Weather in Phuket
Phuket has three seasons, but mostly it is hot and humid. The cool season (actually quite hot), the hot season (very hot), and the rainy season (hot and wet). The climate is dictated by the monsoon winds. That is the direction of the prevailing winds caused by the difference in the way oceans and land heat.
From May to November the great land mass of Asia is warmer than the Indian Ocean. The colder, heavier air over the Indian Ocean pushes into the warm air over the continental land mass. This causes the prevailing winds to come from the southwest over the Indian Ocean, bringing lots of warm moist air and therefore lots of rain.
Then as the Asian continent cools from November to April, the ocean becomes warmer than the land. The prevailing wind direction changes to come from the northeast. That is from the great land mass of China bringing cooler, drier air and good weather.
The result is a fairly predictable weather pattern.
The Cool Season
The cool season runs from November to February. The northeast winds normally start sometime in November bringing an end to the rainy season. The cool season is the best time of year to visit Phuket. There is lots of blue sky, very little rainfall, lower humidity and the temperatures are comfortably hot. This is Phuket's high-season with the peak of visitors coming over the Christmas and New Year period.
The Hot Season
The hot season is from March to early May. The weather is dry and the skies blue but many people find the temperature is a little too hot for comfort. This is mid-season for Phuket. There are still a lot of visitors but the hotels start to lower their rates around this period as demand drops.
The Rainy Season
The monsoon winds from the southwest start in early May and bring the rain. The highest rainfall comes in May, September and October. In these months it can rain for long periods. These three months are definitely not the time of year to come if you want sun or outdoor activities. However, June, July and August are good months to visit. During these three months, there will be plenty of rain but it usually falls in short heavy bursts. In between, the weather is pleasant with comfortable temperatures and a cooling breeze. There will often be periods of several days without any rain.
The six months of the rainy season are Phuket's low-season. Hotel rates are slashed and you can generally get lower prices from tour operators, vehicle rentals, boat trips, etc. The months June, July and August are still a popular time to visit as people take advantage of the low season prices. May, September and October are quiet as it is just too wet for most people.
These monsoon winds bring large waves and dangerous undercurrents to Phuket's western beaches. Every year around 20 people drown during Phuket's rainy season. Red flags are raised on the beaches to warn of dangerous conditions. Do not swim in the sea if there are red flags.