Thailand has three broad seasons: a cool, dry spell from about November to February, a hot season that peaks in April, and a rainy season from about mid-May to October. The big exception is the Gulf coast of the south (Koh Samui and its neighbors), which gets its heaviest rain in October to December, just when the rest of the country dries out.
That split is why there is no single "best month" for all of Thailand. In almost any month, at least one region has good weather. This page shows the pattern region by region and month by month, using the 30-year climate averages (1991–2020) published by the Thai Meteorological Department (TMD). Averages are not forecasts: a single year can be wetter, drier or later than the table, so check the TMD forecast in the days before you travel.
For a short, general answer about seasons and why low season can be worth it, see our older article on the best time to visit Thailand. This page goes into the numbers.
Thailand's climate in brief (TMD averages 1991–2020)
- Driest months, most of the country
- December to February
- Hottest month
- April (Chiang Mai average high 36.7 °C, Bangkok 35.7 °C)
- Rainy season, north, center, Andaman
- About mid-May to October
- Wettest month, Bangkok
- September (335.9 mm), not August
- Wettest month, Koh Samui
- November (444.6 mm)
- Haze season, north
- January to April, worst in March
- Coolest nights
- January in the north (Chiang Mai average low 15.7 °C)
- Weather forecasts and warnings
- Thai Meteorological Department, hotline 1182
Two monsoons explain almost everything
Thailand's weather is driven by two seasonal wind systems. From about mid-May the southwest monsoon blows in from the Indian Ocean. In its monthly summary for May 2025, TMD described rain increasing "due to the influence of the southwest monsoon prevailing over the Andaman Sea and Thailand" and dated the start of the rainy season to 15 May that year. This wind brings rain to the north, the northeast, Bangkok, the east coast and, most heavily, the Andaman coast around Phuket, Krabi and Ranong.
Around the second half of October the wind turns. TMD's October 2025 summary describes the southwest monsoon still prevailing in the first half of the month, followed by a "transition of monsoon system" to easterly winds and the northeast monsoon. That northeast wind is dry and cool over the north and the center, so November to February is the pleasant season for Chiang Mai, Bangkok and the Andaman beaches. But the same wind picks up moisture over the Gulf of Thailand and drops it on the east side of the southern peninsula: Koh Samui, Koh Phangan, Koh Tao, Surat Thani, Nakhon Si Thammarat and Songkhla.
The result is a simple rule of thumb. When Phuket is wet, Samui is often fairly dry, and the other way around. The Tourism Authority of Thailand puts it the same way: when it rains on the Phuket side, it is often sunny on the Samui side. Our guide to the best time for the Thai islands turns that into an island-by-island choice.
Rainfall by region and month
The table gives average monthly rainfall in millimeters at eight TMD weather stations. As a rough guide, under 60 mm is a dry month, 60–150 mm means some showers, and over 250 mm is a properly wet month. The station named Trat lies on the coast of Trat province, the mainland closest to Koh Chang and Koh Kood.
| Month | Chiang Mai (north) | Udon Thani (Isaan) | Bangkok | Pattaya (east) | Trat (east, Koh Chang) | Hua Hin (west Gulf) | Ko Samui (Gulf) | Phuket (Andaman) |
|---|---|---|---|---|---|---|---|---|
| Jan | 11 | 6 | 24 | 16 | 51 | 25 | 127 | 50 |
| Feb | 10 | 21 | 21 | 20 | 82 | 14 | 64 | 24 |
| Mar | 20 | 50 | 51 | 47 | 140 | 51 | 116 | 81 |
| Apr | 54 | 73 | 93 | 68 | 196 | 47 | 83 | 135 |
| May | 168 | 188 | 217 | 121 | 420 | 96 | 132 | 237 |
| Jun | 118 | 219 | 199 | 132 | 777 | 82 | 134 | 249 |
| Jul | 149 | 225 | 190 | 104 | 1,024 | 90 | 117 | 240 |
| Aug | 223 | 299 | 227 | 91 | 970 | 67 | 102 | 309 |
| Sep | 207 | 250 | 336 | 214 | 816 | 130 | 118 | 350 |
| Oct | 124 | 94 | 289 | 225 | 338 | 250 | 295 | 336 |
| Nov | 41 | 14 | 45 | 57 | 85 | 73 | 445 | 178 |
| Dec | 14 | 6 | 12 | 12 | 28 | 12 | 263 | 82 |
| Year | 1,139 | 1,445 | 1,702 | 1,106 | 4,927 | 936 | 1,994 | 2,272 |
Three things stand out. First, the Trat coast is by far the wettest station in the table, with more than a meter of rain in July alone, so Koh Chang and Koh Kood are really dry-season islands. Second, Hua Hin is one of the driest beach stations in the country, with no month above 130 mm except October. Third, Koh Samui never has a truly dry month like Bangkok's December, but its June to August (102–134 mm) is far drier than Phuket's (240–309 mm).
Temperatures: how hot, and how cool the nights get
Heat varies less than rain, but it matters more than many visitors expect. The table shows the average daily high and low in degrees Celsius.
| Month | Chiang Mai high / low | Chiang Rai high / low | Bangkok high / low | Ko Samui high / low | Phuket high / low |
|---|---|---|---|---|---|
| Jan | 30.0 / 15.7 | 28.6 / 13.8 | 32.7 / 23.4 | 29.0 / 24.2 | 33.0 / 25.0 |
| Feb | 32.9 / 16.8 | 31.3 / 14.5 | 33.7 / 24.8 | 29.4 / 24.9 | 34.0 / 25.4 |
| Mar | 35.4 / 20.1 | 33.7 / 17.5 | 34.7 / 26.4 | 30.4 / 25.4 | 34.4 / 25.9 |
| Apr | 36.7 / 23.4 | 34.7 / 21.1 | 35.7 / 27.2 | 31.8 / 26.0 | 34.2 / 26.3 |
| May | 34.7 / 24.2 | 33.5 / 23.0 | 35.1 / 26.9 | 32.9 / 25.7 | 33.2 / 26.1 |
| Jun | 33.2 / 24.5 | 32.5 / 24.0 | 34.1 / 26.4 | 32.7 / 25.3 | 32.6 / 25.8 |
| Jul | 32.1 / 24.3 | 31.3 / 23.9 | 33.5 / 26.1 | 32.3 / 25.1 | 32.3 / 25.6 |
| Aug | 31.6 / 24.0 | 30.9 / 23.7 | 33.3 / 25.9 | 32.4 / 25.1 | 32.2 / 25.6 |
| Sep | 32.0 / 23.7 | 31.4 / 23.1 | 33.2 / 25.4 | 31.9 / 24.8 | 31.8 / 25.1 |
| Oct | 31.7 / 22.6 | 30.9 / 21.5 | 33.0 / 25.2 | 30.6 / 24.3 | 31.8 / 24.9 |
| Nov | 30.8 / 19.9 | 29.6 / 17.9 | 33.1 / 24.7 | 29.7 / 24.1 | 32.1 / 25.2 |
| Dec | 29.2 / 16.9 | 27.8 / 14.7 | 32.3 / 23.2 | 29.3 / 24.0 | 32.1 / 25.0 |
Bangkok and the beaches stay between about 23 and 36 °C (73–97 °F) all year. The north is different: a January night in Chiang Rai averages under 14 °C (57 °F), and TMD recorded an extreme low of 1.5 °C there in December. Mountain areas outside the cities are colder still, so bring a warm layer for the north between December and February, especially for early-morning viewpoints.
The opposite end of the year is the hard part. TMD declared the 2024 hot season from 21 February, and the environment ministry's summary of that forecast put the highest heat index "from April to early May". Thai authorities use a four-level heat index scale, from Caution at 27 °C to Extreme Danger at 52 °C and above; Bangkok's heat index passed 52 °C on 4 May 2026. Our guide to coping with heat and sun covers what to do about it.
November to February: the cool, dry season
November is when most of the country turns dry. Bangkok drops from 289 mm in October to 45 mm, Chiang Mai to 41 mm and Udon Thani to just 14 mm. The Andaman coast is drying out (Phuket 178 mm, still showery), and the national marine parks off Phang Nga reopen in mid-October. But November is the wettest month of the year on Koh Samui, with 444.6 mm on average.
December is the driest month in Bangkok (11.6 mm), Pattaya and Hua Hin, and nights in the north are cool. It is also peak season, when rooms are usually at their most expensive. Samui is still wet (263 mm), though less so than November.
January and February are the most reliable months for the Andaman coast: Phuket averages 24 mm in February, its driest month. Samui improves quickly, and February (64 mm) is its driest month. In the north, January has the coldest nights, and from January onward the haze starts to build.
March to May: heat, haze and the first storms
March is hot almost everywhere and, in the north, it is the worst month for haze. TMD counts days with haze at each station: Chiang Mai averages 18.7 such days in January, 22.1 in February, 26.8 in March and 17.1 in April, and Chiang Rai 25.2 in March. Most of this is smoke from farmers burning fields and forest, which the UK Foreign Office says affects both Chiang Mai and Bangkok and "may worsen bronchial, sinus or asthma conditions". Chiang Mai province declares a burning-control period each dry season, but the haze has not gone away. Our older explainer on Chiang Mai's smoky season covers why the valley traps it.
April is the hottest month in Chiang Mai (average high 36.7 °C), Bangkok (35.7 °C) and inland towns. The Songkran water festival on 13–15 April lands in the middle of it. The sea is usually calm on both coasts, and April is still a good month for the Gulf islands (Samui 83 mm).
May is the turning point. Thunderstorms arrive in the afternoon or evening, the haze clears in the north (Chiang Mai haze days fall to 1.5), and the Andaman Sea gets rougher. The Similan and Surin marine parks close on 15 May each year and do not reopen until 15 October.
Haze in the north is measured, not imagined. If you or a child has asthma or a heart or lung condition, think twice about Chiang Mai, Chiang Rai and Mae Hong Son between February and April. Check a live air-quality index before you book a trek, and keep an indoor plan B.
June to September: the southwest monsoon
This is the rainy season for most of Thailand, but how wet depends on where you are. In Bangkok the rain builds through the season to the wettest month, September (335.9 mm on 21.4 days). The north peaks in August (Chiang Mai 223.3 mm, Chiang Rai 372.9 mm). In Isaan, Udon Thani gets 299 mm in August.
On the Andaman coast the sea is the bigger issue than the rain. The UK Foreign Office warns that people have drowned in strong riptides, especially in monsoon season, and lifeguards fly red flags on Phuket's west-coast beaches when swimming is unsafe. Maya Bay on Koh Phi Phi Le closes every year from 1 August to 30 September. Read our page on rip currents and beach flags before you swim.
The Gulf islands are the dry-ish option: Samui gets 102–134 mm a month from June to August. The catch there is jellyfish. The Department of Marine and Coastal Resources has reported box jellyfish off Koh Samui, Koh Phangan and Koh Tao from July to October; see our page on box jellyfish in Thailand.
The east coast splits in two. Pattaya stays moderate (91–132 mm a month in June to August), while the Trat coast near Koh Chang gets 777–1,024 mm a month. If Koh Chang is on your list, it is a November-to-April island.
October: the month that changes
October is the most mixed month of the year. In the first half the southwest monsoon is usually still active, and October is the wettest month at Hua Hin (250 mm), Krabi (292.9 mm) and Pattaya (225 mm), and still very wet in Bangkok (289 mm) and Phuket (336 mm). In the second half the wind turns, the north and the center start to dry out, and the rain moves to the Gulf side of the south, where Samui jumps from 118 mm in September to 295 mm.
Flooding is a real risk in this period. In late September 2026, heavy rain led the government to declare special holidays on 28 and 29 September in Bangkok, Nonthaburi, Pathum Thani and Samut Prakan, and Khao Yai National Park closed after floods destroyed a bridge. Do not assume floods are a thing of the past. Our page on travelling in the rainy season covers flood risk, ferries and what low season means for prices.
Where to go, month by month
This is our reading of the averages above, not a guarantee. "Good" means low rain and calm seas on average; "Mixed" means showers or rough spells are common; "Wet" means a high chance of rain on many days.
| Month | North (Chiang Mai, Chiang Rai) | Bangkok and center | East coast and Koh Chang | Gulf islands (Samui, Phangan, Tao) | Andaman (Phuket, Krabi, Lanta) |
|---|---|---|---|---|---|
| Jan | Good, cool nights, haze starts | Good | Good | Mixed | Good |
| Feb | Hazy | Good, warming | Good | Good | Good |
| Mar | Worst haze, hot | Hot | Good, hot | Good | Good |
| Apr | Very hot, hazy | Hottest month | Mixed | Good | Mixed, storms begin |
| May | Storms, haze clears | Wet spells | Wet | Mixed | Wet, rough sea |
| Jun | Wet, green | Wet | Wet (Trat very wet) | Mixed, often fine | Wet, rough sea |
| Jul | Wet | Wet | Wet (Trat very wet) | Mixed, often fine | Wet, rough sea |
| Aug | Wettest | Wet | Wet (Trat very wet) | Mixed, often fine | Wet, Maya Bay closed |
| Sep | Wet | Wettest month | Wet | Mixed | Wettest, Maya Bay closed |
| Oct | Drying out | Wet, flood risk | Wet early, drying | Wet | Wet, improving late |
| Nov | Good | Good | Good | Wettest month | Mixed, improving |
| Dec | Good, cool | Good, driest | Good | Wet | Good |
Two months need a word of warning. In November, travelers who booked Samui for a "dry season" beach week often find themselves in the island's wettest month; choose Phuket, Krabi or Hua Hin instead. In September, the reverse applies: Samui and Koh Tao are usually the better beach bet. Public holidays also shape the calendar, so check our guide to Thai public holidays before booking around Songkran or New Year.
Frequently asked questions
What is the best month to visit Thailand overall?
For a trip that combines Bangkok, the north and an Andaman beach, December to February is the most reliable window: low rain, calm seas and cool nights in the north. Expect peak-season prices. February is also the driest month on Koh Samui.
Which month has the most rain in Bangkok?
September. The TMD 1991–2020 average for Bangkok is 335.9 mm in September, followed by 288.7 mm in October. August, which many sites name as the wettest, averages 227.1 mm. December is the driest month, with 11.6 mm.
Is Koh Samui rainy at the same time as Phuket?
No. Phuket's wettest months are September and October, while Koh Samui's are October to December, with November the peak at 444.6 mm. From June to August Samui averages 102–134 mm a month, against 240–309 mm in Phuket.
When is the smoky season in Chiang Mai?
Roughly January to April, worst in March. TMD records an average of 26.8 days with haze in Chiang Mai in March. The timing shifts from year to year with rain and wind, so check a live air-quality index close to your trip.
How cold does it get in northern Thailand?
In January the average night-time low is 15.7 °C in Chiang Mai and 13.8 °C in Chiang Rai. Cold snaps go lower: TMD's lowest reading at Chiang Rai in the 30-year period was 1.5 °C. Mountain viewpoints are colder than the towns.
Sources
- Thai Meteorological Department – Climatological Data of Thailand for 30 Year Period (1991–2020) (checked October 2026)
- Thai Meteorological Department – Monthly climate summary, May 2025 (checked October 2026)
- Thai Meteorological Department – Monthly climate summary, October 2025 (checked October 2026)
- Tourism Authority of Thailand – The best time to visit Thailand's beaches and islands (checked October 2026)
- UK Foreign, Commonwealth and Development Office – Thailand: health (checked October 2026)
- UK Foreign, Commonwealth and Development Office – Thailand: safety and security (checked October 2026)
- Ministry of Natural Resources and Environment – Heat index warning levels (checked October 2026)
- Nation Thailand – Bangkok heat index warning, May 2026 (checked October 2026)
- Department of National Parks – Maya Bay seasonal closure 2026 (checked October 2026)
- Tourism Authority of Thailand – Special public holidays on 28–29 September 2026 (checked October 2026)