[Reading Science] The Heat Island Effect Persists After Summer Heat Waves... Cities Stay Hot Even in Winter

The Lingering Echoes of the Heat Island Effect Persist on Late Autumn Nights

In the past, the heat island effect would end when summer was over. However, the structure of the city has a long memory. Glass, rebar, and asphalt absorb the day’s heat and release it at night. Even as autumn deepens, that memory does not fade easily.

Urban forests absorb this heat, Cool Roofs reflect the sky’s light, and Mist Roads cool the warmth of the ground. Although the seasons change, the city still remembers summer.

Even after summer passes, cities filled with glass, rebar, and asphalt do not release their heat. Though the seasons change, the city still remembers summer. Last summer, citizens walked through Gwanghwamun Square in Seoul. Photo by Yonhap News Agency

Lingering Traces of Heat Continue Into Late Autumn

According to the Korea Meteorological Administration, the average temperature in Seoul in November of last year was 9.1°C, which is 1.6°C higher than the normal average of 7.5°C.

This year is no different. According to the Korea Meteorological Administration’s data portal, at 10 p.m. on November 18, the temperature in Jongno, Seoul was 2.2°C, while in Yangpyeong, Gyeonggi Province, a suburban area, it was 0°C. The city center was 2.2°C warmer. On the following day, November 19, the temperature in Jongno was 2.7°C and in Yangpyeong it was 0°C, again showing higher temperatures in the city center. This “relative temperature difference” is evidence of the Urban Heat Island (UHI) effect.

The urban heat island phenomenon does not only occur during summer heat waves. According to the “Time Evolution of the Surface Urban Heat Island” study conducted by the American Geophysical Union (AGU) on major cities around the world, the surface temperature difference in major cities averages 2-4°C in summer, and a steady difference of 1-2°C persists on autumn and winter nights when wind speed is low and there are few clouds. This means the intensity of the heat island varies by season, but it is a year-round phenomenon.

This study also analyzed the thermal inertia of cities. In large cities with a high “artificial surface coverage rate,” such as Seoul, Tokyo, and New York, where the ground is covered by man-made structures rather than soil or water, the nighttime cooling rate was found to be 20-40% slower than in the suburbs. In short, “cities do not cool down easily, even after a heat wave has passed.”

'Four-Season Heat Island': Models Reveal Winter’s Residual Heat

Kim Gyurang, a researcher at the National Institute of Meteorological Sciences (NIMS), is a leading expert who has analyzed Seoul’s year-round heat island effect using urban meteorological numerical models. He explained, “The urban heat island is a complex phenomenon that results from the combination of wind speed, cloud cover, and surface coverage. In Seoul, the city center is on average 1-1.5°C warmer than the suburbs even in winter.”

In his urban meteorological numerical modeling (based on WRF-LDAPS) conducted from 2011 to 2013, a “nighttime residual heat pattern” was observed, where heat absorbed during the day is stored in building exteriors and road surfaces and is slowly released as radiant heat throughout the night. In particular, when there are few clouds and wind speeds fall below 1.5 meters per second, the cooling rate is about 30% slower than in the suburbs.

Dr. Kim stated, “This is not just a matter of perceived temperature, but it also alters the urban meteorological field, affecting fog, atmospheric stagnation, and fine dust concentration,” adding, “In the long term, a four-season prediction model that reflects the urban heat island is needed.”

Currently, the Korea Meteorological Administration and the National Institute of Meteorological Sciences are working to improve the national, regionally customized forecast model (LDAPS) by including urban parameterization. Once this model is completed, it is expected that “city-customized forecasts” will be possible, taking into account not only summer heat waves but also residual winter heat.

Future Cities: Heating Up Beyond the Seasons

According to a simulation study published in the international journal MDPI (2024) on Zhengzhou, Henan Province, China, changes in the urban heat island effect are predicted for 2030-2060. From the 2030s, the intensity of nighttime UHI in autumn and winter is expected to increase sharply, peaking around 2050.

The reasons are not simply rising temperatures. The strengthening of the “winter heat island” is attributed to a combination of heat emissions from heating, transportation, and industrial complexes, as well as high-density construction. As climate change intensifies, cities are transforming not into “warm cities,” but into “cities that do not cool down.”

How Can We Cool the Residual Heat Left in Cities?

According to an international academic paper (PMC, 2023) that observed 17 urban forests in Seoul over a year, urban forests exhibited a cooling effect, maintaining an average temperature 1-2°C lower than their surroundings throughout the year. Even at night during autumn and winter (October to January), temperatures in urban forests were on average 1.3°C lower than in commercial areas and roads. Grass-type and mixed-forest urban forests showed a maximum temperature reduction of 2-3°C on late autumn nights. The Seoul Metropolitan Government announced plans to expand urban forests as climate buffer zones and use them as infrastructure to mitigate the UHI effect.

Below the city, Mist Roads cool the heat. According to the Seoul Metropolitan Government’s Road Management Division, Mist Roads operate every year from April 1 to October 31, spraying fine mist on roads three to five times a day (more during advisories) using groundwater from subway outflows or recycled water. In April, May, and October, they operate flexibly only during fine dust advisories.

An official from the Road Management Division stated, “During this year’s heat wave period, we shortened the spraying interval to one hour in some sections, such as around City Hall Station, for more flexible operation. However, we adjusted the intervals according to the available groundwater supply.”

In October, due to frequent rain and the absence of fine dust advisories, Mist Roads were not operated. According to external measurements used by the city, the operation of Mist Roads resulted in a reduction of road surface temperatures by 7-9°C in summer, and a decrease in fine dust by 12μg/m³ in spring and autumn.

Now, Mist Roads in late autumn have evolved beyond simply preparing for heat waves; they serve as “four-season cooling devices” that also improve air quality and mitigate residual heat.

The "Mist Road (Cooling Road)" is operating on the road near Seoul City Hall Station. The Mist Road is a water spraying facility installed in small rectangular shapes along the road's center line. It not only cools down the heated road but also helps remove fine dust. Sometimes, the Mist Road is operated even in the cool autumn weather for this reason. Photo by Yonhap News Agency

Daegu: Heat Island Amidst the Cold

Daegu, sometimes called “Daefrica,” remains hot even in winter. Professor Kim Haedong of the Department of Environmental Engineering at Keimyung University explained, “The urban heat island is caused by man-made structures, waste heat, and reduced wind speed due to tall buildings, so unless wind speed is very high and ventilation is active, it develops regardless of the season. In particular, it is more pronounced at night than during the day, and stronger in winter than in summer.” He added, “When the wind is weak, hot air in the city center cannot escape and stagnates, intensifying the heat island effect.”

The average wind speed in Daegu is only half that of Busan. As a basin surrounded by mountains, Daegu has poor ventilation. Therefore, the urban heat island is strongest on winter nights when the wind is weak.

As a practical countermeasure, Professor Kim suggested, “It is most important to reduce the density of man-made structures in the city center and to control the disorderly construction of tall buildings along prevailing wind corridors to secure wind paths.”

Uncooled City, Cooling Science

One technology that helps reverse urban heat is the Cool Roof. This method uses white, highly reflective coatings on roofs to reduce the absorption of solar radiation.

In a demonstration study registered in 2022 in the Korea Citation Index (KCI) under the National Research Foundation of Korea, applying Cool Roofs to public buildings at Gangnam-gu Health Center in Seoul resulted in a reduction of roof surface temperature by 20°C and indoor temperature by 3°C. Although the insulation effect is limited in winter, the benefits of reduced cooling energy use and UHI mitigation are clear in summer and autumn.

As winter approaches, the weather has turned cold. However, the air in the city remains warm. This is because the city is a structure that remembers heat. Urban forests absorb that heat, Cool Roofs reflect light, and Mist Roads cool the ground’s warmth.

The heat wave has ended, but the city has not yet cooled down. And the science that cools these hot cities has now become not just the science of summer, but the science of all four seasons.