Ha Long Bay has a subtropical climate with distinct seasons, and the timing of your visit significantly affects the experience.
October to April is generally the best period. From October through December, the weather is cool, clear, and dry — the finest conditions for visibility on the water and for photography. January and February are slightly cooler (15–20°C) but still good. March and April warm up again with continued good weather.
May to September brings the wet season, with higher humidity, occasional typhoons, and water conditions that can shut down the bay entirely. July and August are the worst months: hot (30–35°C), humid, and prone to sudden storms that force cruise cancellations. The visibility on the water is also reduced by haze and rain. Some cruises are cancelled with no notice during this period.
A note on the mist: The famous low mist that gives Ha Long Bay its ethereal, ink-painting quality is most common in early morning from November through March. The mist burns off by 9 or 10am. An overnight cruise that puts you on the water at dawn in this period is the ideal way to experience the bay at its most atmospheric.
Typhoon season: June through November, with peak risk in September and October. Check weather forecasts and be aware that boats may be held in port for safety during storms.