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Definition of Urban Heat Definition of Urban Heat Island Island and it’s and it’s mechanismmechanism

Department of Civil and Environmental System Engineering University of Incheon, South Korea

Chi-Man, Park

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ContentsContents 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.

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Urban Heat Island?Urban Heat Island?Urban area is densely populated.Most of land is covered by building, road and concrete.There are many high buildingsBuilding, 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

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Black Body Theory Black Body Theory (continued)(continued)Black body

theory?radiation Heat radiates from the stove What an object that become hot emits is radiationWithout 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

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Black Body Theory Black Body Theory (continued)(continued)Black body doesn’t exist for real

Ideal black body

σ = 5.6705 x 10-8 Watt/m2k4

Stefan-Boltzmann LawE= σAT4

Black body give off radiation as a rate proportional to

the 4th power of the temperature

Stefan-Boltzmann’s constant

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Black Body Theory 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 4th power of the temperature.

Wine LawWien's Displacement Law states that the wavelength distribution peaks at a value that is inversely proportional to the temperature.

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Albedo Albedo (continued)(continued)

Heat balance

Maintaining the proper balance between absorbed solar energy and emitted earth radiation energy.

→ ragular average temperature of the earth.

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AlbedoAlbedoAlbedo?

• 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%

Surface Albedo(%)

 

Surface Albedo(%)

Concrete 17~27 Sand (White) 30~60Crops (Green) 5~25 Snow (Fresh) 80~90Forest (Green) 5~10 Snow (old) 7~45Grassy plain

(Green) 5~25 Soil (Dark) 5~15

Farm land 14~17 Soil (Bright) 25~30Road (Black) 5~10 Water 8

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Temperature inversion Temperature inversion (continued)(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?

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Temperature inversionTemperature inversionAs cool air from suburb with much dust and pollutants enter to the urban, pollutants is collected in urban, without diffusion of pollutantsDust and pollutants cover the skies of city - it is called dust roof.

That is, urban heat island is related to photochemical smog.

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Relationship between albedo, black bRelationship between albedo, black body theory and U.H.Iody 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

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Amount of radiation energyAmount 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

1m

1m

T=(60+273.15)K

E=698.46 w

<Asphalt>

1400W/m2

·min

Stefan-Boltzmann’s constant

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ConclusionConclusionAbsorbs many energy if Aledo was lowWhen temperature rises, radiation energy also increaseAs Construction, such as asphalt, has lower albedo than soil, it absorbs more energy and emits radiation energyUrban heat Island is generated by this radiation energy.