Much of Switzerland awoke last Sunday beneath a blanket of smog. The skies have since cleared somewhat, but the air remains far from pristine.
The sun’s dim appearance, filtered through a whitish haze, was not initially due to Saharan dust, as many might have assumed. Instead, the culprit was smoke from wildfires that have plagued Canada for several weeks. Fires in the provinces of Saskatchewan, Manitoba and Ontario have prompted evacuations and health alerts across parts of Canada and the United States.
When wildfires burn intensely over a wide area, smoke can rise high into the atmosphere. There, the jet stream—a fast-moving ribbon of air at altitude—can carry these particles across the Atlantic to Europe, , according to MeteoSwiss.
The suspended aerosols are significantly larger than air molecules, causing sunlight to scatter evenly across all wavelengths. This phenomenon, known as Mie scattering, gives the sky a milky appearance during the day and ochre tones at sunset.
More recently, dust from the Sahara has reached Switzerland, a migration that is ongoing. Canadian wildfire smoke has now dissipated—but in its place is sand from the Sahara.
Recent measurements show that pollution levels exceeded the daily average limit of 50 micrograms per cubic metre, a threshold that is not breached more than three times a year in Switzerland. The concentration of PM2.5 particles was 10 times the healthy limit set by the World Health Organisation.
More on this:
Swiss Meteo report (in French) – Take a 5 minute French test now
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