September is usually the warmest month in Hong Kong, with temperatures ranging from 25°C (77°F) to 28°C (82°F). However, since you asked about the upper part of August in Haikou Port area, I would estimate it to be around 22-24°C (72-75°F).
Please note that these are rough estimates and actual temperature may vary depending on various factors like weather patterns, humidity and time of day.