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.
Koh Samui climate is different from the other parts of Thailand. Rain Season and Hot Season are not so strongly marked. More than that, these seasons do not coincide with the same seasons in other places like Pattaya or Phuket.
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 decrease, 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.