Each year, the Similan Islands close from 15 May to 15 October to allow the national park’s natural resources time to recover. This period coincides with Southern Thailand’s Southwest Monsoon. Heavy rain and strong winds during this time make the Andaman Sea too dangerous for trips to the Similan archipelago.
Weather-Related Closures in the Similan Islands
In addition to the annual closure, there have been temporary shutdowns due to severe weather. For example, on Monday 12 May and Tuesday 13 May, the Similan Islands were closed to tourists because of dangerous sea conditions, as reported by the Bangkok Post. These short-term closures are implemented to protect visitors and preserve the fragile marine environment.
Recent weather-related closures reflect the general deterioration of conditions in Southern Thailand. Similan liveaboard trips have been affected since late April 2025, as thunderstorms and strong winds disrupted coastal areas along the Andaman Sea. Many operators adjusted routes or schedules, and some trips were cancelled entirely.
How The Weather Affected The 2024 – 2025 Similan Diving Season
May 2025 was not the only time poor weather impacted diving operations. Strong northerly winds and high waves have been a persistent issue since the start of the season in October. While such conditions are typical in October and November, this season they did not ease until well into January.
February and March finally brought classic high-season conditions—calm seas and clear skies returned to the Similan Islands. However, the rainy season arrived early in 2025, with heavy rainfall already occurring in early April, weeks ahead of the usual seasonal transition.
Similan Diving Conditions in May 2025
Until early May, underwater conditions around the Similan Islands and Richelieu Rock had been exceptional. The calmer seas were a welcome relief for crews, and underwater visibility was clear and bright, as expected in this region. High productivity in the water brought frequent sightings of larger marine species, including manta rays and whale sharks.
As the weather worsened in April and into May, underwater visibility dropped noticeably. Waters took on a greenish tinge, and the increase in plankton reduced sunlight penetration. While not ideal for diving, cooler temperatures and reduced sunlight help prevent coral bleaching—another positive sign following recent announcements of coral recovery by the Similan National Park.
The Similan Islands will re-open on 15 October 2025.
