Koh Samui's climate is tropical. Wonderfully perfect in both temperature and humidity. Most people find it pleasant with average temperatures of 30 degrees Celsius, or 86 degrees Fahrenheit. More or less summer here year round.
Koh Samui has three seasons :
Dry Season, December - February: Generally very little rain and often refreshing evening breezes cool the island. Great for bodysurfing!
Hot Season, March - August: The weather is still dry but hot. In May, it rains just a little. This is a great period to visit Samui and the temperature is a fairly constant 30-35 degrees.
The Monsoon, September - November: It's still hot with many sunny days. The beauty of finding yourself in a refreshing monsoon shower is in itself invigorating. The showers often last just a short while and refresh the island water supply. A time to enjoy the islands waterfalls and regeneration of the flora on this sceptered isle.