What's the weather like in Hainan in early September?

September is still typhoon season, so it depends on your luck to come and travel. As long as it's not caused by a typhoon, the problem is not big. It won't rain for long in Sanya, and it will avoid getting sunburned. If it is a typhoon, it is not suitable to go to scenic spots. Good luck! My personal suggestion is to book accommodation four to five days in advance, and pay attention to the weather when booking.