Short answer
October and November are the best all-round months: dry, clear and warm enough for the beach until early November. June to August is best for swimming but hot, wet and busy. January to March is cold, grey and often foggy.
01The year in one table
Each column links to that month's page. Darker cells mean better conditions for swimming, boat trips and hiking, while the crowds row turns red when the island is busiest.
| Cat Ba month by month | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Day high | 19° | 19° | 22° | 26° | 30° | 32° | 32° | 31° | 30° | 28° | 25° | 21° |
| Sea | 20° | 19° | 21° | 24° | 27° | 29° | 30° | 30° | 29° | 27° | 24° | 21° |
| Rainfall | 25 | 35 | 50 | 90 | 190 | 250 | 300 | 360 | 300 | 140 | 50 | 25 |
| Swimming | 1/5• | 1/5• | 1/5• | 2/5•• | 4/5•••• | 5/5••••• | 5/5••••• | 4/5•••• | 4/5•••• | 3/5••• | 2/5•• | 1/5• |
| Boat trips | 3/5••• | 2/5•• | 2/5•• | 4/5•••• | 4/5•••• | 4/5•••• | 3/5••• | 3/5••• | 3/5••• | 5/5••••• | 5/5••••• | 4/5•••• |
| Hiking | 4/5•••• | 3/5••• | 3/5••• | 4/5•••• | 3/5••• | 2/5•• | 2/5•• | 2/5•• | 3/5••• | 4/5•••• | 5/5••••• | 4/5•••• |
| Crowds | 2/5•• | 3/5••• | 2/5•• | 3/5••• | 3/5••• | 5/5••••• | 5/5••••• | 4/5•••• | 3/5••• | 2/5•• | 1/5• | 1/5• |
Rounded long-term averages for the Hai Phong coast (Phu Lien and Cat Ba records); sea temperature is a rounded average for Lan Ha Bay. The ratings are our own summary for swimming, boat days, hiking and crowds. Individual years vary, especially in the typhoon months.
02Three seasons, not two
Forget the wet and dry split you may know from southern Thailand. Cat Ba sits on the Gulf of Tonkin and has a cool winter, a short spring, a hot rainy summer and a lovely autumn.
- Summer (May to September). Daytime highs of 30–32 °C, high humidity and short, heavy downpours, often in the afternoon or at night. The sea is at its warmest. This is the domestic holiday season, so weekends are crowded and prices peak in June and July.
- Autumn (October to early December). Humidity drops, skies clear and the sea stays swimmable until about early November. Boats run on calm water and the hiking trails dry out. Our favourite time.
- Winter and spring (mid-December to April). Highs around 19–22 °C, frequent drizzle and sea fog that can hide the karsts entirely, especially in February and March. The water is too cool for most swimmers. Quiet, cheap and atmospheric, if you pack a jacket.
03Typhoons and storm days
The Gulf of Tonkin sees tropical storms most years, usually between July and October. When the authorities issue a warning, boats into Lan Ha Bay and the ferries to the island are suspended, sometimes for two or three days. Typhoon Yagi in September 2024 hit Hai Phong and Quang Ninh head-on, wrecked many floating fish farms and left Cat Ba without power and phone signal while it passed.
A storm does not mean you should avoid these months, but it does mean building in slack. Do not plan to leave Cat Ba the day before an international flight between July and October, and book refundable rooms when you can.
04Holidays that fill the island
- Tet (Lunar New Year). Late January or February. Many local businesses close for several days around the holiday itself, then domestic travel surges in the days after it.
- 30 April and 1 May. Reunification Day and Labour Day form one of the busiest long weekends of the year. Book well ahead.
- June to August. School holidays. Weekdays are manageable, but Friday to Sunday the town, the beaches and the boats are full.
- 2 September. National Day, usually a long weekend, and often the last big rush of the summer.
05Pick your month
Every month has its own page with temperatures, sea conditions and what to expect on the water: January, February, March, April, May, June, July, August, September, October, November and December.
?Questions people ask
What is the rainiest month on Cat Ba?
August, with something like 350 mm of rain on average, followed closely by July and September. Rain usually comes in heavy bursts rather than all-day drizzle, so boat trips often still run.
Can you swim on Cat Ba in winter?
Few people do. From December to March the sea is roughly 19–21 °C and the air is cool and damp. If swimming matters, come between late May and October.
Is Cat Ba foggy?
Often in late winter. February and March bring sea fog and low cloud that can last for days and blank out the limestone scenery. Mornings in December and April can also be hazy.
When are boat trips cancelled?
Mainly during storm warnings in the typhoon season and on rare days of strong winter wind. Operators usually reschedule or refund, but confirm the policy when you book.