It can be quite cool from November to March in Halong Bay, there are some days during December and January can be very cold (about 10 DC) but this does not last very long. If you are planning to travel to Sapa during this time too then it would be good to bring along some good jackets with you!

Please see the best time to visit Halong here: http://www.footprintsvietnam.com/FAQs/Halong/Best-time-visit-Halong.htm

See also other FAQs

What kind of food on boat in Halong Bay?

What do we have to bring with us to the boat?

Do we have to bring passport for sleeping aboard?

Where does the boat anchor at night time?

Can we sleep outside on the top deck of the boat?

What kind of boat do you use?

How many caves in Halong will we visit?

Is one day trip to Halong Bay worth to visit?

Do you have life jackets on boat?

What if the weather turns very bad?

What about seasick on boat?

Can we travel from Hai Phong to Halong?

How far is that from Hanoi to Halong Bay?

When is the best time to visit Halong?