Weather Conditions that Affect Air Quality

The following are some of the weather variables that air quality forecasters monitor to predict the Air Quality Health Index (AQHI):

The greatest potential for high risk AQHI days occurs when several weather conditions come together resulting in a deterioration of air quality.

City blanketed in smog
Photo © istock, 2013.

Wind speed plays a role in diluting pollutants.  Generally, strong winds disperse pollutants, whereas light winds result in stagnant conditions allowing pollutants to build up over an area.

Inversion or 'stagnant' conditions are commonly associated with major air pollution episodes.  Under normal conditions, the air near the surface is warmer.  The warmer air rises and mixes with the cooler air above.  This is known as an ‘unstable’ condition.  Inversions can develop when a warmer, less dense air mass moves over a cooler, denser air mass creating a temperature inversion where the air is now cooler closer to the surface. Pollutants are unable to mix vertically and will stay pooled near the ground due to these ‘stable’ conditions.  Inversions can persist for hours or days

Topography can create conditions that trap pollutants.  At night, cold air tends to drain downhill, settling into low-lying basins and valleys.  Unable to rise, the cool air settles and accumulates in these valleys, trapping air pollutants.

Long-range transport or transboundary transport of air pollution is a significant problem in Canada. Winds coming from the United States and industrialized areas of Ontario and Quebec can result in higher levels of air pollutants in neighbouring Canadian cities. 

Clear, cloudless skies allow more sunlight and UV to reach the Earth’s surface.  This higher intensity of sunlight allows for more photochemical reactions to occur producing high levels of ground-level ozone, which is one of the pollutants measured in the AQHI. 

For more information:

Air Quality and Weather Videos: a series of three short animations that explain how the weather can affect air quality

Weather and Air Quality poster

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