Lan Ha Bay and Cat Ba Island are open all year, and every season has something to offer – but the weather in northern Vietnam changes a lot between a hot, humid summer and a cool, often misty winter. As a team living on Cat Ba, we are on the water every day, so here is what to really expect month by month, and how to plan around it.
The short answer
For most travellers the best months are March to May and September to November: comfortable temperatures, calm water for kayaking and fewer crowds than in summer. Come in June to August for warm-water swimming and glowing plankton, and in December to February for misty landscapes, quiet trails and lower prices.
Month-by-month overview
| Months | Weather | Sea | Crowds | Good for |
|---|---|---|---|---|
| Jan – Feb | Cool, around 15–20°C, often cloudy or misty, light drizzle | Cold for swimming | Quiet, except Tet | Trekking, photography, cosy cruises |
| Mar – Apr | Mild and warming, morning mist clears by midday | Calm, cool | Moderate | Kayaking, cruises, trekking |
| May | Warm and sunny | Warm, calm | Busy around 30 April – 1 May | Everything – one of the best months |
| Jun – Aug | Hot and humid, short showers, risk of tropical storms | Warmest, best for swimming | Busiest, Vietnamese summer holidays | Beaches, swimming, glowing plankton |
| Sep – Oct | Warm, clearer skies, storms become rarer | Still warm | Fewer crowds | All-round – our favourite time |
| Nov – Dec | Dry, pleasant days, cooler evenings | Cooling down | Quiet | Cruises, trekking, cycling |
Spring (March – May): fresh and green
Spring brings mild temperatures and calm seas. Early mornings can be misty, giving the karsts a soft, dreamlike look, while afternoons are usually clear. The forest of Cat Ba National Park is at its greenest – a good time for the hike to Ngu Lam Peak. By May the water is warm enough for a comfortable swim.
Summer (June – August): beaches and glowing plankton
Summer is hot and humid, with short tropical showers that often pass within an hour. The sea is at its warmest and clearest, which makes this the best time for swimming, beach stops and jumping off the boat. It is also the peak season for bioluminescent plankton: on dark, moonless nights the water sparkles blue around your paddle.
Summer is also the Vietnamese school holiday, so Cat Ba Town is lively and busy, especially at weekends. Book tours, buses and hotels a few days ahead. Tropical storms can occasionally reach the coast; when they do, the authorities close the bay for safety. With us you can then move to another date or receive a full refund.
Best tours in summer: the glowing plankton kayaking tour and the floating house trip, both with a night kayak.
Autumn (September – November): our favourite season
As the summer crowds leave, the weather stays warm, skies clear up and storms become rare by October. The sea is still pleasant for swimming in September and October, and the light is beautiful for photos. If you can choose any time, this is it.
Winter (December – February): mist and quiet
Winter is cool, often around 15–20°C, with grey skies and mist wrapped around the karsts. It is too cold for most people to swim, but it is a peaceful, atmospheric time for cruising, trekking and photography, with fewer boats and lower prices. Cruises with indoor dining rooms, like our Hera luxury cruise, are especially comfortable now. Bring a warm layer for the evenings on the water.
How to see glowing plankton
Bioluminescent plankton glow when the water is disturbed, so you see the effect best from a kayak or while swimming. Choose a night with little or no moon, away from town lights, roughly between April and October. The strength of the glow depends on nature and changes from night to night, so it can never be guaranteed – but on a good night it is unforgettable.
Holidays to plan around
Cat Ba gets very busy during Vietnamese public holidays: Tet (Lunar New Year, usually late January or February), 30 April – 1 May, and 2 September. Buses and hotels sell out and a small holiday surcharge applies. If your trip falls on these dates, book as early as possible.
Our recommendations
- First visit, flexible dates: April–May or September–October.
- Swimming and plankton: June–August, booked a few days ahead.
- Quiet and budget-friendly: November–March.
- Trekking: October–April, when the forest is cooler.
Whenever you come, we will match the tour to the weather. Browse our Lan Ha Bay tours, read the best things to do on Cat Ba, or send us your dates for a free tailor-made plan.