Canada

The conditions, however, already had all the ingredients for the kind of fast-spreading wildfire that hours later would force nearly 90,000 people, including nine newborns in hospital, to flee on short notice from the Alberta oil town.

The weather, the type of trees in the local forest, the time of the year and the kind of fire propagation were all indicators that the flames would be hard to tame, jumping over the Athabasca and Hangingstone rivers as they spread northward into the city.

“The fire’s behaviour was beyond all control efforts,” Bernie Schmitte, wildfire manager at Alberta Agriculture and Forestry told reporters later in the night.

To make things worse, there had been an inversion, a weather condition where hot air at higher altitude traps down the smoke. However, by the end of the morning, the inversion reversed, Mr. Schmitte said, which would have fanned the flames and smoke further.

THE CROSSOVER

Brian Burnett, head instructor at Wildfire Specialists Inc., an Ontario training school, noted that the Fort McMurray area was experiencing a weather condition familiar to forest firefighters, the Crossover.

The Crossover happens when the numerical value for the ambient humidity is lower than the recorded temperature.

On Tuesday in Fort McMurray, while the mercury soared towards 30C, the humidity dropped to 15 per cent.

“It’s just a recipe for a wildfire. This is such a difficult situation,” Mr. Burnett said in an interview.

Such hot, dry conditions were made worse by the wind which pushed the flames toward the city.