First we would like to give you brief information about geographic location of Koh Samui. The island is situated in the Gulf of Siam a very calm place which connects with South China Sea in the South-East. Due to its geographic location there can never be a tsunami like the one happened in Phuket. Same location features affect the climate of Koh Samui.
Samui Climate and hence the weather on Koh Samui is significantly different from the climate and weather of the rest of Thailand. And if during the summer in Thailand the weather is rainy, it does not mean that it will also be rainy in Koh Samui - on the contrary, it will be sunny and only in the evening there might be short thunderstorms In general - the weather on Koh Samui favorably differs from the weather in Thailand, making holiday on Koh Samui pleasant throughout the year - both in summer and winter.
Due to its climatic features, Koh Samui does not have so pronounced rainy or hot seasons.
The seasons such as the rainy season - when the tropical rain is usually short and powerful, but virtually inevitable in these latitudes, happens here is a little more often than usual; or the hot season - when the air and water temperature slightly higher than usual - these times of year or seasonal changes in weather on Koh Samui do not coincide with such in the other places in Thailand.
Here is a rather revealing graph of temperature and precipitation on Samui:
Please note that the graph was compiled on the basis of the results of observations of Samui weather during 30 years - from 1961 to 1990. More recent data on Samui climate is here:
And please don't be upset if you have found that some months are promised to be rainy. It is not that bad. If there is going to be a storm it doesn't have to be on the whole island. It might be raining hard on one side of the island while on the other is still sunny. And the rain might end just as quickly as it started.
Speaking about climate in general there are some peculiarities:
- October, November and December are the least hot months. Rainfalls are more often than usual, there are waves, the sea gets rough. - January, February and March. Rains are getting rare. The sea becomes quiet and clears itself from the organic litter which has been flushed into the sea during the rainy season. - In April and May the number of cloudy days decreases, temperature is rising, the water is getting warmer. - June to September. Hot tropical sun, clear blue sky, short thunderstorms and perfectly clean water await you in that period.
Note that there might be some changes in nature's timetable. There might be a very hot week in December as well as a sudden storm in July. But in general everything is as described above.
However, now, in the time of global climate change, it is difficult to give a 100% guarantee that, for example, in March, which is traditionally considered the dry season throughout Thailand, there will not happen a cyclone (as it was in 2011).
Information on the beaches of Koh Samui, high / low tides, currents and more can be found in this section. And here is a very detailed article on the beaches of Koh Samui. Enjoy your vacation in Koh Samui and may the weather only pleases you!