Thailand is largely affected by two monsoon systems, which affect most of South East Asia.
Between May and September the Southwest monsoon brings wetter weather from the Southwest, affecting much of Thailand especially western areas, which include Phuket, Krabi, koh Lanta, Phi Phi Island and Khao Lak.
April and October are transitional months, which see the switch in prevailing monsoons.
November to March sees drier conditions across most of Thailand including the west coast destinations.
Some areas of Thailand don’t fit this pattern entirely. Koh Samui normally has great conditions in July and August, whilst November can see high rainfall. The driest area of Thailand is Koh Samet in the north of the Gulf of Thailand.
Southern Thailand sees much more rain than in the northern areas. In the north temperatures can also be much lower, especially at altitude where temperatures can drop dramatically after dark.