Urban heat effect
: focusing on relative humidity
Urban Heat Island(UHI)
: The relative warmth of a city compared with surrounding rural areas.
The urban heat island (UHI) phenomenon has emerged since the mid-20th century due to rapid industrialization and urbanization. As cities have expanded, there has been a simultaneous increase in residential, commercial, and public facilities, while green areas have decreased. This has resulted in higher temperatures in the urban airspace compared to surrounding areas, primarily due to various artificial heat sources and air pollutants.
Causes of the urban heat island
The causes of the urban heat island phenomenon can be attributed, first, to the emissions from factories and vehicles, as well as heat emissions from heating systems. Second, the proliferation of artificial structures like asphalt and concrete has contributed to this phenomenon. These artificial structures have high heat absorption efficiency, absorbing light and re-emitting it in the form of infrared radiation, thereby increasing the atmospheric temperature. Additionally, the reduction of green areas exacerbates the urban heat island effect. Vegetation helps lower atmospheric temperatures by evaporating moisture and absorbing solar heat, creating shade that blocks solar energy's heat.
Result of the urban heat island
As a result of the urban heat island effect, temperatures rise as one moves towards the city center and decrease as one moves towards the outskirts. It often leads to lower temperatures in surrounding areas compared to the city center, and temperature fluctuations can be significant during calm wind and clear sky conditions. Seasonally, the urban heat island effect tends to be more pronounced in winter compared to summer, and in the summer, it can lead to phenomena like tropical nights, where even after sunset, the atmospheric temperature does not drop. In winter, various pests overwinter in cities, affecting the quality of human life. Furthermore, the urban heat island phenomenon worsens air quality, impacts the bioclimatic distribution within the city, and exacerbates smog-related issues.
Temperature of Seoul, Korea
Heatwave
: a period of abnormally hot weather, often defined with reference to a relative temperature threshold, lasting from two days to months.
A heatwave refers to an extended period of extreme heat, lasting for several days or more. Heatwaves not only impose heat stress on humans but also on ecosystems, including wildlife. They restrict outdoor human activities and disrupt efficient sleep even at night, impacting health. Prolonged heatwaves can have adverse effects on livestock and crop growth. Heatwaves are significantly influenced by the Urban Heat Island (UHI) water-vapor feedback. Moreover, humid regions experience more severe heatwave issues than dry regions.
The relationship between Urban Heat Island and water vapor
In humid environments, a complex and subtle warming feedback intensifies the Urban Heat Island (UHI) phenomenon compared to dry conditions. The humidity effect reveals a significant average temperature difference of 3.3 K when comparing UHI daytime temperatures in humid and dry settings.
In the lower troposphere, UHI convection cooling involves warm water vapor air parcels and their re-radiation effects. If water vapor feedback were not significant, the opposite effect would occur, and urban areas in regions with high precipitation would cool more than dry urban areas. Particularly, water vapor feedback in UHI intensifies heatwaves, leading to prolonged and severe heatwave conditions that have significant impacts on health and economic issues.
UHI index
In “A Study on the Intensity of Urban Heat Islands in the Seoul Metropolitan Area by Weather Conditions(Kim et al., 2012)” quantitatively calculated the intensity of the urban heat island in Seoul using hourly temperature data from Seoul and three surrounding areas (Yangpyeong, Icheon, and Dongducheon). In the preceding research, the urban heat island intensity was defined based on the temperature difference between Seoul and the surrounding areas.
Therefore, in this study, the urban heat island intensity index was defined as the temperature difference between the urban center and rural areas. The urban center corresponds to the location with a latitude of the 50th, while the rural area corresponds to the location with a latitude of the 10th. The temperature of ETA 1 closest to the ground was used.
