Natural conditions

Terrain

Ha Long bay has a terrain of islands mixed with lagoons, mangrove intertidal areas and steep limestone islands. Islands are mainly limestone islands (over 90%) with different heights (from 50 to 200 m). In addition, Ha Long bay has some soil islands where there are people, animals living with rich vegetation. The bottom of Ha Long bay is quite flat with the average depth of 5 -10m,  some channels have the average depth of 15 - 29m.

Climate
Ha Long Bay has a tropical climate, hot and humid, with two main seasons (winter: from November to the next March, average temperature 15^oC - 20^oC; summer: from May to September, average temperature 26^oC - 27^oC) and 2 transitional seasons (spring is in April, autumn is in October, the average annual temperature is 18^oC - 19^oC).
The average annual rainfall is from 2.000 mm to 2.200 mm.
Hydrology
River hydrology:Rivers which have influence on Ha Long bay are the Troi river, the My river, the Man river, the Dien Vong river, the Vu Oai river and a part of the Lach Huyen river. These rivers flow into Ha Long bay through Cua Luc bay.
Oceanography:
+ Waves: because of the shielding of many islands, the average height of waves of Ha Long bay is not high, about 0,5 m (up to 2,5m in stormy days), the common directions are North in winter and South in summer.
+ Flows: the average speed is 0,3 - 0,5 m/s, the common directions are North and Northwest during high tides and South during low tides.
+ Tides: The tidal regime in Ha Long bay is a typical diurnal regime with a high tide and a low tide in a day. The tidal amplitude ranges from 1,9 m to 4,6 m. Low tides are in March, April, August and September, high tides are in January, June, July and December.
Seawater salinity:21 – 22 ‰ in rainy season, 32 – 33 ‰ in dry season.