Weather Conditions that Affect Air Quality

The following are some of the weather variables that air quality forecasters monitor to predict the AQHI:

The greatest potential for high risk Air Quality Health Index (AQHI) days occurs when several weather conditions come together resulting in a deterioration of air quality.

  • Ottawa under smog conditionsWind speed plays a role in diluting pollutants.  Generally, strong winds disperse pollutants, whereas light winds generally 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 above cooler air.  This condition is known as ‘unstable.’  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 allow the trapping of 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 (south) 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 or UV radiation to penetrate the Earth’s surface.  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