Thailand lies within the humid tropics and remains hot throughout the year. Average temperatures are about 29oC, ranging in Bangkok from 35oC in April to 17oC in December. There are three seasons: the cool season (November to February), the hot season (April to May), and the rainy season (June to October), though downpours rarely last more than a couple of hours.

Most of Thailand - with the mountains in the North and the Khorat 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.