Thailand is warm all year, but it does not have the same weather everywhere at the same time. In September, when Bangkok has its heaviest rain, Koh Samui is still in its drier period. In November, when Koh Samui has its wettest month, the rains in Bangkok have mostly ended. If you know the pattern, you can choose where to go, what to pack and how much free time to leave in your plans.
This guide uses the official seasons of the Thai Meteorological Department (TMD) and the 1991–2020 climate averages for Bangkok, Chiang Mai, Phuket, Koh Samui and Pattaya. Averages describe a typical year, not a forecast.
Thailand's three seasons
| Season | Official dates | What to expect |
|---|---|---|
| Hot season | Mid-February to mid-May | Typical highs of 35–39 °C; the hottest days pass 40 °C |
| Rainy season | Mid-May to mid-October | South-west monsoon; showers and thunderstorms |
| Cool season | Mid-October to mid-February | North-east monsoon; drier and less humid, cool nights in the north |
Many travel sites say the rainy season runs from June to October, but officially it starts in mid-May. In 2026, TMD declared the rainy season open on 15 May. The main exception is the Gulf coast of the south (Koh Samui, Nakhon Si Thammarat and Songkhla), where the heaviest rain falls from October to December.
Regional weather differences
Bangkok and central Thailand
| Month | Average daily high | Average rainfall | Average humidity |
|---|---|---|---|
| January | 32.7 °C | 23.6 mm | 67.9% |
| April | 35.7 °C | 93.3 mm | 72.0% |
| July | 33.5 °C | 189.7 mm | 76% |
| October | 33.3 °C | 288.7 mm | 78.0% |
- Nights stay warm: the average daily low is 23.4 °C in January and 27.2 °C in April.
- September is the wettest and most humid month: 335.9 mm of rain, with rain on about 18 days.
- December is the driest month (11.6 mm), and in December and January it rains on only one or two days.
- April is the hottest month. A day of 40 °C is possible but rare: the city's records are 40.0 °C in April and 41.0 °C in May.
Northern Thailand (Chiang Mai)
| Month | Average daily high | Average daily low | Average rainfall |
|---|---|---|---|
| January | 30.0 °C | 15.7 °C | 11.3 mm |
| April | 36.7 °C | 23.4 °C | 54.3 mm |
| July | 32.1 °C | 24.3 °C | 148.6 mm |
| October | 31.7 °C | 22.6 °C | 123.8 mm |
- In the cool season, days are warm and nights are cool. The city's record low is 3.7 °C and its record high is 42.5 °C.
- August is the wettest month (223.3 mm). The total for the year, about 1,139 mm, is much lower than Bangkok's 1,702 mm.
- The mountains are colder. On 27 November 2025, Doi Inthanon (2,565 m, Thailand's highest peak) had frost, with 2 °C at the summit and −2.3 °C at the coldest point.
The south: Phuket and Koh Samui
The Andaman coast and the Gulf coast of the south have almost opposite seasons. This table shows the average rainfall for each month.
| Month | Phuket (Andaman coast) | Koh Samui (Gulf coast) |
|---|---|---|
| January | 49.9 mm | 126.8 mm |
| February | 24.0 mm | 64.3 mm |
| July | 240.2 mm | 117.0 mm |
| September | 350.4 mm | 117.5 mm |
| October | 336.0 mm | 294.9 mm |
| November | 178.2 mm | 444.6 mm |
| December | 82.0 mm | 263.1 mm |
- Phuket: February is the driest month, with rain on about 2 days. September and October are the wettest months, with rain on about 19 days each. The average daily high stays between 31.8 and 34.4 °C all year.
- Koh Samui: February to September is the drier period, and October to December is the wettest period. Humidity stays high all year, at 78–86%.
Eastern seaboard (Pattaya)
Pattaya is drier than Bangkok, with about 1,106 mm of rain a year. The wettest months are October (224.5 mm) and September (213.6 mm), and January is very dry (16.3 mm).
The two monsoons
The south-west monsoon (mid-May to mid-October) brings the rainy season to most of the country, including Bangkok, the north and the Andaman coast. It does not usually rain all day, and many days stay partly dry.
Two famous marine sites close during this monsoon:
- The Similan Islands (Mu Ko Similan National Park) are closed from 16 May to 14 October every year. The 2026–27 season opens on 15 October 2026.
- Maya Bay on Koh Phi Phi Le is closed every year from 1 August to 30 September so that its reef can recover. In 2026 it reopens on 1 October. Boat tours to the Phi Phi Islands continue during the closure, but they may not enter the bay.
The north-east monsoon (mid-October to mid-February) brings the cool season to the north and the centre. The Gulf coast of the south does not follow this pattern: Koh Samui, Nakhon Si Thammarat and Songkhla get their heaviest rain from October to December. In late November 2025, the rains of Cyclone Senyar flooded southern Thailand, and Songkhla, the province of Hat Yai, was declared a disaster zone.
The 2026 rainy season: what TMD expects
TMD's forecast for the 2026 rainy season said:
- Total rainfall about 10% below normal.
- A dry spell from late June to early July.
- The heaviest rain, and the highest risk of flash floods, in August and September.
- One or two tropical storms, most likely in August or September and mainly in the north and north-east.
- Rain in upper Thailand ending about mid-October, and rain on the Gulf coast of the south continuing until December.
For a trip from October to December 2026, the last point matters most: the north and the centre become drier, while the Gulf coast of the south stays wet. The rains start and end at different times each year, so check the TMD forecast in the days before you travel.
Weather hazards and how to handle them
- Thunderstorms: plan outdoor sightseeing for the morning and keep indoor options for later in the day. Do not shelter under trees in a thunderstorm.
- Heat: drink water often and rest in the shade or in air conditioning at midday. Confusion and hot, dry skin are signs of heat stroke; call 1669 for a medical emergency.
- Floods: do not walk or drive through floodwater. After the floods and mudslides in the north in September 2024, including in Mae Sai (Chiang Rai), Thailand built T-Alert, run by the Department of Disaster Prevention and Mitigation (DDPM). It sends a message with an 8-second alarm to phones in the affected area. If a T-Alert message appears, follow its instructions. If you travel to the Gulf coast of the south from October to December, leave free days in your plans.
- Smoke and haze: in April 2026, Chiang Mai, Lamphun and Phayao provinces were declared disaster zones for air pollution, and on 7 April Chiang Mai ranked as the most polluted city in the world. In March and April, check PM2.5 (fine dust) levels on the free Air4Thai or IQAir AirVisual apps before you plan outdoor days in the north.
Month-by-month overview
| Months | Bangkok and the north | Phuket (Andaman coast) | Koh Samui (Gulf coast) |
|---|---|---|---|
| December–February | Cool season, dry; cool nights in the north | Drier; February is the driest month | Still wet in December and January; drier from February |
| March–April | Hot; haze in the north | Rain starts to increase | Drier period |
| May–July | Rainy season from mid-May | Wet; Similan Islands closed | Drier period |
| August–September | Heaviest rain | Wettest period; Maya Bay closed | Moderate rain |
| October–November | Rain ends about mid-October | Still wet; Similan Islands open from 15 October | Wettest period |
Weather and activities
- Beaches: Phuket is best from December to March, and Koh Samui is driest from February to September. Since August 2021, sunscreen that contains oxybenzone has been banned in all marine national parks, such as the Similan Islands and Koh Phi Phi. The fine can be up to 100,000 THB, so bring sunscreen without oxybenzone.
- Mountains and hiking: Phu Kradueng National Park (Loei) closes every year from June to September. The Kew Mae Pan nature trail on Doi Inthanon closes every year from June to October and reopens on 1 November; the summit, the chedis and the waterfalls stay open in the rainy season. For a mountain trip in December or January, pack a warm jacket, long trousers and closed shoes. On cold mornings, the road to the summit of Doi Inthanon can be slippery with frost.
- Packing for the rainy season: quick-dry clothes, sandals with grip, a dry bag for your phone and passport, and a light layer for air-conditioned buses and malls.
Travelling on rainy days
In heavy rain, it is hard to wait at a bus stop with luggage. Smile Taxi Service drives door to door at fixed fares per vehicle, not per person, one way, with expressway tolls included:
- Suvarnabhumi Airport to Bangkok: 600 THB by sedan, 800 THB by SUV, 1,100 THB by van. Book a sedan from Suvarnabhumi Airport to Bangkok.
- Bangkok to Pattaya: 1,500 THB by sedan, 1,700 THB by SUV, 2,000 THB by van. Book a sedan from Bangkok to Pattaya.
- Phuket Airport to Patong Beach: 1,000 THB by sedan, 1,100 THB by SUV, 1,400 THB by van. Book a sedan from Phuket Airport to Patong Beach.
A member of the team checks your flight, and airport pickups include 60 minutes of free waiting, counted from the actual landing time. You pay the driver in cash (THB) at the end of the trip; if you can only pay by card, message the team for a PayPal payment link (a 5% fee is added). Cancellation is free up to 24 hours before pickup. After that, a cancellation or change costs 25% (less than 24 h), 35% (less than 12 h) or 50% (less than 2 h) of the price, and a no-show 100%, also when you pay the driver in cash (details). See all Smile Taxi routes and fares, or read our guides on how to get from Bangkok to Phuket and how to get from Bangkok to Koh Samui.
Weather information and emergency numbers
| Service | Number |
|---|---|
| TMD weather hotline | 1182 |
| Disaster hotline (DDPM) | 1784 |
| Tourist Police | 1155 |
| Police and general emergency | 191 |
| Medical emergency | 1669 |
| Marine Department (boats and piers) | 1199 |
The TMD website in English has 7-day forecasts, radar and heavy-rain warnings. TMD's free apps, "Thai Weather" and "TMD พยากรณ์" (TMD Forecast), are both available in English.
FAQ
When is the rainy season in Thailand?
Officially from mid-May to mid-October; in 2026 it began on 15 May. On Koh Samui and the rest of the Gulf coast of the south, the rain lasts until December.
Which is the wettest month in Bangkok?
September, with about 336 mm of rain on average. October follows with about 289 mm.
Is the weather on Koh Samui the opposite of the weather in Phuket?
Largely, yes. Phuket is wettest from August to October, with about 350 mm in September. Koh Samui is wettest from October to December, with about 445 mm in November.
Does it rain all day in the rainy season?
Not usually. Bangkok has rain on about 18 days in September, its wettest month, so many days stay partly dry.
Do I need warm clothes for Thailand?
Only for the north and the mountains from about November to February. Chiang Mai nights average 15.7 °C in January, and mountain summits can drop below 0 °C.
Planning a trip in any season? Message us on WhatsApp or LINE, or book your transfer online.