The Eight Weeks Nobody Mentions

For about eight weeks every year, between late February and April, the air in northern Thailand becomes some of the worst in the world. Chiang Mai regularly tops the global live rankings during March, above Delhi and Lahore. The mountains that make the region beautiful are the reason: they hold the smoke in a bowl until the rains come.

Almost no itinerary mentions it, and it is the single most useful thing to know about when to go north.

What is burning

Three things at once. Farmers clearing crop residue — maize stubble above all, on hillsides too steep for machinery. Forest fires, some accidental and some set deliberately to encourage the mushrooms and vegetables that grow in burnt ground and are worth money. And the same processes across the border in Myanmar and Laos, where roughly half the smoke in a bad year originates.

The maize matters more than it sounds. Much of it is grown on contract for animal feed, on land that was forest a generation ago, by smallholders with no equipment and a narrow window between harvest and planting. Burning is free and takes an afternoon. Not burning costs money the contract does not pay for. That is the whole problem, and it is an agricultural economics problem wearing an environmental costume.

Every year the government announces a burning ban, and every year the smoke arrives on schedule. A ban does not change the arithmetic on a hillside.

What it means for a visit

PM2.5 readings above 150 are common in March and readings above 300 happen. At those levels, outdoor activity is genuinely inadvisable for anyone, and people with asthma or heart conditions should not be there at all. Views disappear — the mountains around Pai and Mae Hong Son become a white wall.

The season is not fixed and it is not uniform. Some years it starts in mid-February; some years the rains break early. Check a live air-quality reading before committing to March dates, and understand that the islands in the south are entirely unaffected — this is a northern problem, and one of the few cases where "go somewhere else in the same country" is a complete answer.

When to go instead

November to February is the cool dry season and is the north at its best: clear air, comfortable temperatures, and the reason Thai people themselves travel up there in those months. It is also the busiest and priciest window, which is not a coincidence.

June to October is the green season. It rains, usually hard and briefly rather than all day, the hills are green rather than brown, prices fall and the air is clean. For walking country it is a better trade than most visitors assume.