Canadian wildfires have been raging this summer and some of the smoke from those fires is drifting downward into the U.S. In this image collected by the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Aqua satellite on July 24, 2014 a swath of smoke has descended over the Great Lakes region of the United States. What is particularly interesting is the fire image from July 23, 2014 (first image feature highlighted below) clearly shows the path of the smoke as it drifts off southeastward. In the image, it is over Manitoba and parts of Ontario, and by the time this image was taken by Aqua, it had drifted across the Great Lakes.
There have been quite a few image features on NASA.gov recently highlighting the Canadian fires:
Smoke From Northern Territories Fires in Canada - July 24, 2014
Fires in the Northern Territories July 2014 - July 23, 2014
Fires and Smoke in Canada's Northern Territories - July 21, 2014
Wildfires in Canada's Northern Territories - July 15, 2014
Northwest Territories, Canada Wildfires - July 07, 2014
NASA image courtesy Jeff Schmaltz, MODIS Rapid Response Team. Caption: NASA/Goddard, Lynn Jenner