The southwest monsoon arrives between May and July and lasts into November. This is followed by a dry period from around November to May, which begins with lower relative temperatures until mid-February (because of the influence of the northeast monsoon, which bypasses this part of Thailand but results in cool breezes), followed by much higher relative temperatures from March to May. It usually rains most during August and September, though floods in early October may find you in hip-deep water in certain parts of the city. An umbrella can be invaluable – a raincoat will just make you hot.
It’s worth remembering that we’re talking about the weather here, a temperamental beast if ever there was one. So all the dates above are flexible. In 2008, for example, Bangkok was flooded by a major storm in normally dry January, and the cool season stretched well into March.