Prediction Team 3
Urban Heat Island
Urban Heat Island is the phenomenon that the temperature of the city rises due to decreased green area and various artificial heat from the cities and industrialization.
What is Urban Heat Island?
The phenomenon of an Urban Heat Island (UHI) caused by urbanization disturbs the local convection system such as 'Thermal Induced Air Exchange' and 'Structural Wind Circulation'. It interrupts the inflow of cold fresh air from green areas to urban areas and the spread of air pollutants and blocks its fundamental convection function. As the urban microclimate is mainly affected by building environment, many studies of UHI have been conducted especially in the fields of architecture and urban planning since the 1990s. Lots of policies also are being implemented in many cities based on these studies.
What causes Urban Heat Island?
Urban Heat Island phenomenon is known to be related to the density of the urban population, the city's land use and spatial characteristics. Increasing the density of population in cities leads to the large-scaled development of land, which increases the average temperature in cities. For instance, the average temperature in Seoul has been rising over the past 30 years, and the number of days with high temperature over 30℃ has increased since the mid-1990s. Urban areas covered with asphalt or concrete have a high heating effect. Also, artificial heat released in residential, industrial and commercial areas causes heat island phenomenon.
Green areas and water bodies have been considered the main solution as they affect the mitigation of the heat island phenomenon. Especially, green areas play an important role to improve the artificial climate of the city such as decreasing temperature and circulating contaminated air while regulate relative humidity.
Wind is an important weather factor for urban heat island phenomenon as well. The wind blowing in the city can make hot air flow into one place or spread it out. But, the topography of the city is an obstacle to the progress of the wind. In other words, as vertical structure of the city increases, the wind cannot make active atmospheric circulation. Even, this structure causes solar irradiance changes, making the heat capacity and thermal conductivity of the city higher than rural areas.
In addition, air pollutants exacerbate urban heat island phenomenon. Especially, smog, carbon dioxide, and sulfur dioxide gas keep the city warm at night. When the center of a city is severely polluted by dust, etc., the heated air by urban heat island phenomenon forms a dust roof and interferes with the heat exchange by solar energy, blocking the vertical flow of air and causing more severe pollution in the city center. That is, hot air does not mix with the upper atmosphere and is stuck in a layer, and it prevents polluted air from mixing with the clean air above it. As a result, the stored heat of the atmosphere in the city center is released into the city. In short, air pollution is the cause of the urban heat island phenomenon and the vicious cycle of urban heat island with air pollutants continues to repeat itself.