Updated December 14, 2024 at 10:00 p.m.
If you’re under the impression that smog episodes are more numerous during the colder months, you’re right. Explanations.
A combination of factors
When air quality is good, cold air is generally at altitude and warm air is at ground level. However, during a temperature inversion, these air masses exchange places. When an episode occurs on a calm day, weak winds prevent air circulation. As a result, cold air has difficulty rising and forms a sort of lid that prevents pollutants from dispersing into the atmosphere. Remember that since cold air is denser than warm air, the two air masses do not mix.
Temperature inversion is not the only factor contributing to the creation of smog: clear skies, light winds and little precipitation are necessary. Thus, most smog episodes in winter occur when an anticyclone dominates the province and during an absence of clouds.
Good to know: the air quality index (AQI) allows you to know the quantity of atmospheric pollutants present in the air. Three categories are used to assess air quality: “good”, “acceptable” or “poor”. The AQI is calculated and updated every hour, based on measurements of different pollutants carried out at the various stations in the air quality monitoring network, spread across Quebec.
Not to mention, pollution is particularly present during cold days due to energy needs such as heating. Without the mixing of air caused by winds, this pollution remains on the ground and thus contributes to poor air quality.
In the almanac
Certain periods particularly marked Quebecers. During the winter of 2008-2009, the Montreal region experienced 47 days with a smog warning. From January 31 to February 8, 2005, a thermal inversion trapped fine particles at ground level. Some regions of southern Quebec experienced a period of nine consecutive days with an episode of smog. This is the longest streak in the history of the province.
Sources: Environment Canada | Health Montreal
