Title: Bangkok
Content:  Like most of Thailand, Bangkok has a tropical savanna climate (Aw) under the Köppen climate classification and is under the influence of the South Asian monsoon system. It experiences three seasons: hot, rainy, and cool, although temperatures are fairly hot year-round, ranging from an average low of 22.0 C in December to an average high of 35.4 C in April. The rainy season begins with the arrival of the southwest monsoon around mid-May. September is the wettest month, with an average rainfall of 334.3 mm. The rainy season lasts until October, when the dry and cool northeast monsoon takes over until February. The hot season is generally dry, but also sees occasional summer 
Title: Geography of Thailand
Content:  of the south as well as the eastern tip of the east have a tropical monsoon climate. Parts of the south also have a tropical rainforest climate. Thailand has three seasons. The first is the rainy or southwest monsoon season (mid-May to mid-October) which prevails over most of the country. This season is characterized by abundant rain with August and September being the wettest period of the year. This can occasionally lead to floods. In addition to rainfall caused by the southwest monsoon, the Intertropical Convergence Zone (ITCZ) and tropical cyclones also contribute to producing heavy rainfall during the rainy season. Nonetheless, dry spells 
Title: Thailand
Content:   Thailand's climate is influenced by monsoon winds that have a seasonal character (the southwest and northeast monsoon). Most of the country is classified as Köppen's tropical savanna climate. The majority of the south as well as the eastern tip of the east have a tropical monsoon climate. Parts of the south also have a tropical rainforest climate. Thailand is divided into three seasons. The first is the rainy or southwest monsoon season (mid–May to mid–October), which is caused by southwestern wind from Indian Ocean. Rainfall is also contributed by Intertropical Convergence Zone (ITCZ) and tropical cyclones. August and September being the wettest period of the year. The country receives a mean annual rainfall of 1200 to 1600 mm. Winter or the northeast monsoon occurs from mid–October 
Title: Thailand
Content:  mid–February. Most of Thailand experiences dry weather with mild temperatures. Summer or the pre–monsoon season runs from mid–February until mid–May. Due to its inland nature and latitude, the north, northeast, central and eastern parts of Thailand experience a long period of warm weather, where temperatures can reach up to 40 °C during March to May, in contrast to close to or below 0 °C in some areas in winter. Southern Thailand is characterised by mild weather year-round with less diurnal and seasonal variations in temperatures due to maritime influences. It receives abundant rainfall, particularly during October to November. Thailand is among the world's ten countries that are most exposed to climate change. In particular, it is highly vulnerable to rising sea levels and extreme weather events.
Title: Geography of Thailand
Content:  occur for one to two weeks from June to early-July. This is due to the northward movement of the ITCZ to southern China. Winter or the northeast monsoon starts from mid-October until mid-February. Most of Thailand experiences dry weather during this season with mild temperatures. An exception is the southern part of Thailand which receives abundant rainfall, particularly during October to November. Summer, or the pre–monsoon season, runs from mid-February until mid-May and is characterized by warmer weather. Due to its inland nature and latitude, the north, northeast, central, and eastern parts of Thailand experience a long period of warm weather. During the hottest 
Title: Ko Klang
Content:  The tropical climate of Thailand has only two seasons: the dry season from November to May and the rainy season from June to October. The average temperate is 28 degrees Celsius, though temperatures can rise up to 34-37 degrees during the hot season.
Title: Floods in Thailand
Content:   Floods in Thailand are regular natural disasters in Thailand which happen nearly every year during the monsoon season. The monsoon seasons in the country are distinct by region, the southern part mirrors the Malay Peninsula and monsoon begins in Oct and ends in March. The rest of the nation has monsoons and/or frequent thundershowers from April/May through October, but often lasts beyond October. Thailand cycles yearly between drought and flooding. Breaking this cycle was the subject of the "Sustainable Water Management Forum 2016" in Bangkok. The event hosted water management specialists from countries which have dealt with water management challenges such as the Netherlands, Israel, and Singapore. One attendee observed that, "In Thailand, we receive around 754,000 million m3 of rain per year. That is more than enough for the annual water demand of around 100,000 million m3.... However, only 5.7 percent of rainfall, 70,370 million m3, empties into the reservoirs."
Title: 2011 Southern Thailand floods
Content:  The 2011 Thai floods occurred at the end of March 2011, normally the latter part of the dry season (ฤดูแล้ง or หน้าแล้ง nah lang) in tropical Thailand. Heavy rain fell in the southern region, with over 120 cm of rain falling in certain areas.
Title: Geography of Thailand
Content:  southern Thailand and eastern parts of Trat Province receive more than 4500 mm of rainfall per year. The driest areas are the leeward sides of the central valleys and the northernmost portion of south Thailand where mean annual rainfall is less than 1200 mm. Most of Thailand (north, northeast, central, and east) is characterized by dry weather during the northeast monsoon and abundant rainfall during the southwest monsoon. In the southern parts of Thailand, abundant rainfall occurs in both the northeast and southwest monsoon seasons with a peak in September for the western coast and a peak in November–January on the eastern coast.
Title: Chanthaburi
Content:  Chanthaburi has a tropical monsoon climate (Köppen climate classification Am), with little variation in temperature throughout the year. Rainfall, however, varies dramatically by season. Rainfall is light and infrequent in the short dry season (December to February), but the wet season (April to October) features torrential rain. March and November are transitional months, during which significant rainfall may occur in some years, while in other years little rain falls.  The Cathedral of the Immaculate Conception, the principal church of the Roman Catholic Diocese of Chanthaburi, is the largest church in Thailand.

Question: when is the rainy season in bangkok thailand?
Answer: September.