates an inversion situation in which a grey, uniform layer of cloud known as stratus forms, underneath which it is cold and hazy, and above which the air is clear and less cold. Low stratus can also form when ground fog develops during high-pressure conditions after a cold, clear night, and gradually rises.
The top of the fog layer
When a slight pressure gradient from north to south is present in high-pressure conditions, as is typical with a Bise wind, the top of the fog is relatively high (generally above 1,000 meters), because the air builds up at the Alps. If there is no pressure difference, or if there is a gradient from south to north, the height of the fog top sinks to around 600 to 800 metres. The lower the fog top, the greater the likelihood of the fog dissipating during the day.
An inversion is a layer of air in which the temperature increases with altitude rather than decreasing, which it would normally. This occurs in areas of high pressure, in which the air sinks over a large area and warms up in the process. As it warms, the air dries out and becomes clear. Conversely, under an inversion, the air cools and reaches saturation, i.e. it cannot hold any more water vapour. This excess water vapour condenses to form a fog cloud, which can cause drizzle or fine snow crystals. This horizontal inversion layer acts as a barrier preventing the exchange of air on the vertical axis. Underneath this layer, pollutants such as soot and fine particulate matter accumulate. The lower the altitude of the layer, the greater the concentration of the pollutants, as they are accumulating in a smaller volume of air.