Hanoi's Climate
Cradled in the low-lying Red River delta, Hanoi has a humid subtropical climate with four genuinely distinct seasons — the defining rhythm of life in the north. The city receives about 2,203 mm of rain a year, most of it in the May–October wet season, while the cooler half of the year alternates between crisp dry spells, misty drizzle and the notorious indoor dampness northerners call nồm.
Sweltering summers and sudden downpours
From late April the southwest monsoon pushes hot, moist air across the delta, opening a summer that lasts into September. June is the peak, with average highs of 33.4°C, though humidity and the urban heat island make it feel far hotter — rush hour on Nguyen Trai or Giai Phong avenue can feel like standing over a griddle. Afternoons often end in violent thunderstorms that turn a blazing sky black within minutes. Locals catch the breeze along West Lake and Hoan Kiem Lake, or escape to Ba Vi mountain and Dong Mo lake on weekends.
The August–September storm peak
Though well inland, Hanoi still catches the outer bands of typhoons striking the northern coast. September is the wettest month, averaging 441 mm over 22 rainy days — rain two days out of three. Cloudbursts regularly flood the city's low-lying streets and stall motorbikes en masse, while along the Bui and Tich rivers to the southwest, flash floods running off the hills can leave riverside villages waterlogged for a week. When the Red River runs high, the mid-river islets and silt banks below the dykes disappear under rust-red water.
Cold snaps, drizzle and the season of sweating walls
From November, successive surges of the northeast monsoon sweep down the delta. December is the coldest month, with average lows of 14.1°C, but sharper cold snaps push temperatures below 10°C — scarf-and-padded-jacket weather in the Old Quarter. Late winter wraps Hoan Kiem Lake in fine drizzle, and from February to April the nồm sets in: floors and staircases “sweat”, and laundry refuses to dry. Winter is also when air quality bottoms out, as temperature inversions and still air trap fine dust over the city.
When to come and what to pack
Autumn, roughly October–November, shows Hanoi at its finest: blue skies, golden light and a crisp northerly breeze — the season of young green rice from Vong village and the scent of milk flowers along the old boulevards. For the Perfume Pagoda festival, go in late winter but carry a light raincoat for the drizzle. In summer, pack sun protection, drink plenty of water and stay off the streets at midday during heat waves. Winter calls for a proper jacket, especially on a motorbike at dawn or dusk. During the damp nồm weeks, keep the windows shut and run a dehumidifier — opening the doors “for fresh air” only makes it worse. And when the city stifles, the low hills of Ba Vi and Soc Son sit a few degrees cooler.