Fog is formed through a fine distribution of water droplets in the air. It is an everyday meteorological phenomenon - but it is becoming ever rarer. At hundreds of weather stations worldwide experts have been observing a decrease in the occurrence, or the intensity, of fog. "As to the reasons for this, all we could do in the past was speculate," says Prof. Otto Klemm from Münster University. Klemm, a climatologist, and his colleague Prof. Neng-Huei Lin from Taiwan National Central University, have now jointly carried out investigations as to whether climate change - or lower levels of air pollution - might be the cause of the decline. In fact, their conclusion is that both of these could explain the decrease in fog.
For special conditions relating to the physics of fog - i.e. a relative atmospheric humidity of just under 100 percent - the researchers demonstrated that a rise in temperature of 0.1 degrees Celsius can improve visibility by the same amount as a reduction in concentrations of air pollutants by ten percent. Consequently, a decrease in fog can be caused by both climate change and improved emission controls for air pollutants. As far as the latter is concerned, lower levels of harmful emissions lead to a decrease in the concentration of the "pollutant particles" (aerosol particles) in the air on which the water condenses.
"It is important to quantify and understand all changes in the atmosphere. Only then can we learn from what we observe and, possibly, favour the developments required," Otto Klemm emphasizes. Fog provides characteristic conditions in various eco-systems, so that if it occurs to a greater or a lesser extent, this can have serious consequences. For example, fog reduces solar radiation and produces moisture on the surfaces of leaves. There are species of plants - and entire eco-systems - which are dependent on a foggy environment.
Moreover, fog has a direct effect on human life. "Although mist or fog as a consequence of air pollution is no longer a big issue in Germany, it’s quite a different matter from a global perspective," says Klemm. "In São Paulo in Brazil, for example, road and air transport is regularly affected by fog, and repeatedly there are accidents because of it, some of them serious. So for traffic planners it’s important to understand when fog occurs, and in what density."
Measurements taken worldwide document the decrease in fog. In São Paulo, for example, the number of foggy days has halved - from over 144 per year in the early 1950s to just over 70 today. In Los Angeles in the same period, the number has actually dropped from 200 to 30. In many places in China, too, the fog has been decreasing since the 1990s or the early 2000s.
Klemm, O. Lin, N.-H. (2015) What Causes Observed Fog Trends: Air Quality or Climate Change? Aerosol and Air Quality Research, doi: 10.4209/aaqr.2015.05.0353