Hanoi

Many bridges cross the wide Red River.

New trains now run high above some of Hanoi's busy streets.

Many bridges now cross the Red River, and ring roads help link the center with the edges of the city.

The Cat Linh–Ha Dong metro line opened on 6 November 2021. Its trains run on a raised track about 13 kilometers long. A second line, from Nhon toward Hanoi Railway Station, opened its first 7.8 kilometers on 8 August 2024. The whole line is 12.5 kilometers long, partly underground.

Universalis describes how, during the colonial period, roads and railways took over from the river as the city's main routes, and how the city then spread outward like a star along radial roads; today many bridges cross the Red River, and ring roads strengthen links between the center and the outskirts.

UrbanRail.Net records two metro lines. The Cat Linh–Ha Dong Line is a 13.1 km elevated route linking the center with Ha Dong; it opened from Cat Linh to Yen Nghia (12.8 km) on 6 November 2021. The Nhon–Hanoi Line (Line 3) runs from Nhon Street through Cau Dien, Cau Giay, Kim Ma and Giang Vo streets to Hanoi Railway Station: 12.5 km, of which 8.3 km is elevated and the rest underground. Its elevated section from Nhon to Cau Giay, 7.8 km, opened on 8 August 2024.

Winter in Hanoi is cool. Summer is hot and very rainy.

The most rain falls in August.

Hanoi has a cool winter and a hot, rainy summer. The World Meteorological Organization's averages for 1991 to 2020 give January days of about 20 °C and nights of about 15 °C. In June and July, days average about 33 to 34 °C and nights about 26 to 27 °C.

Winter is fairly dry: December gets only about 24 millimeters of rain. Summer is much wetter, and August is the wettest month, with about 335 millimeters. A whole year brings about 1,628 millimeters.

WMO normals for Hanoi (1991–2020) give average highs and lows of 20.0 °C and 14.6 °C in January, the coolest month, and 33.8 °C and 26.3 °C in June and 33.3 °C and 26.5 °C in July, the hottest months.

The monthly averages add up to about 1,628 mm of rainfall a year, most of it from May to September (188 to 335 mm a month, with August the wettest). From November to February each month gets only 19 to 48 mm, yet February still averages 12 rainy days. Winter rain days fall to 7 in November and December.

In the old part of Hanoi, each street once sold one kind of thing.

The river is often higher than the city. Big walls of earth keep the water out.

Hanoi's old quarter is known as the '36 streets'. Long ago, each street was home to people working in one trade, and the quarter linked the citadel with the river landing where goods arrived.

The city sits only about 5.5 meters above sea level, and for six or seven months of the year the Red River is higher than the city. Long walls of earth called dikes, built up as high as 14 meters, protect Hanoi from the biggest floods. Hanoi has many lakes, among them West Lake.

Universalis describes precolonial Hanoi as a city defined by its river. The merchant quarter, the '36 streets' specialized by guilds, joined the citadel to the river landing. During the colonial period traffic turned toward roads and railways, and the city turned away from the river behind its dike. The French-era plan used the many lakes as garden-city features; later housing filled the space toward the To Lich River, and many lakes disappeared.

The city sits at only about 5.5 m, and the Red River stands above it six to seven months a year, so its alluvial levees have been raised up to 14 m to hold back the highest floods. Since the economic opening of the 1990s, shops and services have spread from the old quarter across the city, and high-rise hotels and buildings have multiplied in the colonial districts and around West Lake.