Thailand is a tropical country. As in all of Southeast Asia, its climate is influenced by the monsoon. Monsoon winds are seasonal winds whose direction changes throughout the year due to temperature and pressure differences between land and sea. In spring, a large low-pressure area forms over Southeast Asia, and the winds blow from the relatively cooler sea towards the land. Their relative and absolute humidity is very high. This southwest monsoon brings the rainy season. In winter, on the other hand, the land cools down faster than the sea, and a high-pressure area builds up over Southeast Asia. This winter monsoon leads to an airflow from land to sea, from northeast to southwest. This creates the dry season in Thailand, except on the southeast coast, which the winds only reach after crossing the Gulf and absorbing some moisture there. Based on these changes in wind direction, three seasons can be distinguished in Thailand:

(1) a dry and cool season (the northeast monsoon) from November to February; (2) a transitional season with higher temperatures and changing winds from March to June; and (3) a hot and humid season from July to October (the summer monsoon);

Within this pattern, there are, of course, significant regional differences. For Thailand, which relies on water for its rice cultivation, rainfall (or its lack thereof) is more important than temperature. Temperature fluctuations are generally quite small (below 10°C in Bangkok) and fairly predictable across the country. This is not the case for rainfall, which varies considerably from year to year and from region to region.

In all parts of the country, the rainy season coincides with the southwest monsoon (May to October); however, the amount of rainfall varies considerably from region to region, mainly due to local topography. On the western slopes of the Tenasserim coast, along the Burmese border, there are areas with an annual rainfall of 4 to 5 meters. The influence of the orography is evident in the fact that on the eastern slopes around Kanchanaburi, the annual rainfall is barely 200 mm. Because the winds carry the rain away so effectively from the coastal regions, it decreases steadily inland. This pattern is of great importance for agriculture and vegetation: dense forests in the areas affected by the summer monsoon, open dry forests in the interior, and transitional forests in between.

Thailand's driest regions lie in the western part of the Khorat Plateau, particularly in the rain shadow of the mountain ranges between Khorat and the Menam Plain. Another arid region stretches along the foothills of the western hills from the Prachuap Khiri Khan district, through Ratchaburi, Phetchaburi, Kanchanaburi, and Tak province, to the edge of the Malay Peninsula. The wettest area is the west coast of the Malay Peninsula, where the Andaman Sea monsoon meets the high coastal hills. Takuapa receives over four meters of rainfall annually.

In the most densely populated areas of the country, temperatures range between 25°C (December, January) and 30°C in the hottest months. As mentioned earlier, this small temperature variation is typical for Thailand. In Bangkok, the temperature never drops below 13°C (morning temperatures from November to January are around 18°C) and never rises above 35°C in the shade. During the rainy season, the position of the sun (near its zenith over the Tropic of Cancer) is less decisive for the temperature than cloud cover. Therefore, the temperature variation during the day is at most 3 to 5°C. Since the humidity then reaches almost 100% – and continues to rise throughout the day – and it rains regularly between 3 and 5 pm, the climate between May and October is very hot and humid.

Around October/November, the clouds dissipate and daily temperature fluctuations increase; at this time, it is significantly cooler in the north of the country. The dry winter then begins, a season that is ideal for tourism, especially for European and American visitors.

Temperatures gradually rise in February, and the hot season begins in March, with April and May being extremely hot. The sun beats down relentlessly, and even the nights offer little respite. At most, the sea breeze provides some cooling in the evenings on the plains around Bangkok. In the Nakhon Ratchasima region, with its more continental climate, summers are hotter and winters cooler than in the greater Bangkok area.

The climate of the Malay Peninsula is as unique as its geology and topography. Being located on the coast, the differences between the seasons are significantly less pronounced than in the rest of Thailand; annual temperature variations are minimal, and the climate is sub-equatorial. Occasionally, typhoons, formed by low-pressure systems in the South China Sea, track diagonally across the Gulf of Thailand and make landfall on the peninsula's east coast, sometimes even reaching the Andaman Sea. Cyclones can also track northward, crossing the Annam Mountains to reach eastern or central Thailand, primarily between July and November.

Based on temperature and rainfall, the country can be divided into three climate zones. Most of the interior of Thailand and part of the Malay Peninsula have a dry tropical climate; the higher-altitude areas in the northeast are characterized by a humid subtropical climate; and the rest of the country has a tropical monsoon climate.