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London, Ontario occupies a strategic position in southwestern Ontario's agricultural heartland, situated approximately halfway between Lake Huron to the northwest and Lake Erie to the south. The city lies within the Thames River valley, with the North and South Thames Rivers converging near its downtown core, creating a gentle topographic depression that influences local microclimates. At an elevation of approximately 251 meters above sea level, London sits on relatively flat terrain with subtle rolling hills characteristic of the region's glacial moraine landscape. This urban centre of over 423,000 people represents a significant node within Canada's densest agricultural region, surrounded by extensive farmlands producing corn, soybeans, and livestock. The city's location within the Great Lakes basin creates a modified continental climate, while its position away from major water bodies means it lacks the strong moderating effects experienced by lakeshore communities. London's urban-rural gradient is pronounced, with compact urban development giving way to agricultural lands within short distances, creating distinct air quality dynamics. The city's industrial activities—including manufacturing, food processing, and transportation—are concentrated primarily in eastern and southern sectors, while prevailing westerly winds typically transport emissions away from the urban core. However, the Thames River valley can occasionally trap pollutants during temperature inversions, particularly in winter months when cold air settles in the low-lying areas.

London's air quality follows distinct seasonal patterns shaped by its continental climate and regional geography. Winter months from December through February typically present the most challenging conditions, with cold air inversions frequently trapping pollutants near the surface in the Thames River valley. These temperature inversions, combined with increased residential heating emissions and occasional wood-burning, create periods of elevated particulate matter concentrations. Sensitive groups should monitor air quality forecasts during cold snaps and limit prolonged outdoor exertion when advisories are issued. Spring brings gradual improvement as increased solar radiation breaks up inversions and precipitation helps cleanse the atmosphere, though March and April can experience dust from agricultural activities and road sanding residue. Summer offers generally favourable conditions with good atmospheric mixing, though occasional heat waves in July and August can elevate ground-level ozone, particularly on hot, stagnant afternoons. Autumn represents London's cleanest air season, with September through November benefiting from consistent winds, moderate temperatures, and reduced emissions from heating sources. However, late autumn can see increased particulate matter from agricultural burning and leaf disposal. Throughout the year, westerly winds typically provide natural ventilation, but when winds shift to easterly or become calm, pollutants from industrial areas and transportation corridors can accumulate in the urban basin.