Kagoshima is subtropical and the seasons are pronounced. Spring (late March to May) is the easiest time: cherry blossom in early April, mild days and clear views of Sakurajima. Autumn (October–November) is the other reliable season, with cooler air and the Ohara Matsuri folk-dance festival filling Tenmonkan on the first weekend of November.
Summer (June–August) is hot and humid, with a wet rainy season in mid-June and typhoons possible from late August. The volcano is more active in summer too, and the prevailing wind tends to push ash inland over the city; carry an umbrella for it. Winter is mild — frost is rare and the sea moderates the cold — but cloudy days can hide Sakurajima for a week at a time.
Time a visit for early August if you can: the Kagoshima Kinko Bay Summer Night Fireworks is the largest in southern Japan, with the rockets bursting in front of the volcano from a barge in the harbour.