The weather on Koh Phangan is best described by the average temperature of 25 ℃ – 34 ℃, with lots of sunshine, high humidity and, as the word tropical rainforest implies, the occasional showers. The average annual rainfall is about 1,800 millimeter, February has the lowest average of about 34 millimeter whereas November has the highest average of about 450 millimeter.
The weather is pretty constant during the year but roughly there are three main seasons on Koh Phangan:
The dry season (mid December – April) when it is mostly dry and relatively cool
The hot season (May – September), when it can be very hot with occasional storms
The wet season (October – mid December), when there can be a strong and windy monsoon and a lot of rain
The dry season lasts from late December through to March and this is the main tourist season of Koh Phangan. The tourist season’s weather is hot, bright and not to humid - just right for lounging on the island’s fantastic beaches. February is the driest month and under the Köppen climate classification is the only month which has precipitation levels that are regarded as a dry season.
Due to its proximity to the equator, the climate on Koh Phangan is tropical. Overall we would say the best time to visit Koh Phangan is from January to August, when the weather conditions are usually at their best. However like in many countries the weather on Koh Phangan can be unpredictable and local: on one part of the island you might lie on the beach while on the other side you may have to shelter for a scattered thundershower. If you like a tan, be aware that the sun can be extremely strong. Even on a cloudy day you can get sunburned easily.
Koh Phangan is classified as having a tropical monsoon climate meaning that the island has a dry season and a very wet season. The weather of Koh Phangan realistically has three seasons; the horrid monsoon season which is thankfully short lived, a drier season and a hot season which has frequent heavy showers.
Towards the end of the dry season the humidity of Koh Phangan rapidly increase. From April until the late autumn monsoons the weather of Koh Phangan is the hot and humid with frequent downpours. These rains are short lived and there is always blue sky's afterwards.
Koh Phangan's location means that it avoids much of the south-eastern monsoon that punishes Thailand's Andaman coastline. If you are planning to visit Thailand during the summer months Koh Phangan and Koh Samui would be a sensible location to consider.
Koh Phangan is hit by a much later monsoon, which arrives from the south west and strikes between September and mid November. As a tourist avoid this season due to flooding and high winds that disrupt transport connections. Koh Phangan as an island has less annual rain that the island’s main resort rival of Phuket.