The summer months in Ha Long Bay (June to August), are generally much wetter, due to the tropical rainstorms. Average temperatures range between lows of 17°C to highs of 32°C. In general August is the hottest time of the year to visit, however also the wettest. Rainfall has been recorded at approximately 265 millimetres, with the average rain or drizzle falling on 14 days of the month. Sunshine throughout these months range between 8-9 hours per day. Night time temperatures rarely drop below  25°C, providing a very warm evening.

During the winter months (November-February), Ha Long Bay’s climate tends to be a lot drier and cooler, making it the ‘low season’ to visit. Temperatures range from lows of 0°C to highs of 24°C. There is usually around 5 to 8 hours of sunshine per day, with November being the sunniest month. During these months the humidity is approximately 83% and the average rainfall is 23mm, with the wettest month being January. In general, these months usually present fog, and tend to have low visibility.

Source: Weather 2