Koh Samui has a slightlyt hotter temperature than Phuket, but like Phuket temperatures only vary by about five degrees farenheit throughout the whole year.
 

Average Temperature Monthly Highs & Lows for Koh Samui (°C)

The rainy season in Samui is quite different from most of the other tourist destinations in Thailand. Unlike other places, such as Phuket, the Samui rainy season coincides with the European and North American winters. Still, like the rest of Thailand, the wet season is not an entirely bad time to visit, since the rain normally falls only for an hour or two in the late afternoons. The rain cools down the temperatures a bit and cleans the air quite nicely. With the high temperatures, the rain is not exactly cold and even feels like a hot shower.
 

Average Monthly Rainfall in Koh Samui (mm)