Good to moderate air quality expected

Weather Forecast: Issued on Friday, February 27, 2015

The main weather factors affecting air quality during the next several days include:

Friday:
-a strong high pressure ridge building over the Great Lakes region is causing sunny or mostly skies over all of the forecast regions with light west to southwesterly winds and temperatures significantly colder than late-February values
-Friday night, calm winds and clear skies are expected to allow strong nocturnal inversions to form
-concentrations of fine particles are expected to gradually increase and to cause air quality indices to increase to the good to moderate categories in southwestern regions during Friday, while indices are expected to remain mostly in the good category elsewhere

Saturday:
-the high is forecast to move eastward over southern Ontario and the eastern US, causing sunny or mostly sunny skies, light southwesterly winds over most regions, and temperatures continuing below seasonal values but moderating under the strengthening late-February sunshine
-nocturnal inversions that develop Friday night may cause stagnant conditions at ground level until late Saturday morning that will contribute to increasing concentrations of ground level pollutants
-the sunshine will help snow to sublimate and increase relative humidity of the air, which may contribute to formation of fine particles
-Saturday night, calm winds and clear skies are expected to allow strong nocturnal inversions to strengthen
-concentrations of air pollutants are expected to gradually increase under the light southwesterly winds and cause air quality indices to be in the good to moderate categories in southern Ontario, while indices are expected to remain generally in the good category elsewhere

Sunday:
-the high is forecast to be over the eastern US and an upper level weather disturbance is forecast to move over the eastern US and to spread cloudiness with scattered precipitation northeastward across southern Ontario and all of the forecast regions
-winds are forecast to be southwesterly over northern regions and light and southerly over most regions of southern, central and eastern Ontario
-nocturnal inversions that formed or strengthened during Saturday night, along with light winds and increasing relative humidity of the atmosphere near ground level, may combine to result in increasing concentrations of fine particles and nitrogen dioxide, and air quality indices in the good to moderate categories in southern and eastern Ontario

Air Quality Forecast:

Air quality indices are expected to be in the good to moderate categories in southern regions during Friday and Saturday, and generally in the good category elsewhere. Sunday's air quality indices are forecast to be in the good to moderate categories in southern and eastern Ontario and generally in the good category elsewhere.

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Smog, comprised of Ozone (O3) and/or Fine Particulate Matter (PM2.5), is primarily a summer phenomenon, occurring mostly in southern Ontario; however, PM2.5 can elevate smog levels during all months of the year. For this reason, the ministry reports the AQI hourly, seven days a week; and continues to provide air quality forecasts daily.