Two seasons and what they actually mean
Phuket has no winter or summer in the usual sense — there is a dry season from November to April and a wet one from May to October. Air temperature stays between 28 and 33 °C all year and the sea never drops below 28 °C. The difference between seasons is not heat but wind, swell and rainfall.
The dry season brings the northeast monsoon: calm sea, good underwater visibility, every sea route open. The wet season brings the southwest monsoon: bigger swell on the west coast and some long-range routes closed — but the rain does not fall for days on end, it comes in short heavy bursts of 30–60 minutes, usually in the afternoon.
The practical consequence: what ruins a trip is not rain but the sea being closed. That is why seasonality matters enormously for sea tours and barely at all for land ones.
Month by month
November is the transition into the dry season. The sea settles, the landscape is still green after the rains and there are fewer visitors than in December. One of the best months for weather-to-price ratio. The Similans open on 15 October and Maya Bay on 1 October.
December and January are the peak. Perfect sea, minimal rain, maximum crowds and prices. Over the New Year weeks hotels and boats are booked months ahead and the popular routes get busy.
February and March offer the same weather with lighter crowds. March is noticeably hotter — the warmest month of the year, easily 34–35 °C by day. For land routes, choose an early departure.
April is peak heat with the sea still calm, though swell starts building late in the month. Mid-April brings Songkran, the Thai New Year: a week of street water fights that is either the highlight of the trip or a serious inconvenience, with no middle ground.
May and June open the wet season. Rain appears but is still limited, prices drop sharply and the sea often stays calm through the first half of May. One of the most underrated windows.
July and August are high season by occupancy despite the rain — this is when Europe takes its holidays. Swell on the west coast can be serious and red flags go up regularly.
September and October are the wettest and cheapest months. The Similans are closed until 15 October and some long-range sea routes do not run. Land tours operate as normal, and the jungle is at its greenest.
What closes and when
Similan Islands National Park is closed from 16 May to 14 October. That is an official decision of the Thai national parks department and there is no way around it. If the Similans are non-negotiable, plan for November to April.
Maya Bay on Phi Phi closes annually from 1 August to 30 September for reef recovery. Phi Phi itself remains accessible year-round — only the programme changes.
In a heavy storm the marine authority closes departures from the ports for a day or two. This can happen in any season but is more frequent in the wet one. Land routes — Cheow Lan, elephants, sightseeing, shows — run regardless of sea conditions.
Choosing your month
If the sea, snorkelling and far islands are the point, take January to March: that is what Phuket is loved for. If price matters more and you can live with afternoon downpours, May, June and September deliver the same trip for noticeably less, with the land programme unaffected.
With small children, November and February make more sense: less heat than March–April and a calm sea. For diving, the best visibility runs February to April.
One practical note: in any season, schedule your sea tours for the first days of the trip. If the weather forces a reschedule you still have spare days, rather than choosing between postponing and cancelling.