When does Chiang Mai's burning season actually start?
There is no start date you can pin to a calendar in our data, and that is the most useful thing to know. We model fine particles (PM2.5) from the Copernicus CAMS air-quality data. In each of the three full winters we hold (2023-24 to 2025-26), Chiang Mai's monthly average was above the WHO daily guideline of 15 from November on, and 20 to 30 days of each November were above that line.
What changes is how bad the bad days get. A "burning season" in practice means the weeks when the monthly average climbs above 30 and days above 35 stack up. In our data that happens in February to April, not in November.
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What did each season look like, month by month?
Here are the monthly averages for the last three winters we hold in full, October to April:
- 2023-24 (the lightest): October 16, November 22, December 21, January 18, February 21, March 26.
- 2024-25: October 21, November 24, December 29, January 31, February 31, March 44.
- 2025-26: October 19, November 19, December 27, January 28, February 30, March 38.
April was 30 in 2024, 30 in 2025 and 54 in 2026. The start moves by weeks from year to year, and so does the peak. The 2023-24 winter was the lightest, and the following two were heavier from December.
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Is Chiang Mai air quality bad in January?
January is not the worst month of the cool season, but "safe" is not a word we use. Its average over four years was 29, 18, 31 and 28, always above the WHO daily guideline of 15, and the days above 35 were 4, 0, 8 and 1. January 2024 was the lightest, with an average of 18 and no days above 35; the other three Januaries averaged 28 to 31.
So the honest answer is that January is not a clean month, and some years it already has several very bad days (8 in January 2025). If your plan is sensitive to the air, fly in for October or November, or accept that January can already be hazy. Whether that is acceptable depends on you; we only show the levels.
How much worse do February and March get?
February moves the average up only a little (21 to 32), but the very bad days come back: 13, 2, 7 and 6 days above 35 in the four Februaries. March is where the burning season peaks: 21, 5, 18 and 18 days above 35, with a worst day of 91 in March 2025. The worst single day in our data came later, at 99 in April 2026.
April depends on the year. It was moderate in 2024 and 2025, and extreme in 2026, with 27 days above 35 in the month. Rain at the end of April or in May ends it. We cannot tell you in advance which kind of April you get.
Chiang Mai: Average high and low temperature, °C
Chiang Mai: Rainfall, mm
Averages 2015-2025 · Source: Open-Meteo, CHIRPS · How we measure
Why do guides disagree on the start date?
Because the cause is mixed. Open burning of fields and forest floor is banned for set periods that the province changes from year to year, and the haze that arrives also depends on wind, the dry spell and burning in neighbouring areas. Guides repeat one date because it is easy to remember. Our model is a regional grid about 45 km wide and reads low on the worst days, so treat the numbers as floors: the real streets of the city can be worse than the monthly figure.
When does Chiang Mai become clean again?
June. The monthly average fell under 15 in June in 2023 and stayed there through the rainy season: July and August were the cleanest months in our data. May is the in-between month, with an average of 20 in 2023. If you want Chiang Mai without the smoke, plan the rainy season and accept the rain; see the Chiang Mai weather by month guide for the trade-off.
Where should you be instead in February and March?
The simplest plan is to be in Chiang Mai while the air is moderate and move on before the heavy months. Our cleanest air by month page ranks 17 bases on the same data. Winter is cleanest in Mexico (Oaxaca, Playa del Carmen, Mérida) and on Langkawi, and the Thai coasts are much cleaner than the north in these months. Compare rain and temperature as well as air before you book, and see the older Chiang Mai burning season guide for week-by-week curves.
How sure are these numbers?
PM2.5 is a model value from the CAMS global air-quality data via Open-Meteo, for 2023 to 2026, on a grid roughly 45 km wide. It reads low against ground stations on the worst days. Monthly averages and the count of days above the 15 and 35 lines are our own calculation from daily values. The WHO guideline for 24 hours is 15. Any one year can differ, and this is not a health judgement.
Frequently asked questions
When does the burning season start in Chiang Mai?
There is no single date in our model data. Monthly averages are already above the WHO daily guideline of 15 from November, and the very bad days (above 35) come mostly in February to April. Official burning bans also change by year.
Is Chiang Mai air quality bad in January?
It is not the worst month, but it is not clean. The January average was 29, 18, 31 and 28 in four years, and January 2025 had 8 days above 35. We do not call any month safe or unsafe; these are modelled levels.
When is the worst month for smoke in Chiang Mai?
Usually March: averages of 42, 26, 44 and 38 in the four seasons we hold, with up to 21 days above 35. But April was worse in 2024 (30) and in 2026, which was the worst single month in our data at 54.
When does the air get clean again?
May is still elevated (20 in 2023). The average drops under 15 in June, and July and August are the cleanest months.
Is the air in Chiang Mai worse than Bangkok?
In March and April it usually is: Chiang Mai was worse in 2024, 2025 and 2026, Bangkok in 2023. In January, Bangkok was worse in all four years of our data. See the comparison guide linked below.
Where can I go instead in February and March?
Bases with low modelled air in those months, such as Langkawi and the Mexican bases in our cleanest-air guide. Compare the weather too before you book.
Not sure where yet? Pick your months and what to avoid, and get your 3 best places → Where should I go?
Weather data: Open-Meteo (CC BY 4.0), open-meteo.com; rain totals: CHIRPS (Climate Hazards Center, UC Santa Barbara). Processed by NomadWeather from stored copies of the data.