The climate of Thailand is defined as "humid tropical" which is influenced by the seasonal monsoon and the country’s geographical position. The Indochinese Peninsular is part of the Asian landmass that extrudes between two great ocean bodies – the South China Sea of the Pacific Ocean and the Andaman Sea of the Indian Ocean. While the Peninsular South is bounded by seas on both east and west shores, the mainland is also in comparatively close proximity to two large bodies of water – the South China Sea to the east, and the Andaman Sea plus the Bay of Bengal to the west. The monsoons, resulting from the seasonal differences in temperatures between the landmass and the oceanic body, alternately blow in the southwesterly and northeasterly directions over Thailand. 

     The southwest monsoon which starts in May brings a stream of warm moist air from the Indian Ocean towards Thailand causing abundant rain over the country, especially the windward side of the mountains. Rainfall during this period is not only caused by the southwest monsoon but also by the Inter Tropical Convergence Zone (ITCZ) and tropical cyclones which produce a large amount of rainfall. May is the period of first arrival of the ITCZ to the Southern Part. It moves northwards rapidly and lies across southern China around June to early July; that is the reason for the dry spell over upper Thailand. The ITCZ then moves in a southerly direction to lie over the Northern and Northeastern Parts of Thailand in August and later over the Central and Southern Parts in September and October, respectively. 

     The northeast monsoon which starts in October brings the cold and dry air from the anticyclone in China mainland over major parts of Thailand, especially the Northern and Northeastern Parts which are higher latitude areas. In the Southern Part, this monsoon causes mild weather and abundant rain along its east coast. 

     The onset of monsoons varies to some extent. The southwest monsoon usually starts in mid-May and ends in mid-October while the northeast monsoon normally starts in mid-October and ends in mid-February.

     According to Koppen’s Climatic Classification of the world, the climatic regimes of Thailand are: Tropical Savanna Climate (Aw), for the area above the Peninsular South, excluding the eastern section of the Southeast Coast; Tropical Rain Forest (Af), for the lower East Coast of the Peninsular South; and Tropical Monsoon Climate (Am) for the rest of the Peninsular South, and also the eastern section of the Southeast Coast. In addition, the Humid Subtropical Climate (Cw) is applicable to the area in the far northeastern part of the north at higher elevations.