The 2015 fire season got off to an unusually early start in Canada when blazes broke out in the Northwest Territories, British Columbia, and Alberta in late May. As the season has progressed, the air in western Canada—as well as large swaths of the United States—grew gray and hazy with smoke.

Beginning on June 28, a sharp trough in the jet stream sent a river of smoke streaming south into the United States. By June 29, smoke darkened the skies over much of Saskatchewan, Alberta, Manitoba, North Dakota, South Dakota, Minnesota, and Iowa.

On June 29, 2015, the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite captured this image of smoke from hundreds of wildfires in western Canada. Actively burning areas, detected by the thermal bands on MODIS, are outlined in red, while forests appear dark green. The image below shows shows a closer view of smoke and fires burning in northern Alberta near the Athabasca oil sands. While hundreds of fires are burning throughout Canada, some of the fires producing the most smoke are clustered in this area.

A combination of unusually warm temperatures, parched forests, lightning, and strong winds have fueled the outburst of fire. According to the Canadian government, 168 uncontrolled fires and 273 controlled fires were burning in Canada on June 29. More than than 1,300 people have had to evacuate their homes, and health officials have issued health warnings in several provinces because of the smoke.

NASA image courtesy Jeff Schmaltz, LANCE/EOSDIS MODIS Rapid Response Team at NASA GSFC. Caption and image cropping by Adam Voiland.