In the years after the subsidy era ended, few people owned air conditioners. Even during power outages, a few minutes with a bamboo hand fan was usually enough to fall asleep.

Since the 2000s, both the climate and the city have changed rapidly. Summers today can turn much of the country into a furnace. Hanoi ranks among the hottest cities, with some days approaching 41 degrees Celsius while nighttime temperatures linger at 28-30 C.

The city is not only getting hotter. It is also losing its ability to cool itself after sunset.

Many parts of the world are facing similar heat shocks. South Asia has seen temperatures of 45-46 C. Central Europe endured an unusually hot May, with temperatures climbing above 35 C.

Two layers of causes are at work.

The first is rising global temperatures, amplified by phenomena such as El Niño and heat domes that trap hot air.

The second is the urban heat island effect. During the day, concrete, asphalt, glass, and steel absorb large amounts of heat. At night, they release that heat back into the air, making city centers 4-6 C warmer than surrounding suburbs.

The spread of air conditioning adds another layer. As millions of outdoor condenser units run through the evening, they can push outdoor temperatures up by an additional 1-2 C.

Yet if both Hanoi and Ho Chi Minh City endure intense daytime heat, why does Hanoi often remain stifling at night while Ho Chi Minh City becomes more tolerable?

The answer lies largely in geography, even though both cities face pressure from dense construction and high-rise development. Hanoi has lost much of its natural wind network. Ho Chi Minh City is hot and humid year-round, but it still benefits from a natural cooling system: Proximity to the sea, a dense web of rivers and canals, and regular afternoon rain.

Hanoi was once a city of lakes and waterways with a favorable microclimate.

West Lake, Truc Bach Lake, hundreds of ponds, abundant trees, tiled roofs, and shaded alleys all played a role. The lakes were more than scenic features. They acted as thermal buffers. Water absorbs heat more slowly than concrete during the day. At night, lake surfaces release heat gradually and help regulate the surrounding microclimate. The filling of ponds and lakes, the loss of trees, and the spread of concrete and high-rise buildings have weakened a natural balancing system that existed for centuries.

A woman covers herself while walking on a street in Hanoi under strong heat on May 25, 2026. Photo by VnExpress/Tung Dinh

Many cities have faced similar crises and now treat heat resilience as a matter of survival.

At local levels, many short-term measures can be implemented without waiting for major infrastructure projects.

In Tokyo, some neighborhoods organize community water-splashing events during extreme heat. The practice, rooted in the traditional custom of uchimizu, cools streets through evaporation in the late afternoon and evening. I have taken part myself and found it both enjoyable and effective.

Many countries have expanded tree planting in public spaces and use reflective materials on sidewalks and roads to reduce heat absorption. Permeable surfaces, shaded walkways, and bus stop roofs equipped with radiative cooling panels may seem modest, but together they make a significant difference. Some governments also provide incentives for homes and apartment buildings to install rooftop and wall-mounted solar panels, turning absorbed heat into electricity for daily use.

In construction, India and several other countries have long used white lime coatings or light-colored paint on roofs and walls. The approach is inexpensive yet highly effective. It can reduce roof temperatures by 10-20 C and indoor temperatures by 2-5 C. Rooftop gardens, green walls, and planted balconies help cool building surfaces. Climbing plants on west-facing walls can block intense afternoon sunlight.

Many countries are promoting passive design principles that draw on traditional architectural knowledge found in temples, palaces, castles, and historic homes. Thick walls, courtyards, ventilation corridors, steep roofs, and sun screens can keep interiors much cooler than the outside environment. In apartment buildings, passive design focuses on optimizing airflow and shading. Cross-ventilated units, deep balconies, sun-shading fins, insulated or double-glazed roofs and windows, open corridors, green facades, and rooftop gardens can lower indoor temperatures by several degrees and significantly reduce the need for air conditioning during heat waves.

On a larger and longer-term scale, many cities have taken decisive action.

Seoul in South Korea launched an initiative called "urban wind path forest" to channel cool air from surrounding mountains into the city center. The city also restored the Cheonggyecheon stream by removing an elevated highway, helping lower temperatures in central districts.

Singapore has turned the fight against urban heat into a national planning principle. Wind corridors are strictly protected. Green roofs and urban greenery are standard features. The city has invested in district cooling systems that serve entire neighborhoods.

China has become known for its sponge city model, which relies on water-absorbing green infrastructure that reduces both flooding and heat. Several advanced economies use satellites and sensors to identify deadly heat hotspots in real time and target interventions where they are most needed.

Their common approach goes beyond planting more trees. They redesign airflow and restore the way a city breathes, something Hanoi is gradually losing.

Vietnam's major cities also need broad, long-term solutions.

One example would be a comprehensive "cool Hanoi" strategy that integrates heat mitigation, flood control, lake and river restoration, wind corridors, greener transportation and industry, and public cooling systems.

Hanoi can also learn from countries that have large projects to assess their impact on local microclimates before approval.

Will a project block wind, trap heat, or make surrounding neighborhoods hotter? Singapore applies a clear rule to new developments: If one square meter of ground-level greenery is removed, one square meter of greenery must be replaced elsewhere on the site or at an elevated level.

There are encouraging signs, however. Hanoi has already begun to change, both in policy and implementation. Projects involving urban trees, retention lakes, river restoration, urban forests, sponge-city concepts, and ecological river corridors are moving forward.

The city should also add new benchmarks such as how many square meters of green space each resident has access to, how many minutes it takes to walk to the nearest park, or how far residents must walk to reach a shaded pedestrian route.

Without major reforms, Hanoi and many other Vietnamese cities risk becoming trapped in a vicious cycle: The hotter it gets, the more air conditioners people use, and the more heat they release, making the city hotter.

For decades, many Asian megacities measured modernization through concrete-and-glass towers. In an age of climate extremes, that standard is no longer enough. A modern city must also be ecological and livable.

The capital of the future should value every river and lake, create shade, nurture the wind, and retain the ability to cool itself.