The climate of Ha Long Bay is tropical, being wet and hot. It is devides into four main seasons: winter is from November to March of following year, the temperatures drop to between 15oC – 20oC. Summer is from May to September, the temperatures increases to between 26oC-27oC Two others seaons: Spring and Autumn, climate is cool. The annually average temperature between 18oC – 19oC.
Annual rainfall averages from 2.000mm- 2.200mm. The salinity of seawater divides into between seasons: rainy season 21‰-22‰, dry season 32‰-33‰.. The bottom of bay is quite flat with the average depth of 5 -10m, in some parts 15-29m depth. There are narrow passages such as: Cua Luc(20m), The Vang(22-27m) and the others with 9-10m depth. Thanks to the system of islands and islets, the waves in Ha Long Bay are quite light, here is a uniform daily tide vanation of 3,5-4m. The low spring tides in March, April, August and December, the highest tide is in January, June, July and December.