Chiang Mai is one of Southeast Asia's most popular cities: temples, street food, mountains all around. But every spring, for several weeks, it becomes one of the most polluted cities on the planet — and most tourists only find that out after they arrive.

Chiang Mai looks like a city you can visit in any month: it's warm year-round, rain is hardly a problem, and visas are issued on arrival. But open an air-quality map in March, and the city known for its temples and lush greenery can be buried under a brown haze, with pollution levels reaching twenty to forty times the WHO guideline values.

Here's how Chiang Mai's climate works throughout the year — and why choosing the wrong month is not simply a matter of discomfort, but can pose a genuine health risk.

Three seasons, not two

Chiang Mai sits in a mountain valley in northern Thailand, at an elevation of 310 metres above sea level. The climate is tropical, but its elevation and distance from the coast make it noticeably cooler than Thailand's seaside resorts. The year is divided not into two seasons — dry and wet — but into three distinct periods.

  1. Cool and dry — November to January. Daytime temperatures are +28…+31 °C, while nights fall to +14…+17 °C. Rain is rare, the air is clean and humidity is comfortable. This is the best time of year to visit.

  2. Hot and smoky — February to May. Temperatures rise to +33…+37 °C, rain remains scarce, and Chiang Mai's main seasonal problem begins: the smoke season.

  3. Rainy — June to October. Temperatures drop to around +30…+32 °C, humidity rises and tropical downpours become a daily occurrence. The air is clean because the rain washes the pollution away.

Smoke season: what causes it and why it is so bad

Every year from late February through April, farmers in northern Thailand, Myanmar and Laos burn rice fields and corn plots to clear land for the next planting season. Agricultural fires are joined by forest fires, which spread easily through dry vegetation during the dry season.

Smoke from thousands of fires rises into the air, but has nowhere to go: Chiang Mai lies in a basin surrounded by mountains. Winds are weak during this period, and the mountains restrict air circulation. Smoke builds up in the valley, like water collecting in a bowl, and can remain there for weeks.

On April 7, 2026, Chiang Mai's air-quality index (AQI) reached 180 according to IQAir, and the city once again topped the global pollution rankings. During peak periods in March and April, AQI levels often exceed 300–350, while some areas can rise above 400 — levels at which spending time outdoors without a respirator can be dangerous for anyone.

To put the scale into perspective, the WHO guideline for daily exposure to fine particulate matter, PM2.5, is 15 μg/m³. In the Mae Chaem area, levels of 184.9 μg/m³ were recorded — twelve times the guideline value. On days like these, the mountains surrounding the city disappear behind a brown haze.

Month by month

  • November

This marks the beginning of the best season. The rains have ended, the air has been washed clean and the nights are cool enough to sleep with the windows open. Temperatures are around +29/+17 °C. Loy Krathong, the festival of floating lanterns, usually takes place in November.

  • December–January

This is peak season. The air is clean, temperatures reach +28…+30 °C during the day and fall to +14…+15 °C at night. In the mountains of Doi Inthanon, Thailand's highest point at 2,565 metres, temperatures can drop to +5 °C — a remarkable experience in a tropical country. This is also when tourist numbers and prices are at their highest.

  • February

A transitional month. The first half can still be relatively clear, but during the second half haze and the smell of burning may begin to appear. Temperatures rise to around +33 °C. It's worth keeping an eye on the AQI: if it remains below 100, conditions are still reasonably tolerable.

  • March

The worst month. This is the peak of agricultural burning, with AQI regularly exceeding 200 and sometimes rising above 350. The mountains disappear behind a veil of smoke, while the sun becomes a hazy disc in a yellow sky. Walking around the city can be physically unpleasant: your throat feels irritated and your eyes may water.

Many digital nomads and long-term expats leave Chiang Mai for the entire month of March.

  • April–May

The smoke begins to ease, but the heat reaches its annual peak at +36…+37 °C. In mid-April comes Songkran, the Thai New Year: three days of water fights that briefly cut through both the heat and some of the dust. The first rains arrive in May, and the air can clear within just a few days. By the end of May, Chiang Mai can breathe freely again.

  • June–October

The rainy season. Downpours are common every day, usually arriving in the afternoon and lasting one to two hours. Mornings are generally sunny. The air is clean and the greenery is at its most vibrant. There are fewer tourists and prices are lower. The downside is the humidity, which can reach 85%, and the possibility of washed-out mountain roads.

Why the valley traps the smoke

Chiang Mai lies in a tectonic basin surrounded by mountain ranges reaching up to 2,500 metres. During winter and spring, a temperature inversion can form over the valley: warmer air above traps cooler, polluted air near the ground. Without wind or rain, the smoke cannot rise above the surrounding mountains or disperse, so it hangs over the valley like a sealed container.

It's the same basic phenomenon that turned London into a smog-filled city in the 19th century or can blanket Almaty in winter — except here the "lid" is created by the surrounding mountains rather than a high-pressure system.

Best time to visit: November–January

Taking everything into account — clean air, comfortable temperatures and minimal rainfall — the optimal period for visiting Chiang Mai is November, December and January. These are the three months when the city looks most like it does in the guidebooks: temples framed by green mountains, morning markets in cool air and evening walks without oppressive heat.

For travellers who want to avoid crowds and keep costs down, June–October is a good alternative. The air is clean, the greenery is at its brightest and prices are lower. The rain does not have to be a problem if you accept it as part of the rhythm of the season: use the morning for temples and excursions, and spend the afternoon downpour at a massage or in a café.

March, however, is the one month worth crossing off the calendar entirely. No temple and no street-food tour is worth breathing air that is twenty times more polluted than the recommended level.