Cities often become "heat islands", making the temperature of cities higher than that of suburbs. Under the influence of this factor, where should the near-surface wind between the city and the suburb

Cities often become "heat islands", making the temperature of cities higher than that of suburbs. Under the influence of this factor, where should the near-surface wind between the city and the suburbs come from? Formation of heat island effect

The formation of urban heat island is closely related to the development of urbanization. Li Yanming, a senior engineer of Beijing Institute of Landscape Science, summarized the direct causes of its formation into four aspects:

The first is the influence of the characteristics of the urban underlying surface (the contact surface between the atmospheric bottom and the surface). A large number of artificial structures in the city, such as paving the ground and various building walls, have changed the thermal properties of the underlying surface. These artificial structures absorb heat quickly and have small heat capacity. Under the same solar radiation conditions, they heat up faster than the natural underlying surfaces (green space, water surface, etc.). ), so their surface temperature is obviously higher than the natural underlying surface. For example, in summer, the lawn temperature is 32℃, the canopy temperature is 30℃, the cement floor temperature can reach 57℃, and the asphalt pavement temperature can reach 63℃. These high-temperature objects form a huge heat source, baking the surrounding atmosphere and our living environment. How can they not be spicy?

The second is urban air pollution. Motor vehicles, industrial production and a large number of people's activities in cities produce a lot of nitrogen oxides, carbon dioxide, dust and so on, which can absorb a lot of energy from the thermal radiation in the environment, produce a well-known greenhouse effect, and cause further warming of the atmosphere.

The third is the influence of man-made heat source. Factories, motor vehicles, residents' lives, etc. Burning all kinds of fuels consumes a lot of energy. Countless stoves are burning, emitting heat!

Fourth, the natural underlying surface of the city is reduced. Buildings, squares, roads, etc. It has increased a lot in the city, and the natural factors such as green space and water body have decreased accordingly, releasing more heat and absorbing less heat, and the ability to alleviate the heat island effect has weakened.

Meteorologist and Professor Yu Zhihao of Nanjing University said that the size of heating surface and the difference of heating materials are the main reasons for the temperature difference in suburbs. "The same 10,000 square meters of ground, in the suburbs, may be endless fields, which absorb heat slowly and heat slowly; High-rise buildings in the urban area are dense, and the top and side of the building are heating surfaces, and the heating area is much larger than that in the suburbs. And steel, cement and other building materials, the heat capacity is small, once heated, the temperature rises quickly, and emits a lot of heat to the surrounding, driving the surrounding temperature to rise ... "

Now meteorologists are temporarily absent from urban planning. Urban construction often lacks the basic knowledge of ecological design, which has brought obvious negative effects on urban environment and ecology. A large area of "cement" in the city is the most typical one. "Cement" refers to the phenomenon of hardening cities with building materials such as concrete, asphalt, granite, marble, glazed tile and silicate. Hardened surfaces include ground, walls, roofs and water bodies. The original intention of this hardening design is to reduce urban dust, increase architectural beauty and improve water cleanliness. But the real effect of this hardening design is to aggravate the urban heat island effect, increase the difficulty of dust control, worsen the water quality, drain the rainwater resources, make the urban vegetation unhealthy, aggravate the noise pollution of the city, and make the living comfort of the city worse.

Compared with other factors, the problems brought by "cementation" to the urban environment are the most extensive and diverse:

(1) Cemented building materials can absorb a lot of solar radiation heat. In summer sunshine, the temperature of concrete platform is 8℃ higher than the air temperature, and the temperature of roof and asphalt pavement is 17℃. Therefore, cementation will aggravate the urban heat island effect, make it difficult for suspended particles to settle and improve air quality.

(2) The cement surface will reflect radiant heat and noise, which will further aggravate the thermal effect and noise pollution of the city and directly affect the urban living comfort and residents' health.

(3) Rivers, banks and lakes lined with cement will destroy the water ecosystem and the water body will lose its self-purification function. In this way, the water quality will decline, the water will smell green, and even bacteria, mosquitoes and flies will multiply in large numbers, which will aggravate water pollution.

(4) Cement pavement will hinder the opportunity for urban ground to absorb rainwater and snow water, and it is difficult for urban groundwater level to rise. This kind of ground is extremely dry and dusty when it doesn't rain, but when it rains, it will be full of water and the surface runoff will be seriously polluted. These polluted runoff will directly enter rivers or lakes after entering the rainwater discharge port, thus polluting rivers and lakes. The cement floor is easy to freeze in winter, which leads to the safety problems of walking and driving. In order to melt snow, a large amount of deicing agent is needed, which pollutes the snow water. Therefore, cementation will seriously endanger urban water resources.

(5) Cement pavement will also reduce the survival opportunities of microorganisms in the land, thus destroying the surface ecology, reducing the opportunities for surface soil to replenish organic matter, and thus aggravating the desertification of urban land.

(6) Cement pavement also blocks the biological passage on the urban ground, which is very harmful to the biological species and biodiversity of the city. Hardened pavement will directly affect the root development of urban vegetation. Paving cement on the ground with trees will eventually lead to the death or lodging of trees because their roots can't breathe.

(7) It is difficult for cement cities to bring a harmonious ecological landscape between man and nature.

Harm of heat island effect

Yu Zhihao, an environmental expert, said: "The high temperature in the urban area will easily cause the air to rise, and the colder air in the suburbs will flow to the urban area. If there are factories in the suburbs, the pollutants emitted by the factories will be concentrated in the city center. The emergence of this' urban wind' will aggravate the degree of air pollution in urban areas. "

Li Zongkai, deputy director of the Atmospheric Environment Committee of China Environment Society, believes that this temperature difference is large enough to affect the living conditions of citizens and make the downtown more and more unsuitable for life.

Under the influence of "heat island effect", clouds and fog will increase over the city, which will lead to the accumulation of harmful gases and smoke over the urban area and form serious air pollution. The range of weather changes is intensifying, and the frequency and intensity of disasters are also of concern. This is mainly the related meteorological effect caused by the increase of urban heat island effect intensity. For example, the "rain island effect" and "fog island effect" of the city bring about the "heat island effect" and so on.

Many human diseases are caused by the "heat island effect" Medical research shows that environmental temperature is closely related to human physiological activities. When the ambient temperature is higher than 28 degrees, people will feel uncomfortable; No matter how high the temperature is, it will easily lead to irritability, heatstroke and mental disorders; The temperature is higher than 34℃, accompanied by frequent heat waves, which can also cause a series of diseases, especially the incidence of cardiovascular and cerebrovascular diseases and respiratory diseases, and the mortality rate is obviously increased. In addition, high temperature can also accelerate the photochemical reaction rate, thus increasing the concentration of harmful gases in the atmosphere and further endangering human health.

Under the action of "heat island effect", the temperature in different parts of the city is not the same, but presents a closed high temperature center. In these high-temperature areas, the air density is low and the air pressure is low, which is easy to produce cyclone updraft, so that various waste gases and harmful chemical gases around it can continuously supplement the high-temperature areas. Under the action of these harmful gases, residents in high temperature areas are prone to digestive system or nervous system diseases. In addition, the number of respiratory diseases such as bronchitis, emphysema, asthma, sinusitis and pharyngitis has also increased.

Saving "Heat Island" with "Green Island"

Many experts at home and abroad are actively exploring and studying this aspect, and there are many methods to choose from at present. For example, the surface color of urban buildings can be changed from dark to light to reduce the absorption of solar radiation; Planting flowers and trees on roadsides, gardens and roofs; Strengthen urban planning, choose a reasonable urban structure model, and make overall arrangements for factory areas and residential areas; It is also an important measure to centrally control the dispersed heat sources in urban areas, improve the utilization rate of industrial heat sources and energy sources, and reduce heat loss and release.

Experts call for climate demonstration in urban planning, and three-dimensional greening is the fundamental solution. Because the increase of artificial structures and the decrease of natural underlying surface are the main reasons of urban heat island effect, increasing the proportion of natural underlying surface in cities through various channels is one of the effective ways to alleviate urban heat island effect.

Green space is the most important natural factor in the city, and the coverage rate of urban greening is inversely proportional to the intensity of urban heat island. The higher the green coverage rate, the lower the intensity of urban heat island. Therefore, the establishment of large-scale concentrated green space is the most direct way to weaken the urban heat island effect.

According to scientific statistics, the average daily heat absorbed by each hectare of green space from the surrounding environment is equivalent to the refrigeration effect of 189 air conditioners; The average daily absorption of 1.8 tons of carbon dioxide significantly weakens the greenhouse effect. In addition, each hectare of green space can retain 2.2 tons of dust every year, which can reduce the dust content in the atmosphere by about 50%, which can effectively inhibit atmospheric warming.

Urban greening in the era of "heat island" is not a simple increase in quantity. Professor Tang Gengguo, director of China Forestry Society and professor of Nanjing Forestry University, stressed that the greening of a city depends not only on the green rate, per capita green rate and green coverage rate, but also on the amount of green-the sum of the area of each leaf on the tree. Simply pursuing the green rate is easy to get quick success and instant benefit. "Get a few tons of grass seeds. In a few days, you can add a large green space. " Such a "carpet" is beautiful, but it is difficult to compare with trees in ecological value. "Ten acres of grass is equivalent to one acre of trees." A reasonable urban greening system should be rich in layers: planting forests in the suburbs. Shanghai, Guangzhou, Nanjing and other big cities have all started to take action. Nanjing is prepared to invest more than 654.38+000 billion yuan in the surrounding area and cultivate 654.38+04 million mu of contiguous forest land. Of course, the species ratio in this forest should be reasonable. In a space of 29 square meters, 1 tree, 6 shrubs and 20 square meters of lawn are generally planted. Tang Gengguo emphasized that the city must protect the existing trees. "Like a buttonwood tree with a big crown, there are up to 20,000 plants in some cities. Because to make way for urban construction, there are only more than 2,000 plants left. The amount of green lost here is difficult to calculate! " Secondly, in the middle of the city, we should not only plant trees in the cracks, but also plant as many trees as possible to form a forest of hundreds or even acres. Only when the green coverage rate reaches more than 30%, the green space can alleviate the urban heat island effect; The green coverage rate is over 40%, and the heat island area can be reduced by 3/4; Only when the green coverage rate reaches more than 60% can the heat island effect be basically controlled.

Yu Zhihao, a professor of atmospheric science at Nanjing University, pointed out that "reducing heat emission and increasing green area are the fundamental ways to weaken the heat island effect". Landscape experts said that three-dimensional greening "can absorb air pollutants, humidify, retain dust, reduce noise, curb the aggravation of' heat island' and turn' heat island' into' green island', with good cooling effect. For example, a carpet roof or a wall covered with ivy and ivy can reduce the indoor temperature by 2℃ ~ 4℃ in midsummer, saving 20% ~ 40% of air conditioning power consumption. After calculation, roof greening is equivalent to 60% of the ground greening benefits. "

Natural factors such as wind and water surface can also effectively alleviate the urban heat island effect. Wind is the natural enemy of the heat island effect. As soon as the wind blows, the heat island loses its reason for existence. Through the atmospheric circulation, the heat island exchanges with the surrounding air, thus reducing its own temperature. The water surface not only constitutes the beautiful landscape of the city, but also can cool the air and reduce the ambient temperature by evaporation when the temperature rises. Therefore, it is an effective way to improve the heat island effect by lowering the building level, reasonably dispersing the buildings, making the urban roads spacious, dredging the urban "ventilation pipes" and expanding the urban water surface as much as possible.

Relieving the heat island effect is a long-term and comprehensive systematic project, which requires the efforts and cooperation of all parties. Strengthen urban planning, choose a reasonable urban structure model, establish the concept of urban ecology, especially strengthen greening, increase vegetation area, improve urban microclimate by absorbing plant heat, purify rivers and lakes, monitor air quality and other ecological construction activities.