The smoky fog that formed across the northern Red River Valley and much of Minnesota Monday night and lasted for a day or two was the result of a particular set of circumstances. First of all, the raging forest fires in Manitoba and Ontario have been producing heavy smoke for weeks. Surface winds out of the north blew that smoke southward. Light rain showers generated downward-moving air and also cooled the air in the lower atmosphere, creating a temperature inversion that trapped much of the smoky air close to the ground.
These same conditions tend to produce fog, and the tiny smoke particles became condensation nuclei for humidity in the air to cling to. The end result was a dense, dirty, sooty fog with a very poor air quality reminiscent of a 19th century London smog.