Hi,
Halong Bay belongs to the Northern of Vietnam so it is generally cooler than the Southern of the country. It experiences two main distinct seasons.
+ In winter months, the weather gets colder and colder from November to March and considerably warmer in April (a leap into summer). This is also time to observe much cloud and frequent drizzle. The average winter temperature is 17.2°C (62.9°F). Particularly, deep inside bay is often colder; temperature may keep below 10°C (50°F) for long period.
+ In the summer (from May to October), the whole region is quite hot. Temperature may rise to 37°C (98.6 °F) in the peaks of June and July. Halong City enjoys equitable weather with high average humidity (80%) all year round though they may occasionally be subjected to typhoons and heavy rain in late summers (September to November). The distinct features: pleasant spring (typically March-April) and autumn (October-December), a chilly winter (December- February) and a hot summer (June – July).
Sapa has one of the most favourable climates in Vietnam. The temperature remains stably cool throughout the year though winter may see more severe changes, with frost and even snow (mostly from mid-December to mid-January).Humidity is relatively high all year round, between 83-87% and temperature ranges within 15-30 Celcius degree. If you include Sapa in your itinerary, make sure to bring some warm clothes since it can get quite cold at night