The western coast of Thailand has a very distinct weather pattern and is quite different from the Gulf of Thailand and the east coast. The weather is similar to that of Bangkok with the drier season from November through to May.
At this time of year, the European winter months, he weather is cooler at around 75' F with clear skies and little rainfall. It is hardly surprising that Phuket and the environs attract many North Europeans from Finland, Sweden, Norway and even Denmark, who flock to the beaches of the south of Thailand to get away from the dark cold days in their home countries.
Come April and May, at an almost pre described point in time and very close to the Songkram Festival, the rainy season starts. This is something of a mis named period since the rains are heavier but its not a daily downpour with no sun.
In fact the rains come and go, sometimes heavy, sometimes very short bursts. As the months progress into July and August the rainfall increases and it becomes more humid and hot. At this time of year the best policy is to do as little activity as possible. A trek in the jungle would be quite tiring and unless you enjoy sweating buckets, then its much better to do this kind of activity in the drier period.
For divers and anything to do with sea activity, it's the time of year to stay out of the water. Many of the boat services are restricted or even stopped during July and August as the seas can get very rough with high winds and rain.
This is often described as the Monsoon period, but actual monsoons, in their full blown state are not a daily occurrence.