Air quality and weather

How the weather affects air quality

Several weather conditions can come together resulting in a deterioration of air quality.

Wind can carry pollutants towards us or away from us.  Smoke from forest fires, as well as other less visible pollutants, can be carried over long distances to arrive on our doorstep.  When there is little or no wind, local pollutants build up in the air.  We see this in both summer and winter under temperature inversions that come with light or no wind.

Temperature Inversion. Hot air rises - this is how hot air balloons work.  We normally have warm air at ground level, and cooler air above.  In a temperature inversion, the temperatures are upside down - the cooler air is at ground level, and the warmer air higher up.  The cooler air cannot rise, and the warmer air above acts like a lid, trapping pollutants at the ground where we live and breath. 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 carries pollutants within the air hundreds and even thousands of kilometres from the source. 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.  The intense summer sun causes chemical reactions among the pollutants that are already in the air, leading to the formation of ground-level ozone, a major component of smog.

How the weather affects wildfire smoke

Weather is a critical factor that affects whether a fire will start, how it will behave after it starts, and how the smoke will spread.

Dry conditions

  • In a prolonged dry spell, trees, ground litter and other fuel dries out and ignites more readily.
  • A low snow pack in winter, as well as a lack of rainfall in spring and summer, will increase the chances of dryness.
  • High temperatures will increase evaporation, causing further moisture loss.

Heat and Heat Waves

  • Heat waves often form in association with dry conditions, little evaporative cooling with less moisture making it easier to raise air temperatures.
  • Lightning strikes, a primary wildfire source, are more likely with warmer weather and heat waves.
  • Climate change is causing Canada's summer to become drier and hotter, the increased temperatures are not being balanced with moisture availability therefore wildfires are more common.


  • Strong winds feed the fire with oxygen, and cause it to grow and spread.
  • The wind influences the direction in which the fire travels.
  • Strong wind can disperse the smoke. Light wind can keep the smoke plume intact and the smoke will rise to considerable heights, where it can be carried hundreds and even thousands of kilometres from the fire zone.
  • Light winds can lead to temperature inversions resulting in smoke being trapped near the surface. This can lead to severe air pollution within the local area.

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