Thailand’s year splits into three seasons set by the monsoon, not by a simple hot/cold calendar, and the two coasts don’t share a dry season at the same time. Which season suits your trip depends more on which coast you’re headed to than on the month alone.
The three seasons
The Thai Meteorological Department marks the summer (pre-monsoon) season from mid-February to mid-May, when temperatures climb ahead of the rains. The rainy season runs mid-May to mid-October, driven by the southwest monsoon carrying moist air from the Indian Ocean over most of the country; August and September are the wettest months, and the Intertropical Convergence Zone and passing tropical cyclones add to the rainfall on top of the monsoon itself. A one- to two-week dry spell is common from June into early July, so a rainy-season trip in that window can get lucky. Winter (the cool season) runs mid-October to mid-February, when the northeast monsoon pushes cold, dry air down from China over most of the country.
Why the two coasts don’t match
The far south breaks this pattern. During winter, the same northeast monsoon that dries out Bangkok and the north instead brings mild weather and abundant rain to the east coast of southern Thailand — the Gulf side, including Koh Samui and Koh Phangan. That’s the opposite of the west-coast Andaman islands (Phuket, Krabi, Koh Lanta), which follow the mainland’s dry winter. In practical terms: November through January is peak season on the Andaman coast and the wettest stretch on the Gulf coast, so “the cool season” isn’t a single answer once a beach is involved.
Temperature and rain by region
Away from the coasts, the seasons also feel different by latitude. Southern Thailand stays mild year-round, with smaller swings between day and night and between seasons, thanks to its maritime exposure. The north and northeast see the opposite: summer daytime highs there average 36.1°C and 35.2°C respectively, against a milder 34.1°C average high on the Andaman coast. The all-time extreme for the country is 44.5°C, recorded at Uttaradit in the north on 27 April 1960; the coldest reading on record is -1.4°C at Sakon Nakhon in the northeast on 2 January 1974, while the north’s own record low is a milder 0.8°C — cool-season nights in the northern hills get cold, but not literally freezing. Humidity follows the same regional split: the Andaman coast runs a muggy 84% average relative humidity in the rainy season, while the driest air of the year, as low as 9% at Loei and Chiang Rai on isolated days, turns up inland during the cool and hot seasons.
Rainfall totals vary just as widely: most of the country averages 1,200 to 1,600 mm a year, but windward slopes such as Ranong on the Andaman coast and eastern Trat province get over 4,500 mm, while the leeward central valleys and the northernmost part of the south stay under 1,200 mm. The Andaman coast also logs the most rain days of any region, about 178 a year on average, against 122 in the north and 116 in the northeast.
Tropical cyclones
Thailand sits far enough inland, and behind enough mountain ranges, that it mostly feels tropical cyclones as weakened depressions rather than full storms, and the Southern Part carries the higher risk of an actual tropical storm or typhoon making landfall. Averaged over 65 years of records, a cyclone track crosses the country only about 3–4 times a year, and January through March has never recorded one; the risk climbs from May and peaks in September and October, which is on top of the ordinary rainy-season downpours.
Practical tips
- For Bangkok, the mainland and the Andaman coast, the cool season (mid-October to mid-February) is the most reliable window for dry, comfortable weather.
- If Koh Samui or Koh Phangan is the destination, shift the “best time” forward: aim for the pre-monsoon months or March–May instead, since November–January is that coast’s wet stretch.
- Trekking in the north in December or January, pack warm layers for cold nights (down to single digits °C) even though the daytime is warm.
- A rainy-season trip isn’t a write-off: showers are typically short and intense rather than day-long, except during the August–September peak.
What to check before you go
- Which coast you’re actually visiting — the Gulf and Andaman sides are on opposite rainfall schedules for several months a year.
- Recent rainfall and storm warnings close to departure, since tropical cyclones can add unplanned heavy rain even outside the wettest months.
- Warm layers if travelling to the northern hills between November and February.
- The country’s timezone and other planning basics are in the facts box above.