Tainan is in the south of Taiwan and has a warm, humid climate year-round. The most comfortable months are October through February, when temperatures drop to a pleasant 15–22°C and humidity is lower. Summer (June–September) is hot, humid, and subject to typhoons — temperatures regularly exceed 35°C and the combination of heat and humidity makes extended walking outdoors tiring.
The rainy season runs from May to September, with most rainfall in June and August. Typhoon season overlaps with this period, and a direct hit can disrupt travel plans. However, Tainan is drier than Taipei and the north of the island even in the wet season, and short visits are usually manageable.
The most interesting time culturally is around Lunar New Year (January or February) when the temples fill with celebrations, and during the major temple festivals throughout the spring. The Yanshui Beehive Fireworks Festival — one of Taiwan's most extraordinary and genuinely dangerous events, where crowds stand in the street to be bombarded by fireworks — takes place in the Yanshui district (part of Tainan municipality) on the fifteenth day of the Lunar New Year. It's spectacular, slightly terrifying, and unlike anything else.