Definition of Urban Heat Island and it’s mechanism

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

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. Stefan-Boltzmann Law E= σAT4 σ = 5.6705 x 10-8 Watt/m2k4 Stefan-Boltzmann’s constant Ideal black body Black Body Theory (continued) • Black body doesn’t exist for real Black body give off radiation as a rate proportional to the 4th power of the temperature

6. Black Body Theory

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%

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

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 Solar radiation energy(100) Albedo(5~10) Asphalt road ▣ 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. E=698.46 w T=(60+273.15)K 1m Stefan-Boltzmann’s constant 1m <Asphalt> Amount of radiation energy ▣ E= σAT4 σ = 5.67x10-8 w/㎡k⁴ ▣ Calculation of asphalt road’s radiation (Dimensions:1 m2 , Temperature: (60+273.15)K ←In summer solar constant: 2cal/cm2 ·min) E = 5.67x10-8 w/㎡k⁴ⅹ 1 m2 ⅹ (60+273.15)4K = 698.46 w 1400W/m2 ·min

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.