The three seasons
Cool and dry runs from November to February. It is the peak, the most expensive and the most reliable: 25–32°C, low humidity, calm seas on the Andaman.
Hot season is March to May, with temperatures in the high 30s and Songkran in mid-April. It is quieter and cheaper, and the Andaman is still swimmable.
Green season is June to October. Rain comes in heavy afternoon bursts rather than all day, prices drop 30–50%, and the landscape is at its best.
Two coasts, two calendars
The Andaman coast (Phuket, Krabi, Phi Phi, Koh Lanta, the Similans) is best from November to April. From May to October the swell makes boats uncomfortable and some islands close.
The Gulf coast (Koh Samui, Koh Phangan, Koh Tao) runs later: its wet season is October to December, and it is often excellent in June and July when the Andaman is rough.
What we would actually book
For a first trip combining culture and beach, late November to early February is the safest window. Book six to nine months ahead for the Christmas period.
If you want lower prices and do not mind an afternoon downpour, late June or September is underrated — particularly in the north, where the rice fields are green and the temples are empty.
