urban heat island effect ( ) the hong kong polytechnic university department of applied physics
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Urban Heat Island EffectUrban Heat Island Effect ( (城市熱島效應城市熱島效應 ))
The Hong Kong Polytechnic The Hong Kong Polytechnic UniversityUniversityDepartment of Applied PhysicsDepartment of Applied Physics
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What is Urban Heat Island What is Urban Heat Island Effect?Effect?
• The temperature in cities is higher than the suburbs (郊區 )
– The temperature in cities is higher and so isotherm (等溫線 ) decreases gradually from the cities to the surrounding suburbs (郊區 ).
– The city seems to be an island in relief map (地形圖 ) surrounded by the “suburb” ocean, so it is called Heat Island (熱島 ).
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Urban Heat Island EffectUrban Heat Island Effect• Hot air in city continuously flows up and disperse at a
certain height in the sky, forming low pressure area (低壓區 ) on ground.
• Cool air on ground from suburbs flows into the city causing circumfluence (環流 ).
• Smoke and dust from industrial areas (工業區 ) around the city flow into the town centre instead, causing the concentration of pollutants in town center higher than that of the industrial area.
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Urban Heat Island EffectUrban Heat Island Effect
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Urban Island Effect in Hong Kong…Urban Island Effect in Hong Kong…
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Causes (Causes ( 成因成因 ) of ) of Urban Heat Island EffectUrban Heat Island Effect
• Buildings highly concentrated in cities– Low wind speed near ground level– Poor ventilate (通風 )
• Artificial environment absorbs much radiation from the Sun
– Multiple reflection and absorption between roofs, walls of buildings and the ground
– Large heat capacity (熱容量 ) of reinforced concrete (鋼筋水泥 ) absorbs much radiation during daytime
• Heat release due to human activities – heat emission by air-conditioners and cars
• Air pollution – Such pollutants absorb radiation heat
easily
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Influence (Influence ( 影響影響 ) of) ofUrban Heat Island EffectUrban Heat Island Effect
• The strength of Urban Heat Island Effect is related to the scale of the city (城市規模 ),
meteorology factors (氣象因素 ), etc.
• The annual mean temperature (年平均氣溫 ) of big cities is 0.5 ~ 1 °C higher than that of suburbs
• The lowest mean temperature (平均最低氣溫 ) of big cities in winter is 1 ~ 2 °C higher than that of suburbs
• Cities seems like a warm island, Urban Heat Island Effect is more apparent in big cities and at night.
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Influence (Influence ( 影響影響 ) of) ofUrban Heat Island EffectUrban Heat Island Effect
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Influence (Influence ( 影響影響 ) of) ofUrban Heat Island EffectUrban Heat Island Effect
• Weaken the intensity of solar radiation (太陽輻射 ) and sunshine (日照 )
– Dust in the air is increasing due to air pollution. Therefore, solar radiation reaching the ground is weakened.
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• Low humidity (低濕 ), foggy (多霧 ), rainfall (降雨 ) increases– Cities with good drainage system (排水系統 ) and
concrete ground causing humidity of cities lower than that of suburbs.
– The ground is dry. It cannot help accommodating weather.
– There are many cloud condensation nuclei (凝結核 ) in the sky above the cities, so it is foggy.
– The annual rainfall (年降雨量 ) in cities is about 5 – 10% higher than that of the suburbs.
Influence (Influence ( 影響影響 ) of) ofUrban Heat Island EffectUrban Heat Island Effect
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Urban Heat Island Effect Urban Heat Island Effect aggravated (aggravated ( 加劇加劇 ) by ) by Urbanization (Urbanization ( 城市化城市化 ))
• Development of cities accelerates the changes of climate– rapid urbanization in Hong Kong in recent 50 years causing the
warmer climate– the temperature rises 1 °C in average
Reasons: – A large number of trees are chopped, fewer
plants to remove carbon dioxide (二氧化碳 ) and to adsorb (吸附 ) dust
– Density and height of buildings increases, so wind cannot flow into cities
– Exhaust (廢氣 ) discharged from air-conditioners and cars