Hateruma has a subtropical climate. The beach is swimmable from around April through October; the water is at its warmest in July and August, though the summer months also bring the risk of typhoons. Typhoons can shut down the ferry service entirely and strand visitors for several days — this is worth planning around.
For stargazing, the dry winter months from November through March are best. The Southern Cross rises highest in early spring. Winter nights can be cool by Okinawa standards (temperatures dropping to around 15°C) but rarely cold. Spring and autumn offer the best combination of pleasant weather and clear skies.