A weak high pressure ridge developing over southern Ontario during Sunday afternoon is causing partly clearing skies over southern Ontario, while a low pressure trough moving eastward over eastern Ontario is causing cloudiness over eastern and northern regions. West to southwesterly winds are providing good ventilation and dispersion conditions. The cloudiness and winds are the main cause of air quality indices in the good category during the afternoon, and some indices are expected to increase to the moderate category in southern Ontario due to ozone generation in the late August afternoon sunshine.
During Monday, the weak high pressure ridge is forecast to move eastward over central and eastern Ontario and to cause partly sunny skies over most regions, while a low pressure trough over northwestern Ontario is forecast to cause cloudiness over northern regions and scattered precipitation over northwestern forecast regions. Temperatures are forecast to be warmer than seasonal values for early September with continuing high humidity and southwesterly winds. The partly sunny skies are the main factor expected to cause air quality indices in the good to moderate categories in southern and eastern Ontario. Elsewhere, indices are forecast to be generally in the good category.
During Tuesday, the northern trough is forecast to move eastward over Quebec with a cold front extending southward over southern Ontario. Cloudy skies and scattered precipitation are expected ahead of the front over southern, central and eastern regions with continuing warm and humid conditions and southwesterly winds, veering to westerly following passage of the front. North of the front, over northern regions, partly sunny skies are forecast with westerly winds. Good dispersion conditions provided by the winds, and mostly cloudy skies over the warmest regions ahead of the cold front, are the main factors expected to result in air quality indices mostly in the good category across the province, except for indices in the moderate category in southwestern regions that become sunny by afternoon, due to ozone.
In summary, air quality indices are forecast to be in the good to moderate categories in southwestern regions during Sunday afternoon, in southern and eastern regions during Monday, in southwestern regions during Tuesday, and to remain generally in the good category elsewhere.
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.