Thailand is located very close to the equator and outside the cyclone zone of the Pacific and Indian Ocean. That’s why it is possible to sail all the year around. There are 2 main seasons, the rainy season and the dry season, also called the southwest monsoon and northeast monsoon. Seasonal winds are predictable, but if the winds change the weather can change very fast.
The northeast monsoon (dry season) begins in November and ends in April. The wind speed during this time is between 5 and 20 knots (1 kn = 1.852 km/h) from northeast. The winds are stable and the temperatures are normally around 30°C. Consistently dry and sunny weather are the best sailing conditions. The Similan Islands can only be visited during the northeast monsoon. In January and February winds are usually weak. People who want to sail in this period must use the boats engine because the winds don’t provide sufficient drive.
The southwest monsoon (rainy season) begins in May and takes place until October. Wind speeds are 10 until 30 knots from southwest. During this time there is much wind and better sailing conditions as in the dry season. There are some rain periods, sometimes persistently, but normally for 1 until 2 hours in the afternoon or in the early evening hours. The existing wind direction is important for the anchorage. During the rainy season it is not possible to anchor at the west coast of Phuket. It is recommended to anchor at the east side of islands or in protected bays. Sailing in the Phang Nga Bay right down to Koh Phi Phi is possible at strong sea conditions. The Similan Islands are closed for visitors during the southwest monsoon. In September and October long rain periods are possible and can sometimes take place up to 5 days without a break.