The huge forest fire in the Canadian oil city of Fort McMurray has not been reduced by the recent change of weather.

On Thursday, a weak cold front dropped the temperature, brought imperceptible light rain but more importantly changed the wind.

Over the past two weeks, including at the time the fire began, the wind has come from the southerly quarter with gusts recorded in excess of 70km an hour.

Following the front, the wind swung around to the northwest, gusting briefly to 52km/h, which should, in theory, drive the fire in a different direction. Indeed, smoke is now crossing Alberta province's border into Saskatchewan.

In practice, a fire this big produces its own weather and wind. It builds huge pyrocumulus clouds above it, but any rain generated evaporates before it hits the ground. Wind gusts from fire-generated convection can whip the flames quickly into new areas.

The temperature of 32.8C recorded in the city on May 3 was a record breaker. The average temperature in May in Fort McMurray is 17C. The recent three-day heatwave may have felt like the cause of the fire, but in reality, the underlying conditions of deficient rainfall and low humidity have been preparing the ground for ages.

Average rainfall is low anyway, on a par with central Spain which is a high desert plateau. Fort McMurray should get about 420mm of rain a year, most of which should fall in the summer, but for the past three years the amount of rainfall has been short of the norm.

The year 2015 was particularly deficient with only 65 percent of the long-term average received, most likely as a result of the major El Nino.

With a dry environment underneath the forested slopes, the fire risk was already high. When humidity dropped below 15 percent on May 3, which was on a par with the Arabian desert, all that was needed was a spark.

Although it may be of little help, the chance of rain showers over the weekend is quoted at 30 to 40 percent and temperatures will remain nearer to normal.

Source: Al Jazeera