← Weather & seasons in Thailand
On the thirty-year normal, Chiang Mai’s driest month is February (9.6 mm), wettest is August (223.3 mm), hottest is April (36.7°C mean daily maximum ) and coolest is January (15.7°C mean daily minimum). Chiang Mai Province has proclaimed a total open-burning ban from 1 January to 31 May 2026. The worst air quality historically falls in February–April — which means the hottest month of the year and the worst haze season are the same month.
The season bands, honestly
Chiang Mai does not have a simple two-season calendar. These bands combine the measured rainfall and temperature normals with the province’s own burning-ban dates, so the trade-off is visible rather than buried in one “dry season” label.
| Window | Measured conditions | What that means here |
|---|---|---|
| Nov – Dec | Cool, dry, clear. | The best combination of the year: air is clean, heat has broken, and the open-burning ban has not yet started. |
| Jan | Coolest month (15.7°C mean daily minimum). Still dry. | Good, but the province's burning ban starts 1 January — a precaution, not a promise the air stays clear all month. |
| Feb – Mar | Driest months (9.6 mm and 20.4 mm). Warming fast. | Historically the worst stretch for haze, even with the ban active. Humidity in March averages 53.6%, the driest air in the dataset. |
| Apr | Hottest month (36.7°C mean daily maximum). | Heat and haze risk overlap. Songkran (mid-April) falls here. |
| May | Heat easing, rain building. | Last month of the burning ban. Transitional. |
| Jun – Jul | Rain increasing. | Green season begins; fewer crowds. |
| Aug | Wettest month (223.3 mm). | Lush, but the wettest stretch of the year. |
| Sep – Oct | Rain easing. | Shoulder season; good value, improving weather. |
All figures are 1991–2020 normals for WMO station 48327 (Chiang Mai). The “what that means here” column is our own reading against the burning-ban dates, not a separate data source.
The 2026 open-burning ban
Chiang Mai Province, under Governor Ratthapol Naradisorn, has proclaimed a total ban on open burning of every kind — garden waste, agricultural residue, all of it — from 1 January to 31 May 2026. The proclamation states that violators face strict legal enforcement in every case, and it builds in an exception process: anyone with a genuine need to burn for fuel management must register through the Fire-D system and get approval from a district or subdistrict wildfire-prevention centre before doing so, then report the result.
A province-wide ban is a control measure, not a guarantee of clean air. Enforcement cannot stop every fire, and a meaningful share of the worst haze in this region has historically been attributed to cross-border burning rather than burning inside Chiang Mai province itself — a distinction this page cannot verify further because no single official source we could open apportions the haze between domestic and cross-border sources for any given year.
Why the worst month is a double problem
April is the month to plan around, and for two separate reasons that happen to land in the same thirty days. It is Chiang Mai’s hottest month on the thirty-year normal, at a 36.7°C mean daily maximum — hotter than Bangkok’s April mean of 35.7°C, despite Chiang Mai’s reputation as the cooler northern city. It also falls inside the province’s open-burning ban window, in the stretch that haze reporting consistently identifies as the worst of the year. Songkran, Thailand’s New Year water festival and one of Chiang Mai’s biggest tourism draws, sits in the middle of this same month.
None of that means April should be avoided outright — it is also the busiest and most culturally significant month to be here. It means going in with eyes open: check the air-quality index before booking outdoor activity, and do not assume “cool mountain city” applies in April the way it does in December.
What we could not confirm
- A city-specific PM2.5 exceedance-day count for Chiang Mai. The Pollution Control Department's 2025 briefing gives regional figures for the North only, not a Chiang Mai city number.
- The split between domestic and cross-border burning as a share of any year's haze. Widely discussed, not officially quantified in any source we could open.
- A full monthly rainfall or temperature table beyond the four extreme months (wettest, driest, hottest, coolest) plus the March figures cited above. The archive that would supply every other month could not be reached from this run.
- Whether the 1 January start date, rather than a later February start used in some past years, changes actual burning behaviour on the ground. That is an enforcement question this page cannot answer from a news report of the proclamation alone.
Frequently asked questions
November or December, on the numbers. The northeast monsoon has settled in by then — dry, clear and comparatively cool, with January the coolest month on the thirty-year normal at a 15.7°C daily-minimum mean. Crucially, this window sits before the open-burning season builds, so the air is at its best of the year at the same time the temperature is most comfortable. That combination does not hold for very long — see the next question.
For 2026, Chiang Mai Province has proclaimed a total open-burning ban — garden waste, agricultural residue, everything — running from 1 January to 31 May, signed by the provincial governor, with violators facing strict legal enforcement and a registration process for anyone with a genuine, approved need to burn. That is the province's own control measure, not a claim that the air is clean for five months; the worst air quality is still reported in the February–April stretch, when the ban is already active but enforcement cannot stop every fire, including ones that cross from neighbouring countries.
Yes, and this is the fact most visitor guides leave out. April is Chiang Mai's hottest month by far on the thirty-year normal — a 36.7°C daily-maximum mean, well above Bangkok's April mean of 35.7°C — and it falls squarely inside the province's burning-ban window, which exists precisely because this is also when haze is worst. A visitor planning around heat alone, without checking the air-quality season, can land in the single least comfortable combination of the year: hot and hazy at once.
Less than most people expect for a Thai city: 1,139.2 mm a year on the thirty-year normal, spread over 91.6 rain days. That is less than Bangkok's 1,702.2 mm. August is the wettest month at 223.3 mm; February is driest at 9.6 mm, with March only slightly wetter at 20.4 mm and a mean relative humidity of 53.6% — the driest air of any major Thai station this site has checked against the same normals. The rainy season here is narrower than the coasts, but the dry months are also the ones most affected by burning-season haze.
On the regional data available, the north leads the country. The Pollution Control Department's own 2025 haze briefing lists 2024 days over Thailand's 24-hour PM2.5 standard of 37.5 micrograms per cubic metre by region: North 129, Northeast 113, Central 101, Bangkok and vicinity 97, West 84. That briefing does not break out a Chiang Mai city figure specifically, so this page does not print one — but the North recording the most exceedance days of any region, against Bangkok and vicinity's 97, is itself a sourced answer to the underlying question.
Related guides
- Air quality in Thailand — the PM2.5 standard and what people actually do about the haze season.
- Compare Thai cities — the same normals for twelve stations, side by side.
- Chiang Mai vs Bangkok
- Weather & seasons in Thailand — the country-wide picture this page zooms in on.
- Chiang Mai — the city guide.
- Explore Thailand — the section this guide sits in.
Timed your visit. Now time the move.
If Chiang Mai is more than a trip, see what is actually listed here before the next burning season sets your calendar.
Sources & References
- WMO Climatological Standard Normals 1991–2020 — Thailand station data sheets (NCEI Accession 0253808), Chiang Mai station 48327: annual rainfall of 1,139.2 mm over 91.6 rain days, wettest month August (223.3 mm), driest month February (9.6 mm, with March close behind at 20.4 mm), hottest month April at a 36.7°C daily-maximum mean, coolest month January at a 15.7°C daily-minimum mean, March mean relative humidity 53.6%, annual sunshine 2,436.3 hours
- Chiang Mai News — Chiang Mai Province proclamation, signed by Governor Ratthapol Naradisorn, prohibiting all open burning ("ห้ามมิให้เผาในที่โล่งทุกชนิด"), including garden and agricultural waste, from 1 January to 31 May 2026 (B.E. 2569), with an exception process through the Fire-D registration system for essential fuel-management burning approved by district or subdistrict wildfire-prevention centres, and strict legal enforcement stated for violators
- Pollution Control Department (Ministry of Natural Resources and Environment) — Air Quality Situation and Measures for Haze Prevention and Mitigation in 2025 briefing: Thailand's 24-hour ambient PM2.5 standard is 37.5 micrograms per cubic metre; days exceeding that standard in 2024 by region were North 129, Northeast 113, Central 101, Bangkok and vicinity 97, West 84 — the briefing gives no separate Chiang Mai city figure
Primary and official sources are cited above. Government rules, fees and procedures in Thailand change over time and vary by office; always confirm current requirements with the relevant authority before relying on them. BAANLYY never takes paid placement in editorial content.
