The city of Haikou is on the northern edge of the torrid zone, and is part of the monsoon tropical zone. April to October is the active period for tropical storms and typhoons; most occur between August and September.
The best time to visit Haikou is between November and April. When the weather is fine here this is a gorgeous spot, although on hot days it may be more comfortable outside the city and on one of the island's great beaches.
October to May here is the rainy season and in September especially, expect to experience very heavy rainfall. During June to September typhoons are not unusual and very strong wind affect the city approximately once every ten days during this period. The rains however, do not last for too long and despite their severity if you plan your trip well, it's unlikely the bad weather will affect your plans too much.