Thailand’s climate is ruled by monsoons that produce three seasons in Northern, North Eastern, Eastern and Central Thailand, and two seasons in Southern Thailand. The three season zone, which extends roughly from Thailand’s northernmost reaches to Phetchaburi province on the Southern Peninsula, experiences a ‘dry and wet monsoon’ climate, with the south-west monsoon arriving between May and July and lasting into November.
This is followed by a dry period from November to May, a period that begins with lower relative temperatures until mid-February, followed by much higher relative temperatures from March to May.
It rains more and longer in the South, which is subject to the north-east monsoon from November to January, as well as the south-west monsoon. Hence most of Southern Thailand has only two seasons, a wet and dry, with smaller temperature differences between the two.
Although the rains “officially” begin in July (according to the agricultural calendar), they actually depend on the monsoons in any given year. As a rule of thumb, the dry season is shorter the farther south you go. From Chiang Mai the dry season last 6 months (mid-November to Mau); in most of Central and North-Eastern Thailand, five months (December to May); and below Surat Thani only two months (March and April).
In Central Thailand it rains most during August and September, though there may be floods in October when the ground has reach full saturation. If you are in Bangkok in early October, don’t be surprised if you find yourself in deep water in certain parts of the city. In Phuket it rains the most in May (an average of 21 out of 30 days) and in October (an average of 22 out of 30 days). Travelling in the rainy season is generally not bad, but unpaved roads may be impassable.
Most of Thailand – with the mountains in the North and the Korat Plateau of the North-East notable exceptions – is very humid, with and overall average humidity of 66% to 82%, depending on the season and time of day. The hot part of the dry season reaches its hottest along the north-east plain, and temperatures easily soar to 39 C in the daytime, dropping only a few degrees at night. The temperature can drop to 13 C at night during the cool season in Chiang Mai. If you’re visiting the North during the cooler months, long sleeved shirts and pullovers would be in order.