🇹🇭Thailand · Climate

Thailand — Climate

Thailand's hottest month is April, not July. Station normals for Bangkok, Chiang Mai, Phuket and Samui, plus the northern burning season and recent floods.

Thailand’s hottest month is April, not July, and the region with the coolest nights has the worst air in the country. Which region you settle in decides which months of your year are the hard ones: heat inland, smoke in the northern valleys, or rain that reaches the two coasts at opposite ends of the calendar.

April, not July: how the Thai year really runs

April is the hottest month at every inland station in this chapter, and the gap over July is wide enough to wreck a plan built on northern instincts. Bangkok Metropolis (WMO station 48455) averages 35.7 °C in April against 33.5 °C in July, on the WMO 1991-2020 normals submitted by the Thai Meteorological Department. Chiang Mai reaches 36.7 °C, Udon Thani 36.5 °C. Ko Samui manages only 31.8 °C, the mildest April here.

TMD runs three seasons, not four: the from roughly mid-February to mid-May, the rains mid-May to mid-October, the cool season mid-October to mid-February. Onsets are announced each year rather than fixed, and they drift by days from one year to the next. The hot season opened in February 2024 and in February 2025, the rains in May 2024 and May 2025, the cool season in November 2024 and then earlier, in October 2025.

Learn the forecast vocabulary before your first April. TMD calls a day ‘hot’ at 35 °C and ‘very hot’ from 40 °C, so an average April afternoon in Chiang Mai already clears the first line. The national annual mean of 27.4 °C describes nowhere anyone lives; it matters only as the baseline that the record years further down are measured against.

Sources. Normals: WMO Climatological Standard Normals 1991-2020 as submitted by TMD (NCEI accession 0253808), stations 48455 Bangkok, 48327 Chiang Mai, 48354 Udon Thani, 48550 Ko Samui. Bangkok’s July maximum is stored rounded in the country dataset. Seasons and vocabulary: TMD annual summaries 2567 and 2568. Onsets for 2026 are not published here.

Bangkok: no winter, one long wet season

Bangkok has no cool season in any European sense. December, the coolest month at station 48455, still averages a high of 32.3 °C and a night of 23.2 °C; January nights average 23 °C. April tops the year at 35.7 °C. The spread between the hottest and coolest monthly high is narrower than the gap between a December afternoon and that same night.

Rain marks the Bangkok year where temperature refuses to. The station collects 1,702.2 mm a year: 335.9 mm across 18 rain days in September, and 11.6 mm in the whole of December.

The rains are not uniform inside their own season. A mid-season lull known as usually bites in late June and July, which is why July on the strip above sits below both May and August.

Humidity decides how this feels. Relative humidity averages 73 % across the year and 79.3 % in September, so sweat barely dries on the skin. Sunshine totals about 2,212 hours a year, thinnest in the rainy months. Cooling is a twelve-month line in the budget rather than a summer one, and the cost-of-living chapter carries what it costs to run.

Sources. Bangkok Metropolis (station 48455), WMO 1991-2020 normals submitted by TMD; the annual total is the sum of monthly means. Sunshine rests on 26 to 29 years of record per month, and the strip’s hours per day are our own division of each monthly total.

Chiang Mai, Chiang Rai and the Northeast: cool nights, fiercer Aprils

The north is the only part of Thailand with a winter, and it arrives at night. January minimums average 15.7 °C in Chiang Mai and 13.8 °C in Chiang Rai, inside the band TMD labels ‘cold’, which tops out at 15.9 °C. Bangkok’s January nights average 23 °C. That is the difference between wanting a blanket and never thinking about one.

Higher ground goes further: the coldest reading anywhere in 2025 was 6 °C at Umphang in Tak, a highland district rather than a city.

Those nights are paid for in April. Chiang Mai averages 36.7 °C and Udon Thani 36.5 °C, both above Bangkok’s 35.7 °C, because inland basins heat harder than a delta ventilated by sea air.

Rainfall splits the north from itself. Chiang Mai takes 1,139.2 mm a year, roughly a third less than Bangkok’s 1,702.2 mm. Chiang Rai takes 1,723.5 mm, level with Bangkok and packed into July and August. So the north offers real cool nights and charges for them twice: with the hottest April of all the stations researched for this chapter, and with what happens to the air in between.

Sources. WMO 1991-2020 normals submitted by TMD, stations 48327 Chiang Mai, 48303 Chiang Rai, 48354 Udon Thani, 48455 Bangkok. Cold-band vocabulary and the 2025 national minimum: TMD.

The burning season: what February to April does to northern air

The northern cool season ends in smoke. The Pollution Control Department tracks the as a crisis window from January to the end of May, worst from February to April, and attributes the haze to forest encroachment, maize grown on slopes, and smoke crossing the borders.

Across PCD’s northern aggregate the window mean fell from 47 µg/m³ in 2024 to 35 in 2025, against a Thai 24-hour standard of 37.5. Satellite fell from 79,859 to 55,989. The burned area behind the better year was still 8.57 million rai, about 13,700 km² by our own conversion.

Read the regional day counts carefully. PCD logged 131 days over the standard in the 2024 window and 117 in 2025, but those are days on which at least one station anywhere in the northern provinces exceeded it, not any one city’s days.

Chiang Mai’s own figures come in two scopes, so here are both. ONEP counted 55 days over the standard at province level from January to May 2024, citing PCD Region 1. Across calendar 2024 the city stations recorded 74 days at Si Phum and 77 at Chang Phueak; in 2025 they logged 23 and 49. ONEP put the province’s worst 24-hour reading of the 2024 season at 159 µg/m³, in April, and blames the basin for holding the smoke.

Annual means tell the calmer version, and it is not reassuring. Chang Phueak averaged 30 µg/m³ in 2024 and 23 in 2025; Mae Sai, on the Myanmar border, 35 then 29; Chiang Rai city 24 then 21. The annual standard is 15. Nationally the mean ran 26, 24 and 21 across 2023 to 2025, above the standard every year.

Two cautions before anyone calls that a trend. The 24-hour standard tightened from 50 to 37.5 µg/m³ in June 2023, so counts either side of that date are not like-for-like. One better season is weather as much as policy. If anyone in the household has asthma or a small child, decide about February to April in the north before you sign a lease. Hourly station readings are public on .

Sources. Window means, hotspots, burned area, day counts and station tables: PCD air and noise situation reports 2567 and 2568. Chiang Mai province days and the peak reading: ONEP 2567, citing PCD Region 1. The square-kilometre figure is our conversion of PCD’s rai. Station values: PCD 2567, PDF page 58, and PCD 2568, PDF page 60.

Bangkok's haze runs November to March

Bangkok’s bad air keeps a different calendar and a different cause. PCD’s critical window for the capital runs from November to the end of March, and the driver is stagnant cool-season air over traffic rather than field burning upwind.

Bangkok and its vicinity averaged 24 µg/m³ in 2024 and 24 in 2025, unchanged and above the annual standard of 15. The area logged 132 days over the 24-hour standard in 2024 and 117 in 2025, of which 113 fell inside a critical window averaging 35 µg/m³. These are area counts, triggered by any station in the zone.

What a household breathes depends on the street. Across 2025 the Bangkok stations ranged from 24 days over the standard at Phaya Thai to 63 at the Din Daeng roadside monitor. Two monitors in the same city, a quiet one and a roadside one, sit that far apart.

For contrast, PCD recorded no exceedance of any air-quality standard in Phuket province across 2025. The coasts trade that clean air for water.

Sources. PCD air and noise situation report 2568: annual means, area day counts, critical-window figures and the Phuket provincial statement. Station counts: the Bangkok station table on PDF page 58 of the same report.

Phuket or Ko Samui: two coasts, opposite rainy seasons

The two island shortlists most people draw up are wet in different months. The southwest monsoon, mid-May to mid-October, drives Indian Ocean moisture onto the Andaman side. Phuket town (station 48564) peaks in September at 350.4 mm and bottoms out in February at 24 mm, for 2,271.5 mm across the year.

Phuket figures move depending on which station produced them. The airport monitor (48565), on the island’s north-west, logs 2,664.3 mm a year against the town station’s total. Neither is wrong, and any comparison that mixes them invents a difference.

The , mid-October to mid-February, makes the Gulf side behave backwards. The wind that hands the north its clear cold nights picks up moisture crossing the Gulf of Thailand and dumps it on the eastern shore of the south. Ko Samui (48550) is wettest in November at 444.6 mm, while its May-to-September months stay between 102.2 and 133.6 mm, for 1,994.3 mm a year. A rainy September on the Andaman coast is a moderate one on Samui, well short of its late-year peak, though Samui’s driest months are February to April.

Temperature barely separates the two. Phuket’s January minimum averages 25 °C, meaning no cool season whatsoever, and its April high 34.2 °C; Samui’s April is the mildest here at 31.8 °C. On the mainland Gulf shore rainfall drops further: Hua Hin takes 935.9 mm a year, the driest station here, and Pattaya 1,105.8 mm. Sea air is no defence against heat: Phuket set a March station record of 39.3 °C in 2024.

Sources. WMO 1991-2020 normals submitted by TMD, stations 48564 Phuket town, 48565 Phuket airport, 48550 Ko Samui, 48475 Hua Hin, 48461 Pattaya. Monsoons and the Phuket March record: TMD.

Record heat, record rain: what 2023 to 2025 logged

2024 was the warmest year TMD has on file, in a series 74 years long. The national mean hit 28.5 °C, 1.1 °C above the 1991-2020 normal of 27.4 °C, after 2023 came in at 28.1 °C.

Then 2025 broke the pattern. The national mean landed at 27.4 °C, level with the normal and only 7th warmest of 75 years, and the year turned wet instead: 1,816 mm against a normal of 1,622.9 mm, about 12 % above it. The record leans warm without marching in a straight line, and anyone planning around a single recent year is planning around noise.

The national heat record stands at 44.6 °C, set at Mae Hong Son in April 2016 and equalled at Tak in April 2023. The hottest reading of 2024, 44.2 °C at Thoen in Lampang, did not beat it. Station records fell anyway: in April 2024 Chiang Mai logged an April high of 42.5 °C and Udon Thani 44.1 °C, in a spell TMD describes as far above normal across the North and Northeast. The 2025 peak was gentler, 42.3 °C.

Water did the damage in both years. TMD confirms flooding across the North and Northeast from July to September 2024, with the Chiang Rai agricultural station recording an August total of 662.9 mm against a previous record of 643.9 mm from 1994. In July 2025 the remnant of Tropical Storm Wipha flooded Chiang Rai, Nan and Phayao, and Chiang Rai set a July record of 561.9 mm.

The south got the worst of it last. From November 2025 more than 1,000 mm accumulated over Songkhla and the provinces beside it, and TMD describes the flooding in Hat Yai as unprecedented. That is a northeast-monsoon event, on the coast whose rainy season peaks in November.

Sources. National means, rankings, records and flood events: TMD annual summaries 2566, 2567 and 2568. No casualty or damage figures appear here: the disaster agency’s tallies could not be reached. The Hat Yai rainfall figure is a stated lower bound.

When to arrive, and which climate suits which plan

Thailand has no single best month to arrive; the answer depends on the region you have picked. For Bangkok and the north, the cool season is the gentle landing: TMD opened it in October 2025, earlier than the year before, and it holds until roughly mid-February.

If Chiang Mai or Chiang Rai is the plan, that landing comes with a deadline. The smoke months follow the cool ones immediately, so the question to settle before the lease is whether you intend to spend February to April in the valley at all, or to plan those months somewhere else.

On the coasts the calendar flips depending on which sea you pick. The Andaman side is wettest in September and October; the Gulf islands take their worst in November. Inland, April is the hardest month to arrive.

Two chapters carry the costs this one only points at. The cost-of-living chapter holds what year-round cooling does to a bill, and the healthcare chapter covers respiratory care if the smoke season is a live concern. If climate is one of several things you are weighing, the quiz scores it against six other dimensions, and the Thailand checklist turns a chosen month into a sequence of tasks.

Frequently asked

What is the hottest month in Thailand?

April, at every inland station, and by a clear margin over July. Bangkok averages 35.7 °C in April against 33.5 °C in July on the WMO 1991-2020 normals; Chiang Mai reaches 36.7 °C. TMD’s hot season runs roughly mid-February to mid-May, so March and May are hard too, but April is the peak.

When is the rainy season in Thailand?

Roughly mid-May to mid-October, with the onset announced each year: TMD opened the most recent one in May 2025. In Bangkok the peak is September at 335.9 mm. The Gulf side of the south runs on a different schedule, because the northeast monsoon soaks it later: Ko Samui’s wettest month is November, at 444.6 mm.

When is burning season in Chiang Mai?

Smoke is worst from February to April, inside the crisis window PCD tracks from January to the end of May. The northern provinces averaged 47 µg/m³ across that window in 2024 and 35 in 2025, against a 24-hour standard of 37.5. Chiang Mai logged between 55 and 77 days over the standard in 2024, depending on whether you count the province in season or a city station all year.

Is the air quality bad in Bangkok?

Moderately, and seasonally. Bangkok and its vicinity averaged 24 µg/m³ in 2025, above the Thai annual standard of 15, with the bad stretch running November to March rather than in spring as it does up north. The street matters: in 2025 stations ranged from 24 days over the 24-hour standard at Phaya Thai to 63 at Din Daeng.

Does it get cold in Thailand?

In the north, at night, for about three months. January minimums average 15.7 °C in Chiang Mai and 13.8 °C in Chiang Rai, against 23 °C in Bangkok. Highland districts go lower: the coldest reading anywhere in 2025 was 6 °C at Umphang in Tak. Bangkok and the islands have no cold season.

Is Phuket or Koh Samui better for weather?

Neither wins outright; they take turns. Phuket, on the Andaman side, is wettest in September at 350.4 mm, when Samui is in its moderate season, well below its November peak, holding between 102.2 and 133.6 mm a month from May to September. Samui takes its own soaking in November, at 444.6 mm. Pick by the months you need to be settled.

Does Thailand flood?

Regionally, and in each of the last two full years. TMD recorded flooding across the North and Northeast from July to September 2024, when the Chiang Rai agricultural station logged a record August total of 662.9 mm; severe floods in Chiang Rai, Nan and Phayao in July 2025; and more than 1,000 mm around Hat Yai in late November 2025.

What is the best time of year to move to Thailand?

For Bangkok and the north, the cool season, which TMD opened in October 2025 and which holds until roughly mid-February. For Chiang Mai, weigh that against the smoke months that follow straight after. For the coasts, arrive outside the Andaman peak of September and October, or the Gulf islands’ November. April is the hardest month to land inland.

Verified · 2026-09-15

Open the relocation checklist Back to Thailand

All chapters ↑