Term project 20062006.06.12 1/14

Definition of Urban Heat IslandDefinition of Urban Heat Island and it’s mechanism and it’s mechanism

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

Chi-Man, Park

Term project 2006

Term project 20062006.06.12 2/14

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

Term project 20062006.06.12 3/14

Urban Heat Island?Urban Heat Island?

Densely populated urban area.Land is covered by concrete and bituminous structure like building and road.

Restriction of wind flow

by high rise buildingsFigure: Urban heat island

Term project 20062006.06.12 4/14

Urban Heat Island? Urban Heat Island? (Continue)(Continue)

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 in urban area

than other area is called

Urban Heat IslandFigure: Variation of heat level

Term project 20062006.06.12 5/14

Black Body TheoryBlack Body TheoryBlackbody theoy

radiation

Heat radiates from the stove

Radiation is the emission of energy by hot object

Without medium, energy of one reach other by radiation system

black body

Black Body absorbs light of all wavelength and then radiate

That is, it absorb all light

Term project 20062006.06.12 6/14

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

Ideal black bodyIdeal black body

σ = 5.6705 x 10-8 Watt/m2k4

Stefan-Boltzmann Law

E= σAT4

→ Black body give off radiation as a rate proportional to the 4th power of the temperature

σ =Stefan-Boltzmann’s constant

A= Surface area of object

T= Temperature in Kelvin

Term project 20062006.06.12 7/14

Black Body Theory Black Body Theory (Continue)(Continue)

                                             

                       

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 Law

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

Term project 20062006.06.12 8/14

AlbedoAlbedoHeat balance

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

→ ragular average temperature of the earth.

Term project 20062006.06.12 9/14

AlbedoAlbedoAlbedo?

• The percentage of the incident sunlight that is reflected

• 30% of solar radiation energy which enters 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) 30~60

Crops (Green) 5~25 Snow (Fresh) 80~90

Forest (Green) 5~10 Snow (old) 7~45

Grassy plain (Green)

5~25 Soil (Dark) 5~15

Farm land 14~17 Soil (Bright) 25~30

Road (Black) 5~10 Water 8

Term project 20062006.06.12 10/14

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?

Term project 20062006.06.12 11/14

Temperature inversionTemperature inversion

As cool air from suburban 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.

Term project 20062006.06.12 12/14

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

Term project 20062006.06.12 13/14

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

Term project 20062006.06.12 14/14

ConclusionConclusionAbsorbs many energy if Albedo is 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.