An air quality map shows conditions improving in the U.S. on Friday as smoke from the Canadian wildfires subsides.

Smoke from wildfires in Canada has spread more than 5,000 miles, reaching parts of Russia and Europe during the past week.

AccuWeather meteorologist Brandon Buckingham explained that on sunny days—particularly during stretches of calm, clear weather—sunlight can break down smoke and haze in the atmosphere, causing it to sink towards the surface and increasing the likelihood of ozone action days or air quality alerts.