Vejret i Ko Pha Ngan (Koh Phangan) i september

Forum posters consistently rate September as one of the better months on the Gulf side of Thailand, with the Southern Gulf islands' rainy season not really starting until mid-October or November - meaning Koh Phangan in September still sees long sunny days and generally calm, flat seas. Posters are candid that as a tropical island, brief downpours are still possible and conditions can vary noticeably across the island or even between neighboring islands on the same day, but a tropical shower typically passes quickly rather than settling in. Posters generally recommend September as a strong pick for beach time and boat trips here, ahead of the genuinely wetter stretch that follows in October and November.

Regulars are specific this island stays largely sheltered from the prevailing weather by the Thai mainland, the same effect that makes nearby Koh Samui a reliable choice this time of year, occasional heavy showers the exception rather than the rule. Our row backs that hot, genuinely wet read, 30.8C reading as a scorching 35C, over 26.2C nights feeling 29C, with a substantial 111.6mm of rain across 19.6 days and humidity at 79 percent. Regulars are honest experiences do vary though, some visitors reporting rain most evenings, three or four times a week, occasionally flooding roads and washing sand up onto walkways. Seas generally stay calm enough for good boat trips and water activities regardless. The sea sits at a bath-warm 30C. The record: 9.5 hours of sun, a 25.8 kph wind and AQI 27.

June here has good weather and sea conditions by the board's own account, and one fixed date dominates everything else: the Full Moon Party at Haad Rin draws around thirty thousand people, room rates spike, and regulars say to book three to five days around it well in advance regardless of the rest of the month. Away from that beach it is a normal quiet island. Our row backs a warm, wet-leaning June - 31.3C reading as 36C at 78.3 percent humidity, with 107.4mm across 19.4 days - and their own caution about the party itself is practical: there is no cover on that beach, so a shower there means getting properly wet. The sea is 30.3C. The record: 9.5 hours of sun, a 24.3 kph wind and AQI 30.

April is the hot one and regulars do not soften it: the sun is close to directly overhead, cover up and take the shade, and anything strenuous belongs early or late. Our row gives 30.2C reading as 36C at 79 percent humidity, with nights that never drop below 26.9C and a sea at 31.5C that cools nobody. Two calendar notes. Songkran runs the 13th to the 15th every year and transport before and after it is booked solid, so plan that leg early. The full moon party moves with the moon, so look up the date for your year rather than assuming a week. Rain is building but not yet heavy, 113.8mm across 11.8 days. The record: 10.7 hours of sun, a 19.8 kph wind and AQI 49.

January is still the rainy season on Thailand's Gulf coast, which catches people out after reading about Phuket. Regulars call it hit and miss: some years sunny with calm seas, others rough water and a good number of wet days, and 2017 flooded badly enough to cancel the ferries and the party. Our row's 99.5mm across 8.8 days is about eleven millimetres a wet day, real rain rather than drizzle, though they say it usually falls for a short spell and stops rather than settling in. Two things help. The north-west corner of the island is the sheltered side, often only a half-hour shower in the morning; and the drop into February is steep, from roughly 144mm to 33mm on the charts they quote. The record: 28C over 25.3C reading as 32C, a 22.6 kph wind and a 27.6C sea.