The best time to visit Cat Ba is October and November. The summer storms have passed, rain falls to about 143 mm and then 39 mm a month, the air is at its clearest, and in October the sea is still about 28°C, warm enough to swim off a boat. May to September is the swimming season: hot, busy and wet, with typhoons that can stop the boats to the island. December and January are dry and calm but cool. February to April brings drizzle, haze and fog over Lan Ha Bay.
Cat Ba is the largest island in the Ha Long archipelago and the base for Lan Ha Bay. Every route to it crosses the sea, and the best day of most trips is spent on a boat or in a kayak, so fog, which hides the karsts, and storms, which close the crossings, matter more than rain. The Cat Ba Island guide covers what to do and where to stay.
Cat Ba weather at a glance
I found no published long-run monthly normals for a weather station on Cat Ba itself, so the table uses the nearest official station that has them: Bai Chay in Ha Long, about 25 km north across the bay. Its normals come from the Vietnam Institute for Building Science and Technology, as published in Wikipedia's climate table for Ha Long. Sea temperatures are for nearby Hai Phong, on the same gulf, from Climates to Travel. The last column is the verdict for a Cat Ba trip.
| Month | High / low | Rain (rain days) | Sunshine (h/month) | Sea | For a Cat Ba trip |
|---|---|---|---|---|---|
| January | 19.3 / 14°C | 28 mm (8) | 81 | 21°C | Dry, cool, often grey |
| February | 19.7 / 15.2°C | 25 mm (10) | 48 | 21°C | Drizzle and haze |
| March | 22 / 17.8°C | 43 mm (14) | 43 | 22°C | Foggiest, least sun |
| April | 26.1 / 21.5°C | 86 mm (11) | 89 | 24°C | Warming, still hazy |
| May | 30.1 / 24.6°C | 183 mm (12) | 178 | 27°C | Sun returns, sea warm |
| June | 31.5 / 26°C | 297 mm (16) | 168 | 29°C | Swim season, storms begin |
| July | 31.7 / 26.2°C | 364 mm (17) | 187 | 30°C | Hot, busy, typhoon risk |
| August | 31.3 / 25.4°C | 430 mm (19) | 165 | 30°C | Wettest, typhoon risk |
| September | 30.7 / 24.4°C | 276 mm (14) | 177 | 30°C | Hot, typhoon risk |
| October | 28.8 / 22.2°C | 143 mm (9) | 177 | 28°C | Best: clear, sea still warm |
| November | 25.5 / 18.7°C | 39 mm (6) | 156 | 26°C | Clear, dry, cooler sea |
| December | 21.7 / 15.3°C | 18 mm (5) | 128 | 23°C | Driest, calm, cool |
The year totals about 1,923 mm of rain at Bai Chay. As a cross-check, Hai Phong's Phu Lien station, on the mainland about 45 km west of Cat Ba town, records about 1,679 mm a year from the same source, as Wikipedia's Hai Phong climate table shows, with monthly highs and lows within about half a degree of Bai Chay's. Cat Ba sits between the two, so expect the same seasons and a yearly total somewhere in that range.
October and November: the best months
Rain falls from about 276 mm in September to 143 mm in October and 39 mm in November, over only nine and six days, while sunshine holds at about 156-177 hours a month. Humidity drops below 80 percent, the air clears, and days run about 25-29°C.
October edges ahead on Cat Ba because of the sea. At about 28°C it is still warm for a swim off the day boat or at the Cat Co beaches, and it cools to about 26°C in November. The summer holiday crowds have gone home. The catch is that storms are still possible into October and, less often, November, so leave a spare day before any onward flight.
December and January: dry, calm and cool
Winter is the driest time of year, with about 18 mm of rain in December and 28 mm in January, and the bay is usually calm, so boat days run reliably. Expect highs of about 19-22°C, lows of 14-15°C and frequent grey skies, and bring a warm layer for the boat. Cold snaps from the north can go much lower: Bai Chay's record lows are 1.7°C in December and 4.6°C in January.
The sea, at about 21-23°C, is too cold for most swimmers, though kayaking is pleasant on a still day. Sailings thin out: the Tuan Chau to Gia Luan ferry from Ha Long runs about three times a day in winter against five in summer, as the Hanoi to Cat Ba guide notes.
February to April: drizzle and fog
Spring is the foggy season. Fine drizzle and low cloud settle over northern Vietnam, and Bai Chay records only about 48 hours of sunshine in February and 43 in March, the least of the year, with humidity of 84-87 percent. Rain totals are small, but it is damp on 10 to 14 days a month, and the karsts often fade into grey haze.
Boats still run and the misty bay has its own beauty, but for green towers against a blue sky this is the riskiest season, and at 21-24°C the sea is too cool for most swimmers. April improves as the sun returns.
May and June: warm sea, rising rain
May is Cat Ba's underrated month. Sunshine jumps to about 178 hours, highs reach about 30°C, the sea warms to about 27°C, and the rain, about 183 mm over 12 days, mostly comes in short storms. June is hotter and wetter, about 31.5°C and 297 mm over 16 days, with the sea at about 29°C.
Typhoons become possible from late May, though the riskiest months are still to come, and from June Vietnamese school holidays fill the Cat Co beaches at weekends.
July to September: hot, wet and stormy
Highs run about 31-32°C and the sea is at its warmest, about 30°C. Rain peaks at about 364 mm in July and 430 mm over 19 days in August, often in heavy afternoon storms with sunny spells between, and the bay is at its busiest.
This is the heart of the typhoon season. In northern Vietnam, typhoons generally come from late May to mid-November, most often from July to October, according to Climates to Travel. On Cat Ba, a storm is a transport problem as much as a weather one. In July 2024, Typhoon Prapiroon stopped the high-speed boats, the Dong Bai to Cai Vieng ferry and the Cat Hai to Phu Long cable car, leaving nearly 3,900 tourists, 430 of them international, stranded on the island, as Viet Nam News reported in a story republished by Asia News Network.
In September 2024, Super Typhoon Yagi came ashore over Quang Ninh and Hai Phong. In the Cat Hai island district, which includes Cat Ba, more than 4,700 homes were damaged, 66 vessels sank, nine of them tourist boats, and the island's power and telecoms were knocked out, Tuoi Tre reported. The ferry resumed two days after landfall, according to Tuoi Tre News. Most summer weeks pass without a storm, but keep your dates flexible.
What the weather means for your trip
Choosing the month. October and November are best all round, May and early June suit swimmers, December and January are calm and cool, February to April risks fog, and July to September brings heat, crowds and storms.
Planning around the sea. Every route to the island crosses water, by speedboat, ferry or cable car, and all three stop when a storm approaches, so from July to October keep a spare day on either side; the Hanoi to Cat Ba guide compares the routes. Book boat days for the morning, when the water is usually calmest; the Lan Ha Bay kayaking guide and day trips from Cat Ba cover the options, and the beach water quality atlas covers swim conditions.
Cat Ba or Ha Long. The two share one climate, so the choice comes down to how you want to see the karsts. The Ha Long Bay weather guide covers the cruise side, and the Ha Long vs Cat Ba vs Lan Ha comparison helps you choose.
Limitations
I found no published long-run normals for a station on Cat Ba itself, so the table uses Bai Chay, about 25 km north, with Phu Lien in Hai Phong as a cross-check; conditions on the island can differ, especially wind, fog and local showers. The source does not state the averaging period or rain-day threshold. Sea temperatures are for Hai Phong and serve as a proxy. Typhoon tracks can't be predicted more than a few days out, and single years vary widely.
Workaround: use this page to choose the season, check the marine forecast and the day's ferry and speedboat timetable before you travel, book the Lan Ha boat for a morning, and in the storm months keep a spare day before any onward flight.