Ontario wildfire smoke suppresses irradiance across the Great Lakes

Wildfire smoke over Ontario cut into the sunlight reaching solar panels across the Great Lakes region and parts of the US Northeast in mid-July. From July 10-21, total sunlight (measured as GHI, or global horizontal irradiance) ran up to 10% below normal in the area, according to data from Solcast, a solar forecasting company. Most of that loss came from the smoke itself blocking sunlight, not clouds.

The smoke came from a fast-growing wildfire outbreak in northern and northwestern Ontario, where more than 180 fires were burning at the worst point. Winds carried the smoke southeast across Ontario and Quebec and into the US Midwest and Northeast, giving Toronto, Chicago, Detroit and Minneapolis some of the worst air quality in the world during July 14-17, and triggering air quality alerts in more than 20 US states. Airborne particle levels near Toronto tripled their earlier-July baseline during the worst days, while areas closer to the fires saw levels roughly eight times normal.

For anyone with rooftop solar in the affected region, this means panels likely produced less power than usual during that stretch, both because less sunlight got through and because smoke particles settled on panel surfaces, adding to the shading effect. That soiling buildup did not clear until rain arrived on July 18. If you have solar panels and noticed lower output in mid-July, this smoke event is the likely explanation, and production should have recovered once the rain washed the buildup away.