← Weather & seasons in Thailand
On the thirty-year normal, Koh Lanta’s driest month is February (24.1 mm), wettest is August (342.0 mm), hottest is March (33.9°C mean daily maximum) and coolest is December (23.9°C mean daily minimum). Several outer islands and dive sites — Ko Rok, Mu Ko Ha, Kong Hin Daeng–Hin Muang and a cave and campground — close 1 July to 30 September under the Department of National Parks’ seasonal rule. Ko Lanta itself stays open year-round.
The season bands, honestly
Koh Lanta sits on the Andaman coast and shares its broad wet-season timing with Phuket, though the two islands are not identical — Koh Lanta’s wettest month peaks slightly lower than Phuket’s.
| Window | Measured conditions | What that means here |
|---|---|---|
| Nov – Feb | Dry season. December coolest (23.9°C). | The best window: calm seas, low rain, all outer islands open. Peak tourist season, so also the most expensive. |
| Mar | Hottest month (33.9°C). | Still mostly dry. Good diving and snorkelling visibility. |
| Apr – Jun | Rain building. | Transitional; outer-island trips still running before the July closure. |
| Jul – Sep | Peak southwest monsoon. August wettest (342.0 mm). | Ko Rok, Mu Ko Ha, Kong Hin Daeng–Hin Muang, the Tham Khao Mai Kaeo cave and the Ko Rok campground close 1 July–30 September under the Department of National Parks' seasonal rule. Ko Lanta itself stays open and inhabited year-round. |
| Oct | Rain easing. | Shoulder season; outer islands reopen after 30 September. |
All figures are 1991–2020 normals for WMO station 48566 (Ko Lanta). The “what that means here” column is our own reading against the TAT closure dates, not a separate data source.
What actually closes, and when
If an outer-island day trip is part of the plan, this is the date range that matters most. The Tourism Authority of Thailand’s own published national park status list records Mu Ko Lanta National Park closing Ko Rok, Mu Ko Ha (Tukon Lima), Kong Hin Daeng–Hin Muang, the Tham Khao Mai Kaeo cave and the Ko Rok campground from 1 July until 30 September every year, under the Department of National Parks’ standard monsoon-safety and reef-recovery rule used across its marine parks. Ko Lanta itself — the inhabited main island most visitors and residents mean by “Koh Lanta” — is not part of this closure and functions year-round.
Separately, and worth distinguishing clearly: in June 2026 the park closed three different sites — Laem Tanot, Ko Haa and Ko Ngai — after large numbers of Portuguese man o’war washed ashore, a marine-life hazard rather than the routine seasonal rule above. The Nation’s report on that closure does not state whether the hazard recurs on a predictable schedule, so this page treats it as a real, dated example of an unpredictable risk rather than a fixed calendar item. Always check current park and beach flag status before swimming.
What we could not confirm
- Whether the Portuguese man o'war hazard is seasonal or recurring. The June 2026 report describes one closure; no official source we could open states a pattern across years.
- A full monthly rainfall or temperature table beyond the four extreme months. The archive that would supply every other month could not be reached from this run.
- Ferry frequency or suspensions between Krabi and Koh Lanta during the wet season. This page covers weather and park closures; ferry schedules change operator by operator and are not something we verified against an official source for this page.
- Day-to-day long-stay logistics in the low season (which businesses close, healthcare access off-season). Outside this page's scope — see the Koh Lanta island guide, linked below, for that.
Frequently asked questions
December, on the numbers — the coolest month at a 23.9°C daily-minimum mean, and rainfall is dropping fast from the wet-season peak. The stretch from November to February is the driest and calmest window overall, February being the single driest month at 24.1 mm. It is also the busiest and most expensive time to be here, because the same normals are public to everyone booking a trip.
Not Koh Lanta itself — the main island stays inhabited and open year-round. What closes, per the Tourism Authority of Thailand's own published national park status list, are specific outer sites of Mu Ko Lanta National Park: Ko Rok, Mu Ko Ha (Tukon Lima), Kong Hin Daeng–Hin Muang, the Tham Khao Mai Kaeo cave and the Ko Rok campground, all closed from 1 July until 30 September under the Department of National Parks' seasonal rule for monsoon safety and reef recovery. If a Ko Rok day trip is part of the plan, this three-month window rules it out entirely.
Koh Lanta's wettest month, August, averages 342.0 mm — close to Phuket's own peak of 350.4 mm in September, and both islands share the Andaman coast's broad wet-season timing (roughly May to October) as opposed to the Gulf islands, which peak later in the year. Koh Lanta's annual total of 2,219.5 mm over 132.2 rain days is among the wettest of the twelve Thai stations this site has checked against the same WMO normals.
In June 2026, Mu Ko Lanta National Park closed three specific sites — Laem Tanot, Ko Haa and Ko Ngai — after large numbers of Portuguese man o' war washed ashore, a hazard distinct from the routine July–September monsoon closure above. Officials had already posted red-flag swimming warnings before the closure. We could not confirm from the report whether this is a predictable annual event or an unusual occurrence for that year, so this page reports it as a real, dated example of an unpredictable marine hazard rather than a fixed seasonal rule. Check current park and beach flag status locally before swimming, regardless of month.
This page focuses on weather and park closures rather than day-to-day logistics, so we have not independently verified healthcare access, internet or off-season business hours here. What the sourced weather data does tell you: the wet season is genuinely wet (August averages over eleven times February's rainfall) and several outer-island attractions close for three months, both worth weighing against lower low-season accommodation prices before committing to a long booking. See the Koh Lanta island guide for the fuller day-to-day picture.
Related guides
- Best time to visit Phuket — the other major Andaman coast destination.
- Phuket vs Koh Samui — the two coasts' opposite wet seasons, explained.
- Compare Thai cities — the same normals for twelve stations, side by side.
- National parks in Thailand — the fee bands and the law behind park closures.
- Koh Lanta — the island guide.
- Explore Thailand — the section this guide sits in.
Timed your visit. Now time the move.
If Koh Lanta is more than a trip, see what is actually listed here before the next monsoon window closes your travel dates.
Sources & References
- WMO Climatological Standard Normals 1991–2020 — Thailand station data sheets (NCEI Accession 0253808), Ko Lanta station 48566: annual rainfall of 2,219.5 mm over 132.2 rain days, wettest month August (342.0 mm), driest month February (24.1 mm), hottest month March at a 33.9°C daily-maximum mean, coolest month December at a 23.9°C daily-minimum mean
- Tourism Authority of Thailand (TAT) Newsroom — "The latest opening and closing status of Thailand's national parks" (7 July 2023): Mu Ko Lanta National Park's outer sites — Ko Rok, Mu Ko Ha (Tukon Lima), Kong Hin Daeng–Hin Muang and Tham Khao Mai Kaeo cave, plus the Ko Rok campground — close annually from 1 July until 30 September; the main park area at Ko Lanta itself is listed as open
- The Nation (Thailand) — "Beach red flags fly as Ko Lanta closes sites over Portuguese man o' war": Mu Ko Lanta National Park closed three sites — Laem Tanot (the park headquarters area), Ko Haa and Ko Ngai — from Thursday 11 June 2026 after large numbers of Portuguese man o' war washed ashore, with red-flag warnings already in place beforehand; the report does not state whether the hazard is seasonal or recurring
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.
