[Reading Science] The Heat Dome: The City That Never Cools… The Layer of Radiant Heat Covering Winter Nights

Winter nights are cold, but the city does not cool down easily. The heat absorbed by roads and buildings during the day lingers in the air, creating an invisible “heat dome” over the urban area.

This cover is thin but persistent. As the night deepens, the heat slowly dissipates, and the city retains a warmth as if it were wrapped up and sleeping.

Rooftops of the city seen through a thermal imaging camera. The black area at the bottom is a rooftop with a cool roof installed. The Asia Business Daily DB.

The Roads Cool Down, but the Air Remains

Around midnight in winter, the city center is on average 2 to 3 degrees warmer than the suburbs. The radiant heat absorbed during the day is slowly released throughout the night, and weak winds and clear skies trap this heat in the atmosphere.

The stable layer formed in this way is the “heat dome.” Although invisible to the eye, analysis suggests that this thin layer of air alone can slow the city’s cooling rate by 20 to 40 percent.

Kim Gyurang, a researcher at the National Institute of Meteorological Sciences (NIMS), explained, “In winter, cities experience a complex flow of heat as waste heat from heating, transportation, and commercial facilities mixes with ground radiation heat,” adding, “As a result, nighttime temperature drops are delayed, and even the city’s energy consumption structure changes.” Ultimately, the city is trapped in a cycle where it receives back the heat it generates.

Indoor Heat Is Also Part of the City’s Temperature

Commercial buildings in central Seoul have an average indoor temperature 3 degrees higher than nearby apartment complexes. This is because heat from lighting, equipment, and heating escapes through the walls and roofs into the nighttime air layer throughout the night.

According to building energy analysis data from the Seoul Metropolitan Government, the Korea Institute of Civil Engineering and Building Technology (KICT), and others, the average indoor temperature of commercial buildings in downtown areas during winter is about 21 to 22 degrees Celsius, which is 2.5 to 3 degrees higher than that of nearby residential apartments (around 19 degrees Celsius).

This is because waste heat from lighting, equipment, and heating is emitted through the exterior walls overnight, supporting the city’s “nighttime residual heat.”

The older the building envelope, the greater this effect becomes. According to Seoul’s energy diagnostics, the exterior wall heat loss rate of old commercial buildings was more than 25 percent higher than that of new buildings. As a result, each building acts as a small heater or, on a larger scale, like a greenhouse.

When this heat accumulates, the city cannot fully cool down overnight. The layer of radiant heat thickens on windless days, and people living beneath it experience the paradoxical sensation of “warmth in the midst of a cold wave.”

‘New Attempts to Cool the Winter Urban Heat Island’

Seoul and Daegu have recently been conducting experiments to mitigate residual heat during winter. In the Gwanghwamun and City Hall areas, low-reflectance paving materials have been applied, reducing heat absorption by 30 percent. Daegu is promoting a project to adjust wind corridors between buildings, taking wind direction into account.

In some areas, “heat-dispersing green spaces” are being created between apartment complexes to absorb and disperse nighttime radiant heat as an experiment.

Experts evaluate these efforts as “a new urban science that goes beyond simply blocking summer heat waves to designing the circulation of heat even in winter.”

Beneath the Heat Dome

Several executives from a paint company are participating in the ‘Cool Roof’ campaign, painting the rooftop. Photo by The Asia Business Daily

The warmth of a winter night in the city is not just a climate phenomenon, but a heat circulation system intertwined with urban structure and energy use. Concrete and glass store heat, while heating and transportation add new heat on top of it. In the end, the city functions as a massive heat reservoir.

Dr. Kim Gyurang emphasized, “The heat island effect can no longer be viewed solely as a summer heat wave phenomenon,” adding, “We need to approach it with ‘year-round urban climate management’ that considers energy use, building design, and securing wind corridors together.”

Cooling the city’s heat is now moving beyond cooling technologies to become a scientific field that simultaneously designs for climate adaptation and energy efficiency.

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