Weather

 
The Thais divide the year into three seasons: the cool season, hot season, and rainy season. For most westerners, this translates to "hot," "really hot," and "really hot and wet." Temperatures rarely drop below 25° C (70° F).

Phuket has some of the most constant temperatures of all the popular tourist destinations in Thailand. Highs and lows vary by only about five degrees Fahrenheit over the course of the year.

The rainy season, which coincides with the European and North American summers, is not an entirely bad time to visit, since the rain generally falls within the span of 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. Sometimes it can be like a warm shower.

The rainy season is generally shorter and more pronounced in Phuket than it is in Bangkok. The rain increases sharply in September, but then tapers off just as quickly. January to March are the best months to visit if you want to avoid just about any possibility of rain.
 

It's perhaps worth noting that Samui and Phuket have different weather patterns. At the height of the rainy season in Phuket, Samui will still be relatively dry. Depending on when you want to travel, Samui may be the better option.