Introduction

Thailand's Climate can be described as tropical monsoon climate. It is characterized by strong monsoon influences, has a considerable amount of sun, a high rate of rainfall, and high humidity that makes it sometimes feel quite uncomfortable. 
The annual average temperature ranges from 22°C to 27°C year-round. There are two distinguishable seasons in Thailand, a dry period in the winter and a humid rain period in the summer.
 Required clothing:
Lightweight cotton clothing is advised throughout the year, with an umbrella or raincoat for sudden cloudbursts. Be prepared for high temperatures and humidity, no matter where you go.
koeppen-geiger classification:
The Climate of Thailand can be classified as Aw climate, a hot, tropical climate with all months above 18°C and a dry period in the winter. The southern coast of Thailand has an Af climate, a hot, humid climate with all months above 18°C.

The north

In the northern inland 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. 
Chiang Mai
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 felt in the opposite direction starting from February, when the daytime temperature often reaches 33/34 °C (91/93 °F), and then it 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. 
The monsoon rains go to mid-May to mid October. Annual precipitation in the inland plain is between 1,000 and 1,500 millimetres (40 and 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. The amount of sunshine in Chiang Mai is very good in the dry season, from December to April, when clear skies prevail, while it's not high in the monsoon period, from June to September, since there are several cloudy periods. 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. 

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), so 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 because of 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. Around mid-May, the monsoon arrives, and the temperature starts to decrease. In this period, the weather is 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, in these summer months, in the intervals between periods of bad weather, very hot days may still occur. 
Throughout the year, 1,450 mm (57 in) of rainfall; 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. Also in Bangkok the amount of sunshine is very good in the dry season, from December to April, when clear skies prevail, while it's not high in the monsoon period, from June to September, since there are several cloudy periods. The best time in Bangkok is from December to mid-February, since it is the least hot and outside of the rainy season. 

Pattaya

South-east of Bangkok, Pattaya has a climate similar to that of the capital, but it is located in 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 rainfall per month. Of course, we are still in a hot and muggy season, when the amount of sunshine is not great, and sometimes tropical storms and typhoons can affect the area (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). The coastal stretch to the south-west of Bangkok, down to Ao Noi, is quite sheltered as well, in fact the rainfall in summer is not copious, and the total annual precipitation is about 1,000/1,200 mm (40/47 in). 

 

The rainy coast

South-east of Pattaya, starting from Rayong, the summer rains are 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 fall per year, 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 up to 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.

Peninsular Thailand

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. 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 much so that summer is considered high season, and in this period the area is frequented by many tourists. 

On the contrary, the so-called retreating monsoon, in the early months (October to December), when the sea is still warm and able to provide energy and moisture, brings heavy rainfall, sometimes torrential. 

 

Ko Samui

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 is from the middle or the end of January to April (with a preference for February, since even here March and April are a bit hot). 
Here are the average temperatures in Ko Samui. Despite the different rainfall pattern, the sunshine has a relative minimum in the period June-October on the east coast as well, although it remains quite low even from November to January. 

 

In Ko Samui, and in general in Thailand, the sea is warm all year round, as you can see from the water temperature near Ko Samui. 

Along the west coast, on the Andaman Sea (see the Similan Islands, Phuket, Krabi, the 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. 

Phuket

In addition, during the summer monsoon the sea can be rough, and reaching the islands by boat can be difficult. 
The amount of sunshine on the west coast is quite limited in the May-October period. So here the best time is from late December to March; in April, pre-monsoon showers are more frequent than elsewhere. Here are the average temperatures in Phuket. 

 

The mountains

 

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. 

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 up to 475 mm (18.7 in) in Krabi. In March 2011, a year of a moderate La Niña, things got much worse, in fact 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 up to 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.

When to go

As explained more fully above, the best time to visit Thailand is from December to mid-February in the mainland and in Bangkok, from late December to March on the south-western coastline (see the Similan Islands, Phuket, Krabi, the Phi Phi Islands, Koh Lanta, Koh Lipe), and from late January to April on 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. 
In mid-summer, in July and August, in the eastern part of peninsular Thailand, that is, the coast facing the Gulf of Thailand, the weather is usually comfortable enough, since the area is sheltered from the most intense monsoon rains, which occur on the west side, and generally from tropical cyclones as well. 

What To Pack

In winter: in the north (Chiang Mai, Chiang Rai), pack 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, pack 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. 
When going to the reef, you can bring snorkeling equipment, including water shoes or rubber-soled shoes. 
For the mountains, bring spring/autumn clothes, a warm jacket and a sweater for the evening. 
In summer: tropics-friendly, lightweight clothing, a light raincoat or umbrella, comfortable shoes; a light sweatshirt and a scarf for airconditioned places. 

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