Consisting of nine islands in the middle of the Andaman Sea are the isles of the Mu Koh Similan National Park. The small archipelago of just 14 square kilometres (5 square miles) is strewn over a 140-square kilometer (54 square miles) marine area 60 kilometers (37 miles) away from its mainland province of Phang Nga. Mu Koh Similan straddles a length of 24 kilometers (15 miles) in a straight north-south axis 8º34′ N latitude and 97º38′ E longitude.
The unique land and marine ecosystem coupled with amazing granite rock formations above and below the shorelines prompted the Thai government to declare the group of islands a national park in 1982. Koh Tachai and Koh Bon, two smaller islands to the northeast, were appended to the park due to similar geographical and biological significance.
As a marine park, it should be noted that the whole archipelago is closed annually to tourists during the monsoon season from the latter part of May until the middle part of November. As such, the high season for travel is from the dry months of December to April. The climate here is tropical and the mercury hovers in the middle 20 to 30 degrees Celsius all year round. Still, its balmy climate is somehow tempered by the cooler northeasterly winds from November until March. During this period daytime conditions are excellent with eternal azure skies with gentle breeze while at night temperatures are less humid and become conducive for outdoor relaxation. The border months of April and May clearly demonstrate what the Similans really are–hot and humid tropical hideaway where temperatures range from 30 to 36 °C (86 to 97 °F).
When May sets in the huge land mass of Asia becomes warmer than the Indian Ocean, attracting moisture that generates intense precipitation and unruly waves. The Similans are right smack on the path of the monsoonal winds. September and May are the wettest months of the year and while it seemingly rains perpetually, the surrounding Asian continent shields it from freak cyclones. Considering all this, the whole park is still closed and while diving operators offer you slashed prices everything should be treated with caution.
Probably the best time to visit Similan Islands is from February to April when the winds are gentle, the waters are calm and clear and tourists are few. This is also the period when pelagic marine animals such as the whale sharks, reef sharks and manta rays abound in the area.
Similan Islands Climate Facts:
Rainy season May to October
Dry season November to April
Average whole year temperature 28.1 ºC (82.6 ºF)
Average high temperature 33.4 ºC (92.1 ºF) in April
Average low temperature 23.3 ºC (73.9 ºF) in December
Average annual rainfall 2,316 mm (91.2in) or 193mm (7.6in) per month
Driest months January and February
Wettest months May and September