The Similan islands national park is open from 15th October to 15th of May, the rest of the year it s closed due to weather and it’s illegal to go.
The Similan islands and Thailand is general have 3 season: a bit cool, hot & dry and rainy.
The condition for diving are great between November to May, the great thing about the Similan islands diving season, is that the visibility is always 20-30m, very rare to get less.
The time of the year when we get plankton rich current drops the visibility a bit but attraction the giant Manta Rays and it’s the Whale Sharks season.
Manta ray season is around the same time.
The best dive site to see the giants are Richelieu Rock daytrip and Koh Tachai pinnacle.
There is 2 seasons in the west cost of Thailand, Dry Season and Rainy Season. Dry Season is from October to May with the best weather, hot and sunny. Rainy Season from June to September, with scattered rainfall, but still some sunny days.
The Similan Island liveaboard season runs from mid-October to mid-May.
Scuba day trips and overnight trips run from October through to May, with the best conditions existing from November to April. Outside of these months, surface swells can adversely affect conditions.
From May to August, the weather is often better around Phuket and Phi Phi, during this time our liveaboard trips goes to the best dives around Phi Phi and Phuket.
Check out the Thailand meteorological department website for weather warning (TMD)
The Monsoon is what regulates the Similan islands season weather, to make it simple:
Is a strong wind coming from the south-west, the wind bring humidity from the sea and create cloud and rains near the mainland and at sea.
The west coast of Thailand and the Similan islands is strongly affected with high waves and heavy rains. The South-West Monsoon start en of Mai and last until end of September.
Is a strong wind coming from North-East bringing high waves to the gulf of Thailand and heavy rains between October to May.
The Similan islands and the west coast of Thailand is protected from the North-Est Monsoon, the mainland stops the wind and rains which get stuck on the east coast.
Then the sea is very flat, and sunny weather at the Similan.
This why the East and West coast of Thailand has opposite season.
October to May is the high season and “dry season” for the Similan islands. For October and May these 2 month a transitional and depending on the year is can be more or less rainy.
The best months are from December to April, with stable blue sky, flat sea.
Of course being in a tropical country you always get tropical thunderstorm, most of the time you get a little rain on the mainland in Phuket and Khao Lak due to the mountain, but out at sea, it s nice and sunny.
The Similan islands being 70km from the coast, these islands get there own special weather, more sunny than the mainland.
Check out Windguru one of the most reliable source for detailed weather forecast.
The tides are changing constantly due to the moon attraction.
Tides creates currents so it’s very useful to look at tide tables before going diving, so we can predict current direction and strength.