Which Thai coast for rainy season: Andaman vs Gulf, decided by 10 years of real rain data
The classic mid-year question is "can I still do a beach trip in the rainy season?" — the short answer is yes, if you pick the right coast. Thailand's two coasts don't get their rain at the same time: the Andaman side (Phuket, Krabi, Phang Nga) takes the southwest monsoon head-on from mid-year, while the upper Gulf (Hua Hin, Cha-am, Pattaya) and lower Gulf (Samui, Songkhla) run on their own rain schedules, offset by several months.
This article doesn't argue from "experience" or vibes — every number is a 10-year monthly rainfall mean (2016–2025), the same dataset the weather calendar in rooozt's search bar shows, using the same published thresholds: under 50 mm/month = low rain, over 150 mm/month = heavy rain.
Mid-year (Jul–Aug): the Gulf wins clearly
Through July and August the Andaman coast sits right in the monsoon: Phuket averages 178 and 207 mm, Krabi 202 and 218 mm, and Phang Nga (Khao Lak) is heaviest of all at 240 and 250 mm — all inside our "heavy rain" band.
Cross to the Gulf and the numbers drop visibly: Prachuap Khiri Khan (Hua Hin's province) averages just 146 and 126 mm, Songkhla 131 and 135 mm, and Surat Thani (the gateway to Samui and Phangan) 185 and 161 mm — Hua Hin and Songkhla sit in the middle band and don't even touch the heavy-rain threshold. That's why a July–August beach trip favours the Gulf systematically, not by luck.
Late year (Oct–Dec): the poles flip — the lower Gulf takes its turn
By late October the monsoon changes direction, and now the lower Gulf takes the full hit: Songkhla spikes to 495 mm in November (its highest month of the year) and still runs 375 mm in December, while Surat Thani sits at 316 and 300 mm through October–November — this is the stretch to think hard about before planning Samui or the lower Gulf.
The Andaman side, meanwhile, is winding down: Phuket's November 246 mm falls to 140 mm in December, then full high season arrives January–March (78 → 34 → 62 mm, with February the driest month of Phuket's year in this dataset).
One special warning: Trat–Koh Chang is the mid-year rain champion
Across the whole dataset, the heaviest month of any beach province is Trat in July: a 588 mm/month average, with August–September still at 478 and 529 mm — a rainy-season Koh Chang trip is for people who genuinely want to fall asleep to rain, and some boat routes may pause on rough-sea days.
The easy-to-remember formula
December–March: the Andaman is the main answer (sunny, calm seas) · July–August: switch to the upper Gulf — Hua Hin, Cha-am, Pattaya run at nearly half the Andaman's rainfall · October–November: the transition months when rain peaks almost everywhere — if you must travel then, check each city's weather calendar in rooozt's search bar before picking a destination, and always keep a rainy-day backup plan.
Frequently asked questions
- Can I visit Phuket in the rainy season?
- Yes — just go knowing the numbers: July–September averages 178–241 mm/month, and the rain usually comes in short bursts between sunshine rather than all day, every day. The trade-off is clearly cheaper low-season room rates, and you should expect island tours to occasionally cancel on rough-sea days.
- Which month has the least rain in Hua Hin?
- In this 10-year dataset, Prachuap Khiri Khan's December averages just 37 mm/month, followed by February at 42 mm — both inside the "low rain" band (under 50 mm).
- How much can I trust these numbers?
- They're 10-year means from ERA5, a reanalysis dataset with roughly 9 km resolution — not an on-the-ground weather station, and not this year's forecast. They're excellent for choosing a coast and a month, but check an actual forecast as your travel date approaches.
Where the numbers come from: Every rainfall figure in this article is a 2016–2025 monthly mean from the ERA5 dataset (via the Open-Meteo Historical Weather API) at each province's coordinate — a 10-year statistic, not a forecast for your trip. Any single year can differ a lot from the mean. This is the same dataset the weather calendar in rooozt's search bar displays.