Weather Overview
Located off the west coast of Thailand in the Andaman Sea sits the Similan Islands. The group of 11 islands is part of the Phang Nga province of southern Thailand and became a national park in 1982. It enjoys a tropical climate with consistently warm temperatures throughout the year and a rainy monsoon period that runs from May to October. During the rainy season, the national park is generally closed to public, making November through to April the best time to visit.
The tourist season peaks in December and January, when most people escape the dreary cold of the UK and Europe. You'll find the calmest waters in February and March, ideal for scuba diving and snorkeling. Even if you want to visit in the middle of the monsoon season, it's not a total loss. July and August are warm with the rain concentrated in the evenings, giving you some sunny skies during the day.
The largest of the islands is Ko Similan and several of the smaller islands are closed to visitors because they are part of a turtle sanctuary. Visitors can access the islands from the mainland town of Tab Lamu in Khao Lak. Depending on the boat you choose, the ride can take anywhere from one to three hours. You can get rides daily from October through to May and intermittently the rest of the year. There's only one place to actually stay on the islands themselves, but you can find plenty of hotels on the mainland.
Visitors to the islands enjoy some of the best scuba diving and snorkeling in the world, thanks to the numerous rock formations and coral reefs surrounding the islands. On the east side of the islands, divers are treated to sloping coral reefs, while the west side is made up of large underwater boulders and caves. Famous dive sites around the island include East of Eden, Elephant Head Rock, North Point, Boulder City and Deep Six.
Spring
When spring rolls around in the Similan Islands, the tourist season starts to wind down. The average daily temperature sits between 28°C and 29°C (around 82°F to 84°F), staying nice and warm from the start of the year. During the heat of the day, the highs get up to 32°C to 33°C (about 90°F to 91°F). At night, the lows drop down to 24°C to 25°C (about 75°F to 77°F), offering a pleasant respite from the heat of the day. April has the highest temperatures, longest days and warmest sea temperatures.
The rainfall chances ramp up as the months go along, with 50 mm of rain in March, jumping up to 193 mm by May. The number of rainy days per month varies greatly, from four days at the beginning of the season to 19 days at the end. Visitors get six to nine hours of sunshine each day on average to get outside and enjoy the islands. Swimming, diving and water sports are a definite must-do in the spring, with an average sea temperature between 28°C and 29°C (around 82°F to 84°F).
Summer
During the summer in the Similan Islands, the temperatures stay steady but the rainfall chances jump up. It's not a complete wash however; the rain tends to come in the evening hours in shorter bursts. The average daily temperature is 28°C (about 82°F), consistent from the previous months. The highs get up to a steamy 31°C (around 88°F), but it doesn't feel too hot, thanks to the winds that blow in the monsoon rains from the southwest. At night, the lows drop down to 25°C (about 77°F).
Rainfall chances range from 189 mm to 241 mm over the course of June, July and August. Visitors can expect more rainy days in the month than sunny days, with 17 to 18 days of rainfall each month on average. The area sees between five and seven hours of sunshine each day as well. Plan on getting to spend time in the water, with an average sea temperature between 28°C and 29°C (about 82°F to 84°F).
Autumn
It's the height of the monsoon season in the Similan Islands in the autumn, with ever-increasing rainfall chances and moderate temperatures. The area enjoys a daily average temperature of 27°C to 28°C (about 81°C to 82°C), a welcome change from the heat of the summer. The highs reach up to 30°C to 31°C (around 86°F to 88°F), hot but not unbearably so. Nighttime lows drop down to 24°C (about 75°F), providing relief from the heat of the day.
During the months of September, October and November, the area sees the highest rainfall totals out of the year, with 98 mm to 253 mm of rain falling. The number of days of rainfall ranges from 13 to 21 days, with September having the heaviest and most rain. Daytime hours fall between six and eight hours, ample time to get outside. The average sea temperature drops slightly from previous peak summer months, down to 28°C (around 82°F).
Winter
As the year winds down, the tourist season kicks into high gear, with December and January the most popular time to visit, thanks to moderate temperatures and scarce rainfall. The average daily temperature still sits around 27°C to 28°C (around 81°F to 83°F), offering a welcome change from the frigid winter temperatures elsewhere around the world. December is the "coldest" month of the year in a consistently warm climate year round. The highs get up to a warm and balmy 31°C to 33°C (about 88°F to 91°F), but thanks to the breezes that blow across the Andaman Sea, the humidity stays low. The lows drop down to 24°C to 27°F (about 75°F to 81°F).
During the winter months, the rainfall amounts drop dramatically, down to 17 mm to 48 mm each month. The number of rainfall days each month on average is only three to seven days, so you'll have plenty of dry days with sunshine to enjoy the islands. The days are longer this time of the year as well, with eight to nine hours of sunshine each day. While the average sea temperature has dropped, down to 27°C to 28°C (around 81°F to 82°F), it's still plenty warm to enjoy swimming, water sports and the scuba diving the area is famous for.
Weather Hazards
Because the temperatures are so consistent from month to month, the Similan Islands rarely get extreme or hazardous weather. The islands aren't completely immune to natural disasters however. In 2004, the tsunami that struck the area caused damage to the islands and parts of the national park.
Any thunderstorms or severe weather that make its way across the island are typically short-lived and actually offer relief from the heat. The biggest hindrance the area experiences are choppy seas and high waves during monsoon season, preventing boats from reaching many of the famous dive sites surrounding the islands.
Fortunately, you'll more than likely not have to deal with any extreme weather during your stay. You'll have the same moderate temperatures the islands experience all year round and rain during the monsoon season. Check the local forecast each day to know for sure what to expect.