Thailand sits squarely in the tropical monsoon belt of Southeast Asia and has one of the most clearly defined wet-and-dry rhythms in the region. The country stretches roughly 1,600 kilometres from the mountains of Chiang Rai in the north to the islands off Narathiwat in the south, and this long north–south axis means that weather conditions on any given day can differ dramatically between Bangkok, Chiang Mai and Phuket.
Climate
Thailand has a tropical climate dominated by the Asian monsoon system. Temperatures stay warm year-round — average daytime highs rarely drop below 28°C even in the coolest month — while annual rainfall ranges from around 1,200 mm in the inland north to over 4,000 mm on the Andaman coast. Humidity is consistently high, typically 70–90%, which makes the "feels-like" temperature significantly hotter than the thermometer reading during the wet months.
The country is influenced by two competing monsoon flows. The southwest monsoon, driven by warm moist air from the Indian Ocean, brings the main rainy season from roughly mid-May through mid-October. The northeast monsoon, drier air flowing from mainland Asia, dominates November through February and is responsible for the cool, sunny weather that draws most international visitors. A short transition period in March and April sees temperatures spike — April is reliably the hottest month across most of the country, with peaks above 40°C in the central plains.
Thailand sits outside the main western Pacific typhoon track, but tropical storms and typhoons that cross Vietnam occasionally reach the Gulf of Thailand and bring heavy rainfall to the east coast and northeast regions. The country is generally less typhoon-exposed than the Philippines or Vietnam, but coastal flooding and flash floods from monsoon depressions are regular concerns in the southern peninsula.
Seasons
Cool season (November – February)
The most pleasant time of year across most of Thailand. Bangkok daytime highs sit around 31–33°C with cool, dry mornings that occasionally drop to 20°C in the north. Chiang Mai and the northern hill stations can see overnight lows close to 10°C in December and January. Skies are mostly clear and rain is rare — less than 10 mm per month on average in Bangkok.
Hot season (March – May)
Temperatures climb rapidly through March and peak in April. Highs of 38–42°C are common in the central plains and northeast, and humidity stays high, so heat-stress is a real concern. Afternoon thunderstorms become more frequent toward the end of May as the monsoon approaches. Songkran (13–15 April), Thailand's water-throwing New Year festival, is deliberately timed for the hottest week of the year.
Rainy season (June – October)
The southwest monsoon brings the main wet season. Mornings typically start hot and humid before thunderstorms build through the afternoon, often clearing by evening. September is the wettest month for most of Thailand, with flooding risk in Bangkok and the central plains. The Andaman coast (Phuket, Krabi, Khao Lak) is more exposed to the monsoon than the Gulf coast (Koh Samui, Koh Phangan), which is why the two coasts have opposite peak seasons for beach tourism.
Post-monsoon (October – November)
A short transition period as the monsoon retreats. The Gulf of Thailand (Koh Samui, Hua Hin) actually sees its wettest months in October and November as the retreating monsoon pushes rain toward the east. Inland and on the Andaman side, rainfall tapers off and humidity drops through late November.
Extreme weather events
Thailand has experienced several devastating weather events in the modern record. The 2011 monsoon floods inundated much of the central plains and submerged parts of Bangkok for weeks, killing over 800 people and causing an estimated $45 billion in damages. Tropical Storm Pabuk in January 2019 was the first named storm to hit the country in January in decades, making landfall in Nakhon Si Thammarat. Heatwaves in April 2016 and 2023 set regional temperature records above 44°C.
Regional variation
The northern mountains (Chiang Mai, Chiang Rai, Mae Hong Son) are significantly cooler than the rest of the country. Elevations of 400–1,700 metres produce winter overnight lows near 10°C and occasional frost at higher stations. The dry season here is longer and more pronounced than in the south.
The central plains around Bangkok, Ayutthaya and Nakhon Pathom are flat, low-lying and humid. Temperatures are some of the highest in the country during the March–May hot season, and monsoon flooding is a recurring concern because of the Chao Phraya river system.
The northeast (Isan) is drier than the rest of the country and has the biggest seasonal temperature swing. Winter mornings can be surprisingly cold, while summer afternoons regularly exceed 40°C.
The southern peninsula, stretching from Chumphon down to the Malaysian border, has two distinct coastal climates: the Andaman (west) coast is wetter from May to October, and the Gulf (east) coast is wetter from October to December. This is why Phuket and Koh Samui have opposite "best time to visit" windows.
Best time to visit
The best weather window across Thailand as a whole is November through February, when the northeast monsoon brings cool, dry, sunny conditions. For beach trips, the timing depends on the coast: the Andaman side (Phuket, Krabi, Phi Phi, Khao Lak) is best from November to April, and the Gulf side (Koh Samui, Koh Phangan, Koh Tao) is best from January to August. For the northern hills, November through early February offers the coolest and clearest days. Avoid mid-April if you don't like heat — it's the hottest week of the year.
Climate facts
- Bangkok has been named the "hottest city in the world" by annual average temperature in several climate studies — a title it trades with Khartoum and Mecca depending on the metric.
- The wettest place in Thailand, Ranong on the Andaman coast, averages around 4,000 mm of rainfall per year — more than four times Bangkok.
- Thailand receives more than 35 million international tourists in a typical year, and nearly all of them come during the November–February cool season.
- Chiang Mai and the northern mountains typically see their coolest weather in late December, occasionally dropping below 10°C at night — a shock to visitors expecting uniformly tropical weather.