Rainfall

Thailand's climate is ruled by monsoons that produce three seasons in Northern, Northeastern 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 southwest monsoon arriving around July and lasting into November (the 'rainy season'). This is followed by a dry, 'cool' period (the 'cool season') from November till mid-February, followed by much higher relative temperatures (the 'hot season') from March to June.

It rains more and longer in the South, which is subject to the northeast monsoon from November to January, as well as the southwest monsoon. Hence most of Southern Thailand has only two seasons, a wet and a 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 north the dry season may last six months (mid-November to May); in most of Central and Northeastern Thailand five months (December to May); on the upper peninsula three months (February to May); and below Surat Thani only two months (March and April). Occasional rains in the dry season are known as 'mango showers' (heralding the mango season).

In central Thailand it rains most during August and September, although there may be floods in October when the ground has reached full saturation. If you are in Bangkok in early October don't be surprised if you find yourself in hip-deep water in certain parts of the city. In 1983, when the floods were reputed to be the worst in 30 years, it was that deep in every part of the city! It rains a little less in the north, August being the wettest. The northeast gets less rain and periodically suffers droughts. In Phuket it rains most in May (for an average 21 out of 30 days) and in October (for an average of 22 out of 30 days), as it is hit by two monsoons. Generally, travelling in the rainy season is not unpleasant, but unpaved roads may be impassable.

The arrival of the rains brings relief from the hot, dry weather of the preceding months and softens the soil for planting.

Temperature

Most of Thailand - with the mountains in the north and the Korat Plateau of the northeast notable exceptions - is very humid, with an 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 northeast plateau, and temperatures easily soar to 39°C in the day-time dropping only a few degrees at night. The temperature can drop to 13°C at night during the cool season in Chiang Mai and even lower in Mae Hong Song - if you're visiting the North during the cooler months, long-sleeved shirts and pullovers are in order. Because temperatures are more even year-round in the South, when it is 35°C in Bangkok it may be only 32°C in Phuket.