The climate in Thailand is influenced by monsoon winds, which are seasonal in nature (southwestern and northeastern monsoons). The southwestern monsoon, which begins from May to October, is characterized by the movement of warm humid air from the Indian Ocean to Thailand, causing heavy rainfall in most of the country. The northeastern monsoon, which begins between October and February, brings cold and dry air from China to most of Thailand. In south Thailand, the northeastern monsoon delivers mild temperatures and heavy rainfall at the eastern coast of this region. Most of Thailand has the type of “tropical humid and dry or savannah climate” (tropical savannah climate of Keppen). In the south and east tip of the east tropical monsoon climate.
Thailand is divided into three seasons. The first is the season of rains or south-western monsoons (from mid-May to mid-October), which prevails in most of the country. This season is characterized by heavy rains, and August and September are the wettest periods of the year. Sometimes this can lead to flooding. In addition to precipitation caused by the southwestern monsoon, the intertropical convergence zone (ITCZ) and tropical cyclones also contribute to the formation of heavy rains during the rainy season. However, drought periods typically occur for 1-2 weeks from June to early July. This is due to the northward movement of the intertropical convergence zone into southern China. Winter or northeastern monsoon begins from mid-October to mid-February. In most of Thailand during this season there is dry weather with moderate temperatures. The exception is the southern part of Thailand, where there are heavy rains, especially from October to November. Summer or pre-moon season lasts from mid-February to mid-May, and is characterized by warmer weather.
Because of its inner nature and latitude, the north, northeast, central and eastern parts of Thailand experience a long period of warm weather. During the hottest season of the year (March to May), temperatures usually reach 40 ° C (104 ° F) or more, except in coastal areas where the sea is blowing moderate daytime temperatures. Conversely, outbreaks of cold air from China can lead to lower temperatures; in some cases (especially in the north and northeast) around or below 0 ° C (32 ° F). Southern Thailand is characterized by mild weather all year round with less daily and seasonal variations in temperature due to the sea.
Most of the country receives on average 1,200 to 1,600 mm of rainfall per year (47 to 63 inches). In certain areas, however, the windward slopes of the mountains, like Ranong Province on the west coast of southern Thailand as well as eastern Trat Province, have more than 4,500 mm (180 inches) of rainfall annually. The most arid areas are on the downwind part of the central valleys and the most northern part of southern Thailand, with an annual average rainfall of less than 1200 mm (47 inches).