The climate of most of Thailand is governed by three seasons: rainy (roughly May–Oct), caused by the southwest monsoon dumping moisture gathered from the Andaman Sea and the Gulf of Thailand; cool (Nov–Feb); and hot (March–May). The rainy season is the least predictable of the three, and worth considering when thinking about the best time to visit, varying in length and intensity from year to year, but it’s never a case of the heavens opening in May and not closing again till October: there’ll be rain most days, but often only for a few hours in the afternoon or at night. The rains usually gather force between June and August, coming to a peak in September and October, when unpaved roads are reduced to mud troughs.
The cool season is the pleasantest time to visit, although temperatures can still reach a broiling 30°C in the middle of the day. In the hot season, when temperatures often rise to 35°C in Bangkok, the best thing to do is to hit the beach.
Within this scheme, slight variations are found from region to region. The upland, less humid north experiences the greatest range of temperatures: at night in the cool season the thermometer dips markedly, occasionally approaching zero on the higher slopes, and this region is often hotter than the central plains between March and May. It’s the northeast that gets the very worst of the hot season, with clouds of dust gathering above the parched fields, and humid air too. In southern Thailand, temperatures are more consistent throughout the year, with less variation the closer you get to the equator. The rainy season hits the Andaman coast of the southern peninsula harder than anywhere else in the country: rainfall can start in April and usually persists until November.
One area of the country, the Gulf coast of the southern peninsula, lies outside this general pattern. With the sea immediately to the east, this coast and its offshore islands feel the effects of the northeast monsoon, which brings rain between October and January, especially in November, but suffers less than the Andaman coast from the southwest monsoon.
Overall, the cool season is the best time to come to Thailand: as well as having more manageable temperatures and less rain, it offers waterfalls in full spate and the best of the upland flowers in bloom. Bear in mind, however, that it’s also the busiest season, so forward planning is essential.