Thailand has three seasons: cool and dry (November–February), hot and dry (March–May), and wet (June–October). The Andaman and Gulf coasts have different monsoon timing, which is why the 'best time to visit' question has two answers.
The short version
- November–February: 25–32°C, dry, clear — the reliable window.
- March–May: 33–38°C, humid, the north is smoky in March and April.
- June–October: green season, afternoon rain, lower prices.
- October–December is the Gulf's wet season, not the Andaman's.
November to February
The best window almost everywhere. Bangkok 22–32°C and dry; Chiang Mai cool enough for a jacket in the evening; the Andaman calm and clear.
Christmas and New Year are the busiest and most expensive two weeks of the year. Book by September.
March to May
Hot and dry, then very hot. Songkran in mid-April is a national water fight and a genuine cultural event.
The Andaman is still good for swimming; the north is smoky in March and April because of agricultural burning.
June to October
Green season. The Andaman gets swell and afternoon rain; the Gulf is often better in June and July. Prices fall and the landscape is at its most photogenic.
October is the wettest month overall and the transition between monsoons — avoid it if you have a fixed beach plan.
Reading a forecast in Thailand
Thai weather forecasts are close to useless for planning more than three days out, because the rain is convective rather than frontal. A forecast showing 80% rain does not mean an 80% chance of a wet day; it usually means a 90-minute downpour in the late afternoon and sunshine either side.
The useful signals are different. A 'monsoon' month means the west-coast boats are more likely to be cancelled, not that it rains constantly. Air quality in the north from February to April is a real constraint and is measurable — check the PM2.5 index rather than the weather forecast.
For boats specifically, the marine forecast from the Thai Meteorological Department is what the operators read, and they will cancel on a small-craft warning regardless of how blue the sky looks. As a rule, morning crossings are calmer than afternoon ones in every season.
