Ha Long Bay is a center of a larger zone which includes Bai Tu Long bay to the northeast, and Cat Ba islands to the southwest. These larger zones share similar geological, geographical,geomorphological, climate.
Halong Bay has an area of around 1,553 km2, including 1,960–2,000 islets, most of which are limestone. The core of the bay has an area of 334 km2 with a high density of 775 islets. The limestone in this bay has gone through 500 million years of formation in different conditions and environments. The evolution of the karst in this bay has taken 20 million years under the impact of the tropical wet climate. The geo-diversity of the environment in the area has created biodiversity, including a tropical evergreen biosystem, oceanic and sea shore biosystem. Halong Bay is home to 14 endemic floral species and 60 endemic faunal species.
Halong’s climate is humid and tropical. With a hot, humid summer and dry, cold winter; Halong’s temperature varies from 15ºC to 25ºC.
It is possible to visit Halong Bay year-round, with the best time to visit from October through April, although during the winter months of December, January and February, the weather can be cool and overcast with low visibility. During the summer months from May to September, temperatures rise and storms are more likely during the rainy season.