1 Definition of Urban Heat Island and it’s mechanism Department of Civil and Environmental System Engineering University of Incheon, South Korea Chi-Man, Park

2 Contents Introduction ① What is Urban Heat Island? Main subject ① Black Body Theory ② Albedo ③ A result of U.H.I. Conclusion ① Relationship between albedo, black body theory and U.H.I.

3 Urban Heat Island? Urban area is densely populated. Most of land is covered by building, road and concrete. There are many high buildings Building, road and concrete absorbs, or store, more sun light than earth. A tropical night is also one of Urban heat island phenomenon. The phenomenon of higher temperature of urban area than other area is called Urban Heat Island

4 Black Body Theory (continued)  Black body theory? radiation Heat radiates from the stove What an object that become hot emits is radiation Without medium, energy of one reach other by radiating energy. black body Black Body absorbs light of all wavelength and then radiate. That is, it absorb all light

5 Black Body Theory (continued) Black body doesn’t exist for real Ideal black body σ = x 10-8 Watt/m 2 k 4 Stefan-Boltzmann Law E= σAT 4 Black body give off radiation as a rate proportional to the 4 th power of the temperature Stefan-Boltzmann’s constant

6 Black Body Theory Stefan- Boltzmann Law As temperature is high, Black body emits much energy. That is, black body give off radiation at a rate proportional to the 4 th power of the temperature. Wine Law Wien's Displacement Law states that the wavelength distribution peaks at a value that is inversely proportional to the temperature.

7 Albedo (continued) Heat balance Maintaining the proper balance between absorbed solar energy and emitted earth radiation energy. → ragular average temperature of the earth.

8 Albedo  Albedo? The percentage of the incident sunlight that is reflected 30% of solar radiation energy which enter the earth are reflected by a cloud or surface Therefore an albedo of the earth is 30% SurfaceAlbedo(%) SurfaceAlbedo(%) Concrete17~27Sand (White)30~60 Crops (Green)5~25Snow (Fresh)80~90 Forest (Green)5~10Snow (old)7~45 Grassy plain (Green)5~25Soil (Dark)5~15 Farm land14~17Soil (Bright)25~30 Road (Black)5~10Water8

9 Temperature inversion (continued) Because of Urban heat island, warm air of the urban rise on the city. Cold air of suburbs come to the urban.  Urban heat island’s problem?

10 Temperature inversion As cool air from suburb with much dust and pollutants enter to the urban, pollutants is collected in urban, without diffusion of pollutants Dust and pollutants cover the skies of city - it is called dust roof. That is, urban heat island is related to photochemical smog.

11 Relationship between albedo, black body theory and U.H.I Asphalt road Solar radiation energy(100) Albedo(5~10) ▣ Asphalt roads and concrete structures absorb 90~95 of solar radiation energy. When reaching an absolute temperature, these make radiation energy. It is principle of Urban heat island

12 Amount of radiation energy ▣ E= σAT 4 σ = 5.67x10 -8 w/ ㎡ k ⁴ ▣ Calculation of asphalt road’s radiation (Dimensions:1 m 2, Temperature: ( )K ←In summer solar constant: 2cal/cm 2 · min) E = 5.67x10-8 w/ ㎡ k ⁴ⅹ 1 m 2 ⅹ ( ) 4 K = w 1m T=( )K E= w 1400W/m 2 ·min Stefan-Boltzmann’s constant

13 Conclusion Absorbs many energy if Aledo was low When temperature rises, radiation energy also increase As Construction, such as asphalt, has lower albedo than soil, it absorbs more energy and emits radiation energy Urban heat Island is generated by this radiation energy.