I/  Northern Thailand (Chiang Mai, Chiang Rai):

Three distinct seasons are experienced in northern Thailand. Temperatures during the chilly season, which lasts from November to February, range from 15°C to 25°C (59°F to 77°F). Due to the great weather and beautiful skies, this time is typically regarded as the finest time to explore the area. From March to May, there is a hot season with highs of at least 35°C (95°F). The rainy season, which normally lasts from June to October, is distinguished by frequent drizzle and sporadic torrential downpours.

II/ Central Thailand (Bangkok, Ayutthaya, Pattaya):

There are three seasons in the tropical savanna climate of central Thailand. The chilly season, which lasts from November through February, features pleasant temperatures between 20°C and 30°C (68°F and 86°F). The hot season, which lasts from March to May, may be fairly hot and muggy with frequent highs of 35°C (95°F). Increased rainfall and sporadic storms are common during the rainy season, which lasts from June to October.

III/ Southern Thailand (Phuket, Krabi, Koh Samui, Koh Phi Phi):

Tropical monsoon weather is typical in southern Thailand. The dry season and the wet season are the two main seasons in the area. Usually lasting from November through April, the dry season has lower humidity and less precipitation. The range of temperatures is 77°F to 95°F (25°C to 35°C). Higher humidity, frequent downpours, and sporadic tropical storms are all features of the rainy season, which lasts from May through October. However, there are frequently times of sunshine and clear skies even throughout the rainy season.