The smoky season is caused by several different factors, although we can’t say for certain which is the biggest contributor.
It’s well-known that local farmers burn fields and brush to get ready for a new season of crops. We’ve driven past farmland just outside Chiang Mai’s Old City and smoke trails can be easily seen from the road. We’ve also seen smoke wafting up from the hills and nearby mountains. Locals also burn their trash, but that is arguably a year-round thing.
Additionally, it’s hard to miss the dark grey smog spewing from vehicles’ exhaust pipes, particularly from Chiang Mai’s songthaews and tuk tuks. When the high season rolls around between October and February, the increased traffic adds to the air pollution.