Map from Google - Thailand

Thailand's climate is tropical, with three distinct seasons: a hot season from March to mid-May, a rainy season caused by the southwest monsoon, which generally runs from mid-May to October, and a dry and relatively cool season from November to February, when the north-east monsoon, coming from the Asian continent, prevails. However, the relatively cool season is felt in the north and in inland areas, while on the coasts and in the south it's hot even in winter.
In the southern peninsular region, where beaches and renowned resorts are found, the rainy season has a different pattern than in the continent, which will be discussed later.
The map below shows the climate zones of Thailand, based mainly on the rain pattern.

Thailand - climatic zones


Thailand - inland plains

In inland northern plains, and in the valleys of the north-western hilly area, the climate is hot for most of the year, except in winter, when it's sunny but a bit cool at night, especially in December and January, when night-time temperatures drop to around 15 °C (59 °F) in the centre and 12/13 °C (54/55 °F) in the north. When cold air masses arrive from China, in northern cities such as Chiang Mai, in December and January the night temperature can drop to around 5 °C (41 °F), and in some cases even below. But the continentality is evident also in the opposite direction, as regards the heat, starting from February, when the daytime temperature often reaches 33/34 °C (91/93 °F), then rises up to 35/37 °C (95/99 °F) between March and May, with peaks above 40 °C (104 °F). Here are the average temperatures of Chiang Mai.
Average temperatures - Chiang Mai
Chiang MaiJanFebMarAprMayJunJulAugSepOctNovDec
Chiang MaiJFMAMJJASOND
Min (°C)141619222424242423221915
Max (°C)293235363433323132313028
Min (°F)576166727575757573726659
Max (°F)849095979391908890888682

The monsoon rains go to mid-May to mid October. Annual precipitation in the inland plain goes from 1,000 to 1,500 millimetres (40 to 60 inches), with a similar pattern: scarce and rare rains from mid-November to April, and heavy rains during the monsoon period, with a peak at the end (August and September). Here is the average precipitation in Chiang Mai.
Average precipitation - Chiang Mai
Chiang MaiJanFebMarAprMayJunJulAugSepOctNovDecYear
Chiang MaiJFMAMJJASONDY
Prec. (mm)1010205515511515522520012050201125
Prec.(in)0.40.40.82.26.14.56.18.97.94.720.844.3
Days112615161821171252115

The best time in this area corresponds to the months of December and January, since it often happens that in February the heat is already intense, especially in the south-central part of the plain.


Thailand - Bangkok and surrounding areas

In the capital Bangkok, the climate is hot all year round. Between mid-November and January, the weather is good, usually with clear skies. Highs are around 30/32 °C (86/90 °F), and lows around 20/22 °C (68/72 °F). In fact it's hot, even though it's winter. In February, temperatures start to rise; the weather remains sunny, although occasionally a few short thunderstorms can develop.
Between March and mid-May, before the arrival of the monsoon, the heat becomes intense: highs can reach 37/38 °C (99/100 °F), and even at night it continues to be hot, in fact the night-time temperature may remain around 28/29 °C (82/84 °F). The capital is not as hot as the north-central inland areas, but in return it's more humid, due to the proximity to the sea, moreover the heat is trapped within the big city (a phenomenon known as urban heat island effect). There is nothing left but to take refuge in air-conditioned rooms.
Here are the average temperatures.
Average temperatures - Bangkok
BangkokJanFebMarAprMayJunJulAugSepOctNovDec
BangkokJFMAMJJASOND
Min (°C)212325262625252525242321
Max (°C)323334353433333332323231
Min (°F)707377797977777777757370
Max (°F)909193959391919190909088

Around mid-May, the monsoon arrives, and the temperature starts to decrease; the weather becomes often cloudy, the humidity rises further, but at least the rains, in the form of showers and thunderstorms, sometimes intense, bring a bit of relief. Anyway, even in these summer months, in intervals between periods of bad weather, very hot days may still occur.
Throughout the year, 1,450 mm (57 in) of rain fall; the rainiest months are September and October, at the end of the monsoon season, with 345 mm and 240 mm (13.5 and 9.5 in) of rain respectively.
Here is the average precipitation.
Average precipitation - Bangkok
BangkokJanFebMarAprMayJunJulAugSepOctNovDecYear
BangkokJFMAMJJASONDY
Prec. (mm)1030306522015015519534524050101500
Prec.(in)0.41.21.22.68.75.96.17.713.69.420.459.1
Days122413121315181451100

The best time in Bangkok is from December to mid-February, being the least hot and outside of the rainy season.

Bangkok

South-east of Bangkok, Pattaya has a climate similar to that of the capital, but it is located on a stretch of coast (down to Sattahip) relatively sheltered from the summer rains: from June to August, an average of about 100 mm (4 in) of rain per month fall.
Here is the average precipitation.
Average precipitation - Pattaya
PattayaJanFebMarAprMayJunJulAugSepOctNovDecYear
PattayaJFMAMJJASONDY
Prec. (mm)151555651501209510020521570101115
Prec.(in)0.60.62.22.65.94.73.73.98.18.52.80.443.9
Days1346121111121718101106

Of course, we are still in a hot and muggy season, where the amount of sunshine is not great, and sometimes tropical storms and typhoons can pass (see below), in short the weather in these months is not so inviting, but if you are lucky it may be acceptable. The rainiest months in Pattaya are May, September and October, that is, the beginning and the end of the rainy season. The best time here is from December to February, with March and April still dry and sunny, but a little hot (though not as hot as in the interior).
Here are the average temperatures.
Average temperatures - Pattaya
PattayaJanFebMarAprMayJunJulAugSepOctNovDec
PattayaJFMAMJJASOND
Min (°C)232425262727262625242322
Max (°C)313132333232313131313030
Min (°F)737577798181797977757372
Max (°F)888890919090888888888686

Even the coastal stretch to the south-west of Bangkok, down to Ao Noi, is quite sheltered, in fact rainfall in summer is not copious, and the total annual precipitation is about 1,000/1,200 mm (40/47 in).


Thailand - southeast coast

South-east of Pattaya, starting from Rayong, the summer rains become more abundant, and are plentiful in the easternmost portion (see Chanthaburi, Trat, Koh Chang), where summer is definitely not recommended. In Rayong, 1,700 mm (67 in) of rain per year fall, and in Koh Samet 1,340 mm (53 in), while in Chanthaburi, precipitation reaches as high as 2,800 mm (110 in), and in Trat even 3,500 mm (138 in).
Here too, the best time is from December to February; in March and April it's warmer, but the weather is sunny, despite the first short pre-monsoon showers.


In Peninsular Thailand, the climate is hot all year round. The heating between March and May is less intense than in the continental region, and on average these months are slightly warmer than in the rest of the year: highs are around 33/34 °C (91/93 °F) instead of 31/32 °C (88/90 °F), although sometimes it can get very hot here too. Additionally, in the peninsular area, the rainy season occurs at different times depending on the slope. Let's see when.

Thailand - east coast

Along the east coast, on the Gulf of Thailand (or Gulf of Siam), from Chumphon to the south, the summer monsoon is not felt in a particularly intense way (so that summer is considered high season and in this period the area is frequented by several tourists), while the so-called retreating monsoon, in the early months (October to December), discharges heavy rains, sometimes torrential, because the sea is still warm, and provides energy and moisture.
Here is the average precipitation in Ko Samui.
Average precipitation - Ko Samui
Ko SamuiJanFebMarAprMayJunJulAugSepOctNovDecYear
Ko SamuiJFMAMJJASONDY
Prec. (mm)1254055851701201451351402754651901945
Prec.(in)4.91.62.23.36.74.75.75.35.510.818.37.576.6
Days94481716161618202015163

Sometimes the rains continue until the beginning or the middle of January, so in this area, which includes Ko Samui, Pattani and Songkhla, the best time goes from the middle or the end of January to April (with a preference for February, because even here March and April are a bit hot).
Here are the average temperatures in Ko Samui.
Average temperatures - Ko Samui
Ko SamuiJanFebMarAprMayJunJulAugSepOctNovDec
Ko SamuiJFMAMJJASOND
Min (°C)242526262626252525242424
Max (°C)293031323332323232313029
Min (°F)757779797979777777757575
Max (°F)848688909190909090888684

Thailand - coast of the Andaman Sea

Along the west coast, on the Andaman Sea (see Similan Islands, Phuket, Krabi, Phi Phi Islands, Koh Lanta, Koh Lipe), summer is very rainy, since the monsoon hits the coast directly from the sea, while there's little rain from December to March. Here is the average precipitation in Phuket.
Average precipitation - Phuket
PhuketJanFebMarAprMayJunJulAugSepOctNovDecYear
PhuketJFMAMJJASONDY
Prec. (mm)354075125295265215245325315195802210
Prec.(in)1.41.634.911.610.48.59.612.812.47.73.187
Days43615191917171919148160

In addition, during the summer monsoon the sea can be rough, and reaching the islands by boat can be difficult.
So here the best time is from December to March; in April, pre-monsoon showers are more frequent than elsewhere, even though the amount of sunshine is not affected by much.
Average temperatures - Phuket
PhuketJanFebMarAprMayJunJulAugSepOctNovDec
PhuketJFMAMJJASOND
Min (°C)232324252525252424242424
Max (°C)313233333131313130313131
Min (°F)737375777777777575757575
Max (°F)889091918888888886888888

Phuket

In Thailand, the sea is warm all year round, as you can see from the water temperature in Ko Samui.
Sea temperature - Ko Samui
Ko SamuiJanFebMarAprMayJunJulAugSepOctNovDec
Ko SamuiJFMAMJJASOND
Sea (°C)282829303030292929292928
Sea (°F)828284868686848484848482


Thailand - mountainous areas

In the western part of Thailand, along the border with Burma, and in the north along the border with Laos, there are mountain ranges, with peaks exceeding 2,000 metres (6,500 feet). Above 1,000 metres (3,300 feet), the climate is cool in winter, with mild days and cold nights, and comfortably warm in the rest of the year. In mountainous areas, the monsoon rains are generally more intense, and even in the period before the monsoon itself, some showers may occur in the afternoon, so this area is usually covered by rainforests. In the valleys between the mountains, major rivers such as the Mekong and the Salween flow, which afterwards reach the plain.


Let's finally mention tropical cyclones, which can hit Thailand from May to November, but more often from August to October. Cyclones coming from the Pacific, known as typhoons, follow a trajectory from east to west: when they pass to the north, they affect the continental part of Thailand after discharging the bulk of energy on Vietnam, Laos and Cambodia, however, they can still bring heavy rains; when they follow a more southern trajectory, they can affect more directly the peninsular part of Thailand.
On the other hand, cyclones coming from the Indian Ocean, move from the southwest to the northeast, and can hit the continental part of Thailand after having affected in a more direct way Burma, or they can directly hit Peninsular Thailand.
In general, Thailand is not hit as hard as other countries of Southeast Asia, such as the Philippines or Vietnam. A typhoon that hit the country in a particularly hard way was Gay, which affected the Gulf of Thailand on early November 1989; another one was Forrest in mid-November 1992.
If it were not for the risk of cyclones, November would be a good month in many parts of the country.

Cyclones and typhoons in Thailand

El Niño and La Niña

The climate of Thailand is affected by the cycle that includes the periods of El Niño and La Niña (called ENSO). In particular, during the years of El Niño, the December-February period is drier than normal in the part of Peninsular Thailand exposed to the rains (the eastern one), while the rest of the country, which in this period is normally dry, becomes hotter than normal. On the contrary, in the years of La Niña, the December-February period may be more rainy in Peninsular Thailand, even in areas that are normally dry. For example in January 2012 (a year of a weak La Niña), 325 mm (12.8 in) of rain fell in Ko Samui (which, however, is on the east side, which in this period is still quite rainy), and even 475 mm (18.7 in) in Krabi. In March 2011, a year of a moderate La Niña, things got much worse, so much so that there were floods in the south of the country: 435 mm (17.1 in) fell in Krabi, of which 185 mm (7.3 in) in a single day, 230 mm (9 in) in Ko Lanta, 375 mm (14.8 in) in Phuket, 560 mm (22 in) in Songkhla, 785 mm (31 in) in Surat Thani, and even 1,135 mm (44.7 in) in Ko Samui! Especially in the last two weeks of the month, the rains occurred almost daily, and some tourist resorts were closed. Also in January, however, it had rained enough: 135 mm (5.3 in) in Krabi and 290 mm (11.4 in) in Ko Samui.


As explained more fully above, the best time to visit Thailand is from December to mid-February in the mainland and in Bangkok, from December to March in the south-western coastline (see Similan Islands, Phuket, Krabi, Phi Phi Islands, Koh Lanta, Koh Lipe), and from late January to April in the southern coasts on the Gulf of Thailand (see Ko Samui, Pattani and Songkhla), bearing in mind, however, that March and April are hot months.

What to pack

In winter: in the north (Chiang Mai, Chiang Rai), light clothes for the day, a jacket and a sweater for the evening; in Bangkok, light clothing, a light sweatshirt for the evening, a scarf for the breeze, possibly a light jacket or sweater for cooler evenings. In Peninsular Thailand, light clothing, a light sweatshirt for the evening, a scarf for the breeze, a sweatshirt for airconditioned places. In the Gulf of Thailand, you can add a light raincoat or an umbrella.
For the reef, equipment for snorkeling, water shoes or rubber soled shoes.
For the mountains, spring/autumn clothes, a warm jacket and a sweater for the evening.
In summer: tropics-friendly, lightweight clothing, light raincoat or umbrella, comfortable shoes; a light sweatshirt and a scarf for airconditioned places. In the mountains, light clothes for the day, raincoat, hiking shoes, sweater for the evening and for mountain peaks.
Before entering pagodas, it is customary to remove shoes, dress neatly and cover a little.