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For Chiang Mai and other northern provinces which experience prolonged high levels of microscopic dust, the season usually begins in late February, when dry weather and an atmospheric inversion layer cause airborne particles to accumulate.

This pushed air downwards, creating an inversion layer that trapped the smog close to the ground.

Increased winds Tuesday pushed out a smoke inversion layer that had been stifling the blaze and gave fresh oxygen to the flames, McLean said.

If the inversion layer is strong, he said, “it’ll trap smoke.”

And “we don’t expect a repeat tonight” of the heavy inversion layer and dense fog.