Koh Samui is situated in the Gulf of Siam and its annual weather cycle is ruled by two opposing wind systems, namely the Northeast Monsoon, and the Southwest Monsoon. Visitors to Samui who follow the general guidebooks (which as far as the description of Samui's weather pattern is concerned are usually wrong) are often taken by surprise by the weather here upon their arrival. The weather patterns on the southeast coast of Thailand (and therefore Koh Samui) are the opposite of those elsewhere in the country:

WET/ COOL: Mid October through mid March, when the north-easterlies blow, is considered the rainy season on Koh Samui. However, the peak of stormy and rainy weather is usually at the onset of the season, and by mid December the weather generally clears up with extended periods of dry and often sunny weather from Christmas onwards. The rainy season is also our cool season, with temperatures ranging between a very pleasant 25 and 30°C.

DRY/ HOT: By March the north-easterlies are dying down, and the weather starts warming up. Wind-free days are more the rule than the exception, and as the sun on its path north passes through its zenith during this time of the year, the days become hot and dry. Daytime temperatures during April and May can rise to 35/ 36°C.

DRY/ WARM: Once the south-west monsoon sets in in June the temperatures drop again to a comfortable 30°C or slightly above. While the rest of the country suffers through its hot season, Koh Samui, due to its unique geographic position, is only marginally affected. Indeed the island experiences some of its finest weather during this period. A steady westerly breeze and a lot of sunshine should be the rule, only occasionally upset by thunderstorms or strong winds with rain.

Of course, the above are general guidelines based on a climatic pattern which has prevailed and ruled the lives of Southeast Asia's communities over thousands of years. Of course there are always exceptions to these general patterns. It might storm in the so-called dry season, or we could have months of dry weather during the 'rainy' season.

Here are some links to sites with more climatologic details:

Please remember Koh Samui is considered part of the southern province of Surat Thani on the east coast of Thailand. Often a search for just ‘Samui’ might not give you any joy.

Here are some hopefully helpful links we’ve found for you.

For a ten day forecast and satellite map
http://www.weather.com/forecast/

For yearly temperatures, rainfall
http://www.weather.com/temperatures/

For real weather junkies, there’s Thailand’s detailed meteorological site, with an English link. Here are satellite images, radar and animated maps. However, you need a fast modem to download anything.
http://www.tmd.go.th/

For the daily weather forecast:
http://www.tmd.go.th/dailyforecast
There is even more detail here if you like – daily, monthly and seasonal forecasts.

For a five day forecast with forecast, temperatures and satellite maps
http://weather.cnn.com