The bay is one of sea islands in tropical wet with 2 seasons: hot and moist summer, dry and cold winter. Average temperature is from 15°C- 25°C. Annual rainfall is between 2000mm and 2200mm. Ha Long Bay has the typical diurnal tide system (tide amplitude ranges from 3.5-4m). The salinity is from 31 to 34.5MT in dry season and lower in rainy season.
The bay consists of a dense cluster of 1,969 limestone monolithic islands, each topped with thick jungle vegetation, which rise spectacularly from the ocean. Several of the islands are hollow, with enormous caves. Hang Đầu Gỗ (Wooden stakes Cave) is the largest grotto in the Ha Long area. French tourists visited in the late 19th century, and named the cave Grotte des Merveilles. Its three large chambers contain large numerous stalactites and stalagmites (as well as 19th century French graffiti). There are two bigger islands, Tuần Châu and Cat Ba, that have permanent inhabitants. Both of them have tourist facilities, including hotels and beaches. There are a number of wonderful beaches on the smaller islands.
Some of the islands support floating villages of fishermen, who ply the shallow waters for 200 species of fish and 450 different kinds of mollusks. Many of the islands have acquired their names as a result of interpretation of their unusual shapes: such names include Voi Islet (elephant), Ga Choi Islet (fighting cock), and Mai Nha Islet (roof). 989 of the islands have been given names. Birds and animals including bantams, antelopes, monkeys, and lizards also live on some of the islands.
Almost these islands are individual towers in a classic fenglin lanscape which height is from 50m to 100m and height/width ratios up to about 6.
Another specific feature of Halong Bay is the abundance of lakes inside the limestone islands, for example, Dau Be island has six enclosed lakes. All these island lakes occupy drowned dolines within fengcong karst.
