Some city residents say it looked like Armageddon, but it was really the result of an intense thunderstorm and a wildfire that's burning about 30 kilometres away.
Ella Stinson with the N.W.T.'s environment and natural resources department says as the rain from the storm passed through the smoke on Wednesday afternoon, the droplets picked up ash.
Stinson says that made the rain appear dirty.
The sky began to clear later in the evening and is now back to normal, although there has been a smoky haze from fires for several weeks.
Environment Canada meteorologist Brian Proctor says despite the intensity of the storm, not a lot of rain fell.
"We got these really interesting photographs of lightning appearing to glow red in the sky, if I can put it that way, as the smoke refracted the light from the lightning," Proctor said Thursday.
"They appear to be, as some people have put it, almost apocalyptic in nature."
The darkness was particularly unusual for Yellowknife for this time of year, as the city's northern latitude means the sun doesn't set until about 10:30 p.m. and rises at just around 5 a.m.
Stinson said the smoke was so thick on Wednesday afternoon, it was even hazy in her home.
"It got very dark, very quickly. It was basically like the winter," Stinson said. "I know some people were scared."
The fire that was generating the smoke is about 200 square kilometres in size and has been burning since July 16.
Stinson said the department is still assessing whether the rain that fell had any affect on it.
It's a different fire from one that's caused periodic closures of Highway 3, the route to Yellowknife, over the past few weeks.
That fire has prompted officials to ask people living in a few dozen households along the route to evacuate as a precaution, he said.
— By Rob Drinkwater in Edmonton