**5. Spatial structure of urban heat island**

Spatial structure of the urban heat island in Seoul was investigated based on temperature data measured at 23 automatic weather stations (AWSs) in the Seoul metropolitan area.

**Figure 4** shows the average spatial distribution of air temperature in the Seoul metropolitan area for each season and the whole year for 20 years from 1998 to 2017. A relative warm area extends in the east-west direction, and warm cores are pronounced in residential and commercial area with high-story buildings and heavy traffics. A relative cold area is observed in mountainous areas, which is near the borderline of Seoul except the southwestern and southeastern borderlines where the sprawling expansion of urbanization has already progressed.

Cities are often referred to as urban heat islands, with the urban center having the highest temperatures. This is primarily due to the low amount of vegetation in urban center compared to the suburbs and beyond (**Figure 1**). Cities also use large amounts of energy, and emit this energy as waste heat, further exacerbating the urban heat island effect.

On the other hand, forests and other vegetation types use large amounts of solar energy and evaporate water by means of transpiration to cool leaf surfaces. Evaporative water used through transpiration also contributes in reducing air temperatures in urban areas. Forests and other vegetation can also contribute indirectly to temperature reduction by reducing urban energy consumption

**77**

**Figure 4.**

*Forest Decline Under Progress in the Urban Forest of Seoul, Central Korea*

through intercepting and using solar energy and by reducing building energy demand through shading and reducing wind speed. Therefore, low vegetation coverage in urban center results in larger temperature gradients between urban center and urban fringe or beyond [67–69]. Indeed, Seoul's heat island effect is very large [27, 70], as the temperature difference between the urban center and

*Spatial distribution of air temperature in the Seoul metropolitan area averaged for each season and year.*

The urban heat island is closely linked to the land-use pattern (**Figure 3**). Buildings, asphalt, and concrete pavement absorb solar radiation and emit long-wave radiation that warms the atmosphere [71–73]. Moreover, those artificial structures hold heat for extended periods. This heat moves from buildings, asphalt, and concrete pavement to the cool air as the air temperature decreases after sunset to form atmospheric temperature inversion (warm air over cold air) [74–77]. Therefore, many cities located in basins with limited ventilation like Seoul experience serious air pollution problems. The ventilation of an urban basin can be limited not only by orographic barriers but also by urban heat island-induced circulations and/or the capping effect of temperature inversions. Furthermore, land-use and land-cover changes caused by urbanization alter the dynamics of temperature inversions and

urban heat islands, thereby affecting air quality in an urban valley [78].

boundary was about 5°C (**Figure 4**).

*DOI: http://dx.doi.org/10.5772/intechopen.86248*

*Forest Decline Under Progress in the Urban Forest of Seoul, Central Korea DOI: http://dx.doi.org/10.5772/intechopen.86248*

*Forest Degradation Around the World*

**5. Spatial structure of urban heat island**

metropolitan area.

*from Seoul City [59]).*

**Figure 3.**

urban heat island effect.

Spatial structure of the urban heat island in Seoul was investigated based on temperature data measured at 23 automatic weather stations (AWSs) in the Seoul

*A map showing spatial distribution of vegetation and land-use types in the Seoul metropolitan area (redrawn* 

**Figure 4** shows the average spatial distribution of air temperature in the Seoul metropolitan area for each season and the whole year for 20 years from 1998 to 2017. A relative warm area extends in the east-west direction, and warm cores are pronounced in residential and commercial area with high-story buildings and heavy traffics. A relative cold area is observed in mountainous areas, which is near the borderline of Seoul except the southwestern and southeastern borderlines where

Cities are often referred to as urban heat islands, with the urban center having the highest temperatures. This is primarily due to the low amount of vegetation in urban center compared to the suburbs and beyond (**Figure 1**). Cities also use large amounts of energy, and emit this energy as waste heat, further exacerbating the

On the other hand, forests and other vegetation types use large amounts of solar energy and evaporate water by means of transpiration to cool leaf surfaces. Evaporative water used through transpiration also contributes in reducing air temperatures in urban areas. Forests and other vegetation can also contribute indirectly to temperature reduction by reducing urban energy consumption

the sprawling expansion of urbanization has already progressed.

**76**

through intercepting and using solar energy and by reducing building energy demand through shading and reducing wind speed. Therefore, low vegetation coverage in urban center results in larger temperature gradients between urban center and urban fringe or beyond [67–69]. Indeed, Seoul's heat island effect is very large [27, 70], as the temperature difference between the urban center and boundary was about 5°C (**Figure 4**).

The urban heat island is closely linked to the land-use pattern (**Figure 3**). Buildings, asphalt, and concrete pavement absorb solar radiation and emit long-wave radiation that warms the atmosphere [71–73]. Moreover, those artificial structures hold heat for extended periods. This heat moves from buildings, asphalt, and concrete pavement to the cool air as the air temperature decreases after sunset to form atmospheric temperature inversion (warm air over cold air) [74–77]. Therefore, many cities located in basins with limited ventilation like Seoul experience serious air pollution problems. The ventilation of an urban basin can be limited not only by orographic barriers but also by urban heat island-induced circulations and/or the capping effect of temperature inversions. Furthermore, land-use and land-cover changes caused by urbanization alter the dynamics of temperature inversions and urban heat islands, thereby affecting air quality in an urban valley [78].

Temperature inversions are frequently observed in most urban areas including Seoul. Temperature inversion results in poor dispersion of pollutants. Strong thermal inversion induces pollutant accumulation and thereby become a primary cause of the heavy air pollution. In addition, Seoul is backed by mountains, which intensified the accumulation of pollutants generated in the city itself and blown from other regions, particularly China, which is relatively closely located to Seoul.

In recent decades, East Asia has been significantly industrialized and urbanized through its rapid economic growth. The industrialization and urbanization have resulted in adverse effect on air quality not only in this region but also in neighboring countries [78].
