Halong Bay Weather in December

The short answer: Halong Bay weather in December is cooler and drier than most of the year. The climate dataset shows a 21.7°C high, 15.3°C low, 18.1 mm of rain over 5.2 days and a 21.7°C sea. Choose it for cool-air sightseeing, not guaranteed clear skies or warm-water swimming.

Those are climate normals, not a forecast for every December. They are useful for choosing a month; the official marine forecast is what matters for a particular sailing day.

Halong Bay temperatures in December: the numbers (checked July 2026)

The air figures below are the official climate normals for the Bai Chay station, published by Vietnam's Institute for Building Science and Technology. The sea figure comes from a NOAA/Copernicus-derived temperature dataset. Keeping those sources separate matters because the December numbers in the top-ranking guides disagree by several degrees.

What	December
Daytime high	21.7°C
Daily mean	17.9°C
Night/morning low	15.3°C
Rain	18.1 mm over 5.2 days, the lowest monthly total of the year
Humidity	76.3%, the lowest monthly average of the year
Sunshine	127.7 h (about 4.1 h per day)
Sea temperature	21.7°C average
Daylight	About 10.8 h; sunset from 5:09 to 5:20 p.m.

December is the driest and lowest-humidity month in the normals. That does not make it the clearest: the crachin season begins in late November and peaks in February. Rainfall, humidity and visibility are different measurements.

Daylight is not sunshine. December gives about 10.8 hours between sunrise and sunset, but only 4.1 hours of measured sunshine per day on average. A dry day can still spend hours under cloud or mist.

What December weather does to a Halong Bay cruise

The short day changes the format calculation. A day trip from Hanoi typically lasts 12 to 13 hours door to door, with 4 to 5 hours on the highway. An overnight cruise keeps you in the bay after a December sunset and through the following morning. Exact schedules vary, so verify the departure, return and route when you book.

Sunrise moves from about 6:11 a.m. at the start of December to 6:28 a.m. at the end; sunset shifts from 5:09 to 5:20 p.m. If morning or evening light is the reason you are choosing a format, compare those times with the seller's current schedule rather than assuming the word “overnight” guarantees both.

The recurring winter review is not about rain. It is the traveler who packed for “Vietnam equals tropics” and met a 15.3°C morning on an open deck.

The cold is manageable with layers. A T-shirt may work around the 21.7°C average high; the deck before breakfast calls for a fleece or light jacket. Add a wind-resistant outer layer and closed shoes with grip. That is the article's one packing rule worth remembering.

The 21.7°C average sea temperature makes swimming a matter of cold tolerance, not a reliable December plan. Check the day's conditions and whether your route offers a swimming stop. Kayaking also depends on the actual wind, waves and operator decision. Cruise brochures have mastered the infinity-pool photo at golden hour; in December, golden hour is around 4:40 p.m. and the pool is mostly for looking at.

December sits outside the main late-May-to-autumn tropical-cyclone planning window, but no month is storm-proof. The latest NCHMF annual outlook still warns of strong marine winds from tropical systems and cold-air outbreaks. There is no reliable public cancellation percentage. Check the official NCHMF forecast and the seller's current terms close to departure.

What the December averages do not tell you

Climate normals cannot tell you whether December 27 will be sunny, foggy or windy. They also say nothing about availability, event programming or prices; those belong to the current seller listing.

For December 2026 specifically, the NCHMF outlook issued July 15 projects temperatures in northern Vietnam from November through January at 1.0 to 1.5°C above the long-term average. It also projects rainfall 10 to 20% above average for the wider northern region during those three months. That seasonal signal does not replace a Bai Chay forecast, but it is a useful warning against treating 21.7°C and 18.1 mm as a promise for this year.

Who December is for, and who should skip it

Winter Halong and summer Halong are different trips, not better and worse versions of one trip. December's normals are 15.3 to 21.7°C, 18.1 mm of rain and a 21.7°C sea. July runs 26.2 to 31.7°C, 364.3 mm of rain and a 30.5°C sea. You are choosing between cooler sightseeing conditions and warmer water with much more rain.

December fits if your priorities are the limestone scenery, photography, caves and cooler deck time, and if possible mist will not spoil the trip. If you are still deciding whether the bay belongs on your route at all, our is Halong Bay worth visiting guide takes a side.

Skip December if warm-water swimming and hot weather are central to the trip. If fog would ruin the view for you, remember that the crachin season has already begun even though its peak comes later. December's low rainfall improves the odds of a dry day; it does not guarantee visibility. Our Halong Bay weather and best time guide compares all 12 months.