The weather in Thailand follows certain patterns which are pretty easy to understand. As divers, we are naturally concerned about what the weather will be doing when we are diving. We like sun and calm seas, but diving continues all year.
For most people Thailand's weather is quite warm all year round, especially in the southern parts of Thailand. The southern areas of Thailand in general have less variation in temperatures and between April to November do experience a high average rainfall. In the north temperatiures are a bit milder and if you are visiting this area between November to February you might be advised to pack a pullover or light jacket. Thailand weather is a warm and sometimes mild climate, as Thailand is just south of the Tropic of Cancer in the Southeast Asian monsoon region. In general, the temperatures in Thailand are on average around 29°C.
Northern Thailand Weather:
In the northern regions of Thailand there are mainly three seasons. November to May usually is the dry period getter warmer towards May.
The northeastern monsoons can touch this area around this time of year bringing cool breezes from November to February it can sometimes also bring sleet to the higher regions.
Between May to November the north of Thailand experiences its highest rainfall as the southeast monsoons push their way north.
Southern Thailand Weather:
Most travellers to Thailand would say that the southern region of Thailand has only two seasons - the wet and the dry, but the locals also would include a third around November to February which they call Winter. The temperatures would rarely get below the low 20's, in the evenings, but for the locals this is cold!
Each side of the peninsular experiences opposite seasons, when it is raining and the seas are rough on the western side the eastern side is enjoying blue skies and crystal clear waters, and visa versa.
The west coast experiences monsoon weather from April to October while the east coast the rains are from September to January.
Koh Phi Phi and Phuket Weather:
The rainy season usually runs from May until October. The highest rainfall month is September and although it has the highest rainfall it doesn't rain every day. It usually rains in the afternoons, or at worst there is about three to four days of continual rain at a time. More often than not the drop in temperature at the end of the day will bring about evening storms which completely dissapear by morning. There is not a season that can not be enjoyed. Sometimes the staff here are baffled by the amazing weather and distinct lack of tourists at certain times of the year.
The weather in Phi Phi has three distinct seasons:
1) The hot season from April to May
2) The wet season from June to October
3) The "cold" season from November to March
Every year is different. We had 2 years with very little rain in 2001/2002 but August/September 2003 were very wet, which was good for the islands water supply. The weather changes to "wet" sometime between end of April to June. Normally the summer months are pretty good - certainly more sunny days than rain, but the weather in unpredictable and you can get some really wet and windy days. Rain is usually sporadic short showers and can be very localised - rain in Patong beach but not Karon, just a few km away for example.
The "good" weather starts again sometime in October/November and lasts until April/May. The months November - February are a few degrees cooler due to fresh NE winds. March and April can be really hot, with very little wind.
Phuket climate on offshore islands (Similans, Racha etc.)
It is worth noting that during the rainy season, even if it's raining on the island of Phuket, you are likely to have more sun when you are out at sea. The mountains on Phuket top 500m, so trap clouds and promote thunderstorms. Often a day's diving is sunny, with a rain shower speeding over, you'll get a suntan, but you get wet back on land!