Thailand Weather by Month: When Phuket Is Wet in August, Koh Samui Is Dry — Two Seas, Two Rainy Seasons

Book Phuket for an August holiday, search the weather, and "rainy season" is the first thing you see. August brings 207 mm of rain to Phuket, and the monsoon off the Andaman Sea whips up the surf. Yet on the other side of the peninsula, in the Gulf of Thailand, Koh Samui gets 84 mm that month. Southern Thailand's two seas have opposite wet seasons.

The wind explains it. From May to October the south-west monsoon throws Andaman moisture at Phuket, Krabi and Phi Phi, while Samui, Phangan and Koh Tao sit in the peninsula's lee. From October to December the north-east wind drives Gulf moisture onto Samui. Hence the rule of thumb: Samui for July–August, Phuket for December–March. Bangkok and Chiang Mai follow the south-west monsoon, with rain from June to October.

Bangkokians split the year into three. The cool season, November–February, brings nights of 22–24°C and the most comfortable days. The hot season, March–May, peaks in April at 34.3°C with 21.7 days above 33°C. The rainy season, June–October, brings afternoon downpours, peaking at 297 mm in September.

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There's a reason Songkran, the water festival on 13–15 April, falls in the hottest week of the year: everything dries in minutes.

Up in the northern hills, Chiang Mai's December–January nights fall to 16–17°C. Bring only T-shirts and you'll be cold at dawn on Doi Suthep or at the night market; a light long-sleeve and a windbreaker will do. The Loy Krathong / Yi Peng lantern festival around November's full moon combines great weather with a big event, making it the hardest week of the year to find a room.

Koh Samui flips it, with its rain concentrated in October–December (286 mm in November). February to August is mostly dry, so during Korea's summer holidays Samui has the better odds of sunshine. Both islands sit at 28–32°C all year, so pack the same.

Packing checklist

• Basics: T-shirts and shorts, sandals, sunscreen, a hat.
• For temples: light trousers or a long skirt covering the knees, a shirt covering the shoulders. The Grand Palace (Wat Phra Kaew) checks dress strictly.
• Chiang Mai in December–January: a light long-sleeve and a windbreaker.
• Rainy season: a poncho beats an umbrella; sandals that can get wet.
• Everywhere: a thin layer for arctic air-conditioning in malls, buses and cinemas.

• Period and source: daily values from Open-Meteo's historical weather data (ERA5 reanalysis), 2016–2025, averaged by month at each city-centre coordinate.
• Versus official normals: reanalysis temperatures usually land within 1–2°C of station normals, but downpours get smoothed out, so monthly rain totals come out lower and rainy-day counts slightly higher.
• Rainy day: at least 1 mm of precipitation. Reanalysis picks up even light showers, so counts run high — especially in the tropics, where monthly millimetres are the better guide. Most tropical rain is an hour or two of downpour, not a washed-out day.
• Best months: a first pass from temperature, rainy days and humidity, then adjusted for seasonal factors such as Songkran, the northern smoke season and the two seas' monsoons. It is a judgement call.