It can be seen that the urban heat island embodies the concept of temperature difference. As long as there is an obvious temperature difference between the city and the suburbs, it can be said that there is an urban heat island. So urban heat island may appear all year round. However, the impact on residents' life is mainly the heat island effect of high temperature weather in summer. Medical research shows that environmental temperature is closely related to human physiological activities. When the ambient temperature is higher than 28℃, people will feel uncomfortable. No matter how high the temperature is, it will easily lead to irritability, heatstroke and mental disorders; When the temperature is higher than 34 degrees, frequent heat waves will also cause a series of diseases, especially the incidence of cardiovascular and cerebrovascular diseases and respiratory diseases, and the mortality rate will increase significantly. In addition, high temperature will accelerate the photochemical reaction rate, thus increasing the concentration of O3 in the atmosphere, aggravating air pollution and further endangering human health.
So, how is the urban heat island formed? Or what caused the urban heat island? The formation of urban heat island is obviously inseparable from the development of urbanization, and there are four direct reasons for its formation:
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 main reason 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 main reason 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!
The fourth reason is that the natural underlying surface in the city has decreased. 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.
With the rapid development of China's economy and the intensification of urbanization, the scale of many cities has expanded rapidly, the population has increased sharply, the buildings have become more and more dense, and there are more and more motor vehicles. In addition, industrial production has caused a serious urban heat island effect, especially in summer, the high temperature and summer heat have affected people's normal life and work, which has become a restrictive factor for further improvement of people's quality of life and further development of cities. Therefore, it is of great significance to study the technical methods to reduce the urban heat island effect and take various measures to alleviate the influence of the heat island effect for improving people's quality of life and maintaining the sustainable development of the city.
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.
Urban green space is the main natural factor of the city, and vigorously developing urban greening is the key measure to reduce the influence of heat island. Green space can absorb solar radiation, and most of the absorbed radiation energy is used for transpiration and heat consumption of plants and converted into chemical energy in photosynthesis, so the heat used to raise the environmental temperature is greatly reduced. Garden plants in the green space constantly absorb heat from the environment through transpiration, reducing the temperature of the surrounding air. Every hectare of green space can absorb 8 1.8 MJ of heat from the surrounding environment every day, which is equivalent to the cooling effect of 189 air conditioner. Garden plants can absorb carbon dioxide in the air through photosynthesis. One hectare of green space can absorb 1.8 tons of carbon dioxide on average every day, which weakens the greenhouse effect. In addition, garden plants can retain dust in the air, and each hectare of green space can retain 2.2 tons of dust every year, which reduces the dust content in the ambient atmosphere by about 50% and further inhibits the rise of atmospheric temperature.
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