While there's really no bad time to visit Thailand's capital, rain and extreme heat are less frequent between November and February. April is only recommended to those with portable air conditioners. The rains typically start in July and October brings the heaviest downpours with frequent flooding in the capital. The tourists flock to Bangkok in December and August, while the least crowded months tend to be May, June and September.

Bangkok and Central Thailand are well within tropical latitudes and experience alternating periods of a dry and wet monsoon climate. The south-west monsoon arrives between May and July and lasts into October. This is followed by a dry period from November to May, a period that begins with lower relative temperatures until mid- February (because of the influences of the north-east monsoon, which bypasses this part of Thailand, but results in cool breezes), followed by much higher relative temperatures from March to May. According to the official Thai agricultural calendar, the rains begin in July; however, the arrival of the monsoon can vary. Occasional rains in the dry season are known as 'mango showers'. In Bangkok it usually rains most during August and September, though it can flood in October since the ground has reached full saturation by then. If you are in Bangkok in early October, you may find yourself in hip-deep water in certain parts of the city.

During the cool/dry season (November to February), night-time temperatures may dip as low as 12°C (54°F), with normal daytime temperatures averaging around 28°C (82°F). During the rainy months (June to October), the temperature averages 32°C (89°F) in the daytime, and 26°C (79°F) to 28°C (82°F) at night. Add four or five degrees to the latter temperatures for the hot season (March to May) average. As the city climate is very humid for most of the year, perceived temperatures are often higher than thermometer readings. During the hot season the humidity is compounded by air pollution - the high level of particulate keeps the moisture in the air from evaporating. The lowest humidity occurs between November and May, especially when the occasional upland breeze arrives from the Khorat Plateau to the northeast and pushes back the humid delta air.