Thailand Has a Season for Everything, and You Are Choosing One Right Now
The monsoon is not a national thing. It hits the Andaman in May and the Gulf in November, the air is worst in March, and the month you pick decides which country you actually visit.
A couple booked Koh Samui for the second week of November because it looked cheap on a flight comparison site and because someone had told them the Gulf is sunny in November. They landed to four days of unbroken rain, a sea the colour of dishwater, and a hotel manager who apologised approximately forty times a day. They were not unlucky. They had made the single most common timing mistake in this country, and it is not a small one, because the word "wet season" in Thailand is not a season at all.
Thailand does not have a wet season. It has two completely separate monsoons that do not overlap, one for each coast, and a third problem in the north that has nothing to do with rain. Get that straight and the whole question becomes simple. Get it wrong and you will spend a very expensive fortnight watching water come off a roof.
Last verified 10 October 2026.
The one question that decides everything: which coast
The Andaman coast — Phuket, Krabi, Khao Lak, the Phi Phi islands, Koh Lanta, Koh Yao Noi — gets its rain from the southwest monsoon, and that is May to October. The wettest stretch is September and October, and September is the one to avoid: the Thai Meteorological Department's average for the wettest part of the south-west is over 400mm. By late October it is easing, and by November the Andaman is the driest, clearest part of the country.
The Gulf coast — Koh Samui, Koh Phangan, Koh Tao, Hua Hin, Rayong, Trat — rains on the opposite clock, October to December, peaking in November. This inversion catches people every single year because the Gulf and the Andaman sit on opposite sides of the country and behave like two different countries. A person who loved Krabi in April and wants the same trip again should be thinking about Krabi in January, and a person who loved Koh Samui in August should think about Koh Samui in February.
Bangkok is a third thing again, because it is not on either coast. The capital runs hot and then very wet from roughly May to October, with the heaviest months around September and October, then dry and comparatively cool from November through February.
So the rule is not "go in the dry season". The rule is: on the Andaman aim for December to March, on the Gulf aim for January to May. Those windows overlap in January, February and March, which is exactly why the high season is high season and why November is the awkward compromise where one coast is finishing its rains and the other is starting.
The clearest illustration of all this is a single trip split in two. Eleven months apart I was in the same fortnight of the year: eleven days of flat grey sky in Phuket with longtail boats cancelled one after another, and then a stretch on Koh Tao where it did not rain once. Same country, same two weeks on the calendar. I had not outsmarted the weather — I had simply gone to the wrong side of the peninsula.
November to February: the good window, with one real trap
November, December, January and February are the best months in the country for weather — dry, mostly clear, and cool enough at night to be pleasant rather than oppressive. This is why hotels in the north sell out for Chiang Mai's Yi Peng lantern festival on 24 and 25 November 2026, and why prices there roughly triple for those two nights.
The trap in this window is the north at night, and it catches almost everyone who is surprised by it. In November Chiang Mai still runs around 31°C by day and 20°C at night, but in December and January the northern valleys drop into the mid-teens and the low-20s after dark, and Mae Hong Son and the high passes go colder still. This is not England-cold, but it is genuinely cold by the standard people arrive with, it sits on your motorbike ride at midnight, and the temples are open at six in the morning in a draft. Bring a jacket. This is the single most common packing mistake in the season, and it is not close.
December and January are also the driest and busiest stretch, which is good if you want the country running at full volume and bad if you want the cheap version of it. Prices on the coast peak around the Christmas and New Year fortnight specifically, not evenly across the season.
March and April: hot, and in the north, genuinely difficult
March is the month experienced travellers quietly avoid, and the reason is not really the heat. It is the air. Agricultural crop residue burning across the northern plains peaks in the second half of February and the first half of March, and the resulting PM2.5 regularly reaches levels where you can see the Doi Suthep from the city and almost nobody can. Schools in affected areas close. It is a genuine public health event, not a scenic inconvenience, and it affects Mae Hong Son, Chiang Rai, Lamphun and the northern edge of Chiang Mai city.
April is better for air and still very hot. The burning stops, and what replaces it is the hottest stretch of the year: April averages around 36°C in Bangkok, runs 35-38°C across the central plain, and is worse in the Isan basin, plus Songkran, the water festival, from 13 to 15 April, which is a wonderful thing to be in the middle of and an exhausting thing to be travelling through — because half the country is being hosed with water by strangers on purpose.
If your dates are fixed and they are March or April, put them somewhere with a sea breeze and skip the north. The Gulf islands in April are hot and dry and genuinely lovely, and far better than Chiang Mai in the same fortnight.
May to October: a monsoon that is regional and a north nobody visits
May to October is not one thing. On the Andaman it is the wet season proper and many beach operations and island tours simply do not run. On the Gulf it is the dry season and Ko Samui in July is one of the best cheap weeks you will find all year. In between, the central plain gets the heaviest rain of its year in September and October.
The north in this window is the genuinely underrated option. June to October is green season in the mountains — rice terraces, waterfalls at full volume, the forest genuinely lush — and the rain mostly comes in the late afternoon rather than all day. It is the best cheap season in Thailand and almost nobody from outside the country knows it. The catch is that you will need a rain plan for every day, and the guesthouses that shout about waterfalls in July are not wrong about the waterfalls.
What October 2026 looks like from today
If you are reading this in October 2026, you are reading it at the hinge. The Andaman's worst is over and improving week by week, though early October still has heavy days. The Gulf is starting to turn. The burning season has ended and northern air quality is recovering. High season is starting, and the long-haul bookings that were held back through the summer are landing right now.
The practical consequences: the north is open and improving, so Chiang Mai and the national parks are live and cheap. The Andaman is improving but not yet reliable. The Gulf is the one to avoid until December. And the thing that genuinely dominates the next six weeks is not weather at all — it is the festival calendar.
The festivals that should pick your dates for you
If you are choosing a trip for one specific thing, work backwards from these. Everything below is a fixed, known date rather than something that drifts.
Loy Krathong and Yi Peng, 24-25 November 2026. The nationwide water festival on the full moon of the 12th lunar month, and the Chiang Mai sky lantern release on the same nights, with the Tourism Authority's official Chiang Mai programme running 24-26 November and a separate ten-night festival at Sukhothai from 15-24 November. Book flights and rooms early, and note that Chiang Mai's airport adjusts schedules around those nights.
Songkran, 13-15 April 2027. The water festival. Prepare for it being participatory and unpleasantly thorough.
The cool-season window, November to February. Not a date so much as the reason most people choose it.
And the kite season, January to March. The big kite festivals run through the hot dry months, with Pathum Thani and Chiang Mai among the better-known ones. Not an October story, and worth knowing now if you are planning a few months out.
How to actually pick
Work backwards from the coast, not forwards from the month. Decide Andaman or Gulf or mountains first, then look at what that combination does in your window. Almost every bad trip I have seen the shape of was a person who picked a month from a flight price and then picked a coast that disagreed with it.
Accept that you cannot have Christmas weather and Loy Krathong lanterns in the same comfortable trip. You can — late November in the north is exactly that — but you will be there in the cold, at festival prices, on the busiest nights of the year. It is a real choice, not a compromise.
And if you are genuinely flexible, the best answer is late November in the north or February on either coast. February is dry almost everywhere, the burning has not started, and prices have come off the New Year peak.
