The weather in Thailand is generally hot and humid across most of the country throughout most of the year however the climate is controlled by tropical monsoons so its seasons are generally divided into the hot season, cool season, and rainy season. The weather in central, northern, and northeastern Thailand (the landlocked provinces) is determined by these three seasons, whereas the southern, coastal regions of Thailand feature only two, making the weather in Thailand quite easy to understand and plan a trip around.
In Thailand’s inland provinces the seasons are clearly defined: Between November and May the weather is mostly dry and the cool season and hot season occur from November to February and March to May respectively. The other inland season, the rainy season, lasts from May to November and is dominated by the southwest monsoon, during which time rainfall in most of Thailand is at its heaviest.
The southern, coastal region of Thailand really has only two seasons – rainy season and dry season. Fortunately, for those planning a beach holiday, Thailand’s two coasts have slightly different rainy seasons, allowing visitors to find sunny beaches nearly year round.
On the Andaman or west coast, where Phuket, Krabi, and the Phi Phi Islands lie, the southwest monsoon brings heavy storms from April to October, while on the Gulf of Thailand or east coast, where Koh Samui, Koh Phangan, and Koh Tao lie, the most rain falls between September and December.