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Koh Tao-forum threads comparing it to Koh Lanta for early January make the geography plain: the Gulf coast, where Koh Tao sits, runs its monsoon roughly November through March, while the Andaman coast opposite is already in its dry season by November - so 'Thailand in January' means two different climates depending which coast you pick. Regulars note the monsoon has historically been mostly a November event here, but recent years have seen it linger later, cold and rainy conditions and rough, low-visibility diving stretching into mid-February in a bad year. Early January specifically can still carry wind and chop left over from December, improving gradually week by week rather than all at once. The peak diving season proper doesn't arrive until February through May. The record: 26.1C days feeling like 29C, 25.1C nights, 101mm across 8.7 days, a 27.3C sea, 78 percent humidity, wind averaging nearly 23 km/h and 10.3 hours of sun.

February is the month the Koh Tao forum names first: the northeast monsoon has passed, the skies come clear, and February to April is the island's driest run. Visibility is good - regulars say the only month really worth avoiding for diving is November - and the crowd has thinned from the 15 December to 15 January peak, so it is high season without being the high of it. The wind is the local variable rather than the rain: in February it runs east to west, and under about 10 knots is the threshold people use for good snorkelling. Our row has it at 22.2 kph. The record: 26.5C days that feel like 30C, 25.3C nights, 7.8 precipitation days carrying 49.6mm, 10.7 hours of sun and a 28.2C sea.

Ko Tao's forum calls March a good window as the seas calm after the rainy season — flat water and great visibility for diving, with only occasional cloudy days and the odd rain shower rather than a washout. Regulars flag the humidity as climbing as the hot season sets in, though it's not yet the peak heat of April-June.

April is the hottest month in Thailand and regulars say Koh Tao takes the edge off it - an island with a sea breeze rather than a mainland city - while still being stifling through the middle of the day. February to April is the driest stretch here, and what rain there is tends to be a quick shower in the late afternoon or early evening rather than anything that costs you a day. Some divers name late April and May specifically, on the grounds that the weather is usually settled and the island is quieter, and accept the heat as the price. Our row's 28.2C reads as 34C at 78 percent humidity, with a 30.6C sea. The record: 26.7C nights, 11.6 precipitation days carrying 104.5mm and 10.7 hours of sun.

Regulars are specific this island's own monsoon doesn't actually arrive until October or November, this month instead falling within a Gulf-coast stretch that regulars call usually pretty good, cooled slightly from the hottest months and genuinely quieter than peak season. Our row backs that hot, genuinely wet read, 28.2C reading as a hot 32C, over 26.3C nights feeling 29C, with a substantial 111.8mm of rain across 18.8 days and a real 24.2 kph wind. Regulars are honest the weather stays genuinely hard to predict day to day, capable of being fantastic, bringing a few rainstorms, or just turning windy, with most water flat except a sheltered stretch on the east coast worth seeking out. The sea sits at a bath-warm 30.1C. The record: 9.3 hours of sun, humidity 79 percent and AQI 33.

Koh Tao-forum threads on July weather call it hot and mostly sunny, with dramatic electrical storms visible out over the mainland most nights - a genuine spectacle rather than a disruption, since they rarely reach the island itself. Rain does show up, but posters are split on how much: some describe it as roughly half sun and half rain across July and August, others call it noticeably cooler with occasional evening showers that come as a welcome break from the heat. The consistent thread is that any given shower rarely lasts more than an hour, clearing back to sun and good snorkeling visibility quickly. Regulars recommend Koh Tao over Thailand's west-coast islands this time of year, since the Andaman side takes the brunt of the real monsoon. The record: 27.8C days feeling like a sticky 31C, 26C nights, 135mm across 20.5 days, 80 percent humidity, a 29.9C sea and wind averaging over 25 km/h.

Forum posters consistently rate September as one of the best months on Koh Tao - long sunny days, calm flat seas, and good diving and snorkeling conditions, with whale shark sightings a real possibility this time of year. Posters are clear the Gulf coast's monsoon doesn't really arrive until late October into November, so September stays comfortably ahead of the wetter stretch. As a tropical island, posters note short rain bursts are still normal, typically in the afternoon and passing quickly, with conditions varying enough that rain on a neighboring island doesn't necessarily mean rain on Koh Tao itself. Posters generally recommend September as a strong pick for diving specifically, ahead of the genuinely rainier months that follow.