Batam is hot, humid and tropical year-round, with daytime temperatures hovering around 28–32°C. The driest months — and the best for beach trips and bridge drives — are roughly May to September. The wet season runs November to March, with heavier downpours in December and January; ferry crossings can get rough. Avoid the worst of the haze if you can — smoke from forest fires in Sumatra sometimes drifts over between July and October.
Worth timing your visit around: the Nongsa Regatta in January between Singaporean and Batam sailors at Nongsa Point Marina; the Mid-Autumn (Mooncake) Festival in September, which the local Chinese community celebrates with lanterns and pastries; and Hari Raya Puasa at the end of Ramadan, when the island's Muslim majority puts on family feasts and prayers.