The climate is tropical and humid for most of the year. The best time to visit Bangkok is November-February, when the northeast monsoon is blowing cool, dry air that serves as a respite from the heat. Temperatures at that time of year range 65-90 F/18-32 C. Nights in December can be particularly cool.
The worst time to visit is in April and May, when the heat has even the Thais complaining. Air pollution is also at its worst then. In the hot season, temperatures average about 92 F/34 C, although it's not uncommon to exceed 100 F/37 C. The wet season (late May-October) is characterized by heavy rain almost every day. The city has a poor drainage system, and even brief spells of rain can result in moderate flooding in some parts of the city.