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Koh Samui takes its heaviest rain from the tail of the northeast monsoon, roughly October through December, the opposite of the Andaman coast. On the thirty-year normal, November is wettest (444.6 mm) and February is driest (64.3 mm). May is hottest by daily-maximum mean at 32.9°C; December is coolest by daily-minimum mean at 24.0°C. A named storm — Tropical Storm Pabuk, January 2019 — has closed the airport and suspended ferries here before.
The monthly rainfall, measured
Every figure below is a 1991–2020 normal for WMO station 48550 (Ko Samui), from Thailand’s own submission to the World Meteorological Organization, archived by NOAA. It is a thirty-year average, not a forecast for any single year.
| Month | Rainfall normal | What that means here |
|---|---|---|
| Jan | 126.8 mm | Tail end of the northeast monsoon rains; still a wet month by Gulf standards, and the month a named tropical storm has hit before (see below). |
| Feb | 64.3 mm | Driest month on record for Samui. |
| Mar | 115.8 mm | Dry season proper. One of the better months to be here. |
| Apr | 82.9 mm | Getting hot, rain still low. A strong window for a visit. |
| May | 131.6 mm | Heat builds toward the May peak (32.9°C mean daily maximum); rain starts to pick up as the southwest monsoon reaches the Gulf side indirectly. |
| Jun | 133.6 mm | Moderate rain, hot. |
| Jul | 117 mm | Moderate rain, hot — while Phuket's Andaman coast is deep in its own wettest stretch. |
| Aug | 102.2 mm | One of Samui's driest mid-year months, roughly a third of what Phuket records in the same month. |
| Sep | 117.5 mm | Still relatively dry on the Gulf side, while the Andaman coast is at its wettest. |
| Oct | 294.9 mm | Rain more than doubles from September. The northeast monsoon starts reaching the Gulf. |
| Nov | 444.6 mm | Wettest month on record, and by a wide margin — higher than any single month Phuket's Andaman coast ever records. |
| Dec | 263.1 mm | Still the second-wettest month; the northeast monsoon is fully established on the Gulf side. |
The “what that means here” column is our own reading of the measured figure against the published monsoon calendar, not a separate data source.
Opposite coast, opposite season
This is the fact that trips people up most often when they plan a Thailand island holiday around one generic “rainy season.” In September, when Phuket’s Andaman coast is at its wettest (350.4 mm), Samui’s normal is a comparatively dry 117.5 mm. Two months later the position reverses: Samui’s November normal of 444.6 mm is higher than any single month Phuket’s station has ever recorded in this dataset, while Phuket in November is down to 178.2 mm.
The practical read: a Gulf island and an Andaman island are not interchangeable on a shared calendar. If a trip has to land in September or October, the Andaman side is the better bet weather-wise. If it has to land in November or December, the Andaman side, not the Gulf, is the better bet.
When it goes wrong: Tropical Storm Pabuk
Rainfall normals describe an average year, not the risk of a genuine severe-weather event. The clearest documented case for this stretch of the Gulf is Tropical Storm Pabuk, which made landfall at Nakhon Si Thammarat on 4 January 2019. In the days before landfall, officials evacuated more than 28,000 people into shelters across seven provinces. Koh Samui’s airport closed with every flight cancelled, and ferry services to the southern Gulf islands were suspended. The storm weakened to a depression as it crossed land, and Bangkok Airways resumed normal service at Koh Samui on the morning of 5 January, with ferries resuming shortly after.
January is not Samui’s wettest month on the thirty-year rainfall normal — that is November. Pabuk is the honest reminder that a named storm can disrupt travel and ferries outside the months a rainfall chart flags as worst, and that anyone with a fixed departure during the October –January window should hold a flexible booking where possible.
What we could not confirm
- A full monthly temperature series. Only the hottest (May) and coolest (December) months are sourced from the same normals archive; the other ten months are not stated rather than estimated.
- Sunshine hours. The Ko Samui station reported no sunshine-duration series in the 1991–2020 submission, so none appears here.
- Any current, published ferry-cancellation policy or threshold for the northeast monsoon season. Operators set their own weather calls; no government schedule of cancellation criteria was found.
Frequently asked questions
By the thirty-year normal, February — 64.3 mm, the driest month on record for the island. March and April are close behind and add rising warmth without the heaviest heat. This is the opposite calendar to Phuket and the Andaman coast, where February also happens to be a strong month, so a plan built around "Thailand's dry season" being one single window works for both coasts at once in this narrow February-to-April stretch.
Because they face different monsoons in practice. Both sit under the same two named monsoon systems the Thai Meteorological Department describes for the whole country — the southwest monsoon roughly mid-May to mid-October and the northeast monsoon roughly mid-October to mid-February — but the Gulf islands, including Koh Samui, take their heaviest rain from the tail of the northeast monsoon rather than the southwest one that soaks the Andaman coast. The result, in the measured normals: Samui's wettest month is November at 444.6 mm, while Phuket's wettest month, September, is 350.4 mm — meaning Samui's single wettest month is wetter than Phuket's ever gets.
On the rainfall total, yes — 444.6 mm is the highest monthly figure this site has recorded for any Thai coastal station in the WMO 1991–2020 normals, Andaman or Gulf. October, at 294.9 mm, and December, at 263.1 mm, bracket it on either side, so the real bad stretch runs from roughly mid-October through December rather than being one isolated month. Anyone planning a Gulf-island trip around late-year dates should build in flexibility, both for weather and for the possibility of ferry disruption on short notice.
Yes, and it is worth knowing as more than an abstract risk. Tropical Storm Pabuk made landfall at Nakhon Si Thammarat, on the Gulf coast, on 4 January 2019. Officials evacuated more than 28,000 people into shelters across seven provinces in the days before, Koh Samui's airport closed with every flight cancelled, and ferry services to the southern Gulf islands were suspended. The storm weakened to a depression as it crossed land, and Bangkok Airways resumed normal operations at Koh Samui the following morning, 5 January, with ferries resuming shortly after. January is not Samui's wettest month by the normals — that is November — but Pabuk shows the risk is not confined to the wettest months on a rainfall chart.
There is no winter in the temperate sense. By the same thirty-year normals, May is the hottest month by mean daily maximum at 32.9°C, and December is the coolest by mean daily minimum at 24.0°C — a narrower range than most of mainland Thailand, because the sea moderates both ends. What changes through the year is rain and humidity far more than temperature.
Related guides
- Best time to visit Phuket — the Andaman coast, and its opposite wet season.
- Compare Thai cities — the same normals for twelve stations, side by side.
- Cost of living in Koh Samui
- Weather & seasons in Thailand — the country-wide picture this page zooms in on.
- Koh Samui — the city guide.
- Explore Thailand — the section this guide sits in.
Timed your visit. Now time the move.
If Koh Samui is more than a trip, see what is actually listed here before the next northeast monsoon closes your travel dates.
Sources & References
- WMO Climatological Standard Normals 1991–2020 — Thailand station data sheets (NCEI Accession 0253808), Ko Samui station 48550: monthly precipitation totals, annual rainfall of 1,994.3 mm over 120.6 rain days, hottest month May at a 32.9°C daily-maximum mean, coolest month December at a 24.0°C daily-minimum mean
- Thailand government portal, citing the Thai Meteorological Department — the southwest monsoon covers Thailand from about mid-May to mid-October, bringing humid air and general cloud and rain; the northeast monsoon runs from about mid-October to mid-February, bringing clear skies, cool air and dry conditions on the Andaman side — the Gulf islands take the opposite pattern from the same two monsoons
- Malay Mail, wire reporting on Tropical Storm Pabuk — landfall at Nakhon Si Thammarat on 4 January 2019, more than 28,000 people evacuated into shelters across seven provinces, Koh Samui airport closed with all flights cancelled ahead of the storm and Bangkok Airways resuming normal operations there on the morning of 5 January, ferry services to the southern islands suspended and then resumed as the storm weakened to a depression
Primary and official sources are cited above. Government rules, fees and procedures in Thailand change over time and vary by office; always confirm current requirements with the relevant authority before relying on them. BAANLYY never takes paid placement in editorial content.
