The tropical island of Koh Lanta is located in the Krabi Province of southern Thailand, close to Malaysia and Sumatra (Indonesia) and sitting just 8 degrees above the equator. With a typical tropical climate, the weather on Koh Lanta ranges from cool and dry in the winter to warm and wet in the summer months.
The water temperature is around 28 - 31 degrees throughout the year, varying slightly depending on the time of year. Generally the cooler months are November, December and January, with the hottest and most humid months typically being March and April.
During the high season months, the winds are generally from the east, blowing from the South China Sea into the Gulf of Thailand. The Thai-Malay Peninsula acts as a buffer and here on Koh Lanta we usually experience extended periods of sunny days with flat calm seas and little rain, though we can have the occasional thunderstorm and the winds can sometimes be a little stronger or change direction.
During the rainy season ('low season'), the westerly monsoon winds from the Indian Ocean bring periods of rain and the occasional storms, however Koh Lanta can also experience long periods of sunshine and calm seas throughout the summer months.
Unfortunately the monsoon winds that control Koh Lanta's weather do not change direction on time every year. This means that the rainy season can start later in June, with a break for several months of fine weather and then start again at the beginning of 'high season' to continue well into January.
Diving conditions vary throughout the year, the best conditions typically being October to early May, though of course the weather can be unpredictable at any time and we occasionally have a few days of stormy weather at some point during the high season months. Diving in low season is possible, with the best conditions likely between early July and late August.
Visibility varies throughout the year, ranging from 5 - 10m on Koh Phi Phi after stormy weather to 30m+ at Koh Haa and Koh Bida during periods of fine weather. Currents can bring plankton at any time of year which reduces visibility and makes the water more greenish and this provides food for larger sea creatures. This is important because (for example) we are more likely to see manta ray at Hin Daeng & Hin Muang with greenish water and slight currents than with blue water and 40m+ visibility. Thermoclines can appear at any time and can also affect visibility.
Water temperatures are quite stable, varying from 28 - 31 degrees throughout the year and a 3mm shorty wetsuit is suitable for most people.
Please remember that the above is only a rough guide based on our observations over the past few years. We don't pretend to be able to forecast the weather and often the weather forecast is wrong (please do not ask us to predict the weather in advance of your trip to Koh Lanta!).
For those of you looking for more information, there are many weather resources on the internet to help with your planning: