Bangkok is hot year-round (27–35°C) with three distinct seasons. The cool season (November–February) is the best time to visit: temperatures are manageable, humidity is lower, and skies are blue. The hot season (March–May) is brutal — temperatures regularly exceed 38°C in April, the hottest month.
The rainy season (May–October) brings heavy afternoon thunderstorms (usually 1-2 hours of intense rain, then clearing). June and July can have flooding in low-lying areas. The BTS and MRT avoid flooded streets. October is the transition month — some rain but cooling down. Avoid Songkran (Thai New Year, April 13-15) if you don't want to be soaked in a city-wide water fight — or embrace it fully.