Presentation on theme: "Urban Heat Island Effect ( ) The Hong Kong Polytechnic University Department of Applied Physics."— Presentation transcript:
Urban Heat Island Effect ( ) The Hong Kong Polytechnic University Department of Applied Physics
What is Urban Heat Island Effect? The temperature in cities is higher than the suburbs ( ) –The temperature in cities is higher and so isotherm ( ) decreases gradually from the cities to the surrounding suburbs ( ). –The city seems to be an island in relief map ( ) surrounded by the suburb ocean, so it is called Heat Island ( ).
Urban Heat Island Effect Hot air in city continuously flows up and disperse at a certain height in the sky, forming low pressure area ( ) on ground. Cool air on ground from suburbs flows into the city causing circumfluence ( ). Smoke and dust from industrial areas ( ) around the city flow into the town centre instead, causing the concentration of pollutants in town center higher than that of the industrial area.
Urban Heat Island Effect
Urban Island Effect in Hong Kong…
Causes ( ) of Urban Heat Island Effect Buildings highly concentrated in cities –Low wind speed near ground level –Poor ventilate ( ) Artificial environment absorbs much radiation from the Sun –Multiple reflection and absorption between roofs, walls of buildings and the ground –Large heat capacity ( ) of reinforced concrete ( ) 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
Influence ( ) of Urban Heat Island Effect The strength of Urban Heat Island Effect is related to the scale of the city ( ), meteorology factors ( ), etc. The annual mean temperature ( ) of big cities is 0.5 ~ 1 °C higher than that of suburbs The lowest mean temperature ( ) 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.
Influence ( ) of Urban Heat Island Effect
Weaken the intensity of solar radiation ( ) and sunshine ( ) –Dust in the air is increasing due to air pollution. Therefore, solar radiation reaching the ground is weakened.
Low humidity ( ), foggy ( ), rainfall ( ) increases –Cities with good drainage system ( ) 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 ( ) in the sky above the cities, so it is foggy. –The annual rainfall ( ) in cities is about 5 – 10% higher than that of the suburbs. Influence ( ) of Urban Heat Island Effect
Urban Heat Island Effect aggravated ( ) by Urbanization ( ) 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 ( ) and to adsorb ( ) dust –Density and height of buildings increases, so wind cannot flow into cities –Exhaust ( ) discharged from air- conditioners and cars