Siwan experiences a subtropical climate with distinct seasonal variations. During spring, temperatures rise gradually from March to May, averaging around 29-30°C. Summer months of June to September are hot and humid, with temperatures often reaching up to 38°C. Autumn brings a slight drop in temperature from October to November, but it's still warm, ranging from 22-29°C. Winter is the coolest period, with December being the chilliest month, averaging around 19°C.