Compare the similarities and differences of atmospheric characteristics between cities and suburbs

Influence of human activities on climate

There are two kinds of impacts of human activities on climate: one is the unconscious impact, that is, the side effects of human activities on climate; One is to take certain measures to consciously change the climate conditions for a certain purpose. At present, the first influence is absolutely dominant, which is most obvious in the following three aspects: ① Greenhouse gases and various pollutants discharged into the atmosphere in industrial and agricultural production have changed the chemical composition of the atmosphere; (2) changing the nature of the underlying surface in the development of agriculture and animal husbandry, such as destroying forest and grassland vegetation and marine oil pollution; ③ The influence of urban climate on cities. In the 200 years since the world industrial revolution, with the rapid increase of population, the development of science and technology and the rapid expansion of production scale, the adverse effects of human activities on climate have become more and more serious.

I. Changing atmospheric chemical composition and climate impact

Industrial and agricultural production releases a lot of pollutants such as waste gas and dust into the atmosphere, mainly including carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and chlorofluorocarbons (CFCs). Before the development of refrigeration industry, there was no such gas component in the atmosphere. The industrial emission of CFC 1 1 starts at 1945, and CFC 12 starts at 1935. By 1980, the CFC 1 1 in the lower troposphere is about 168× 10-3mL/L, and CFC 12 is 285×10-3.

Table 3 Comparison of Climate Characteristics between Urban and Suburban Areas

As shown in the figure. It can be seen that except CO2, the contents of other greenhouse gases in the atmosphere are extremely small, so they are called trace gases. But their warming effect

It should be extremely strong, with large annual increment, long decay time in the atmosphere and great influence.

Ozone (O3) is also a greenhouse gas, which is influenced by natural factors (ultraviolet radiation in solar radiation) on oxygen molecules in the upper atmosphere.

Photochemical effect), but it is destroyed by gases emitted by human activities, such as chlorofluorocarbons, haloalkyl compounds, N2O, CH4, CO and other gases that can destroy ozone. Among them, CFC 1 1 and CFC 12 are dominant, followed by N2O. Since the early 1980s, the amount of ozone has decreased sharply, with the lowest value of-15% in Antarctica and -5% in the Arctic. Globally, under normal circumstances, the oscillation should be within 2%, and this year, according to 1987, it has reached more than -4%. From 60 N to 60 S, the total ozone began at 1978, and decreased from more than 300 units on average to less than 290 units at 1987, that is, decreased by 3-4%. In terms of vertical variation, the maximum drop is at the height of 15-20km, and it rises slightly in the lower troposphere. The reduction of Antarctic ozone is the most prominent, forming a polar circle near the center of Antarctica, which is called "Antarctic ozone hole". From 1979 to 1987, the minimum value of the ozone minimum center decreased from 270 units to 150 units, and the area less than 240 units was expanding, indicating that the Antarctic ozone hole was constantly strengthening and expanding. Although the total amount of O3 increased at 1988, it expanded again at 1989. 1994 10 According to a research report published by the World Meteorological Organization, the ozone over three quarters of Antarctica's land and nearby sea surface has decreased by more than 65% compared with that of 10 years ago. However, some data show that ozone in the troposphere has increased slightly.

The increase of greenhouse gases in the atmosphere will lead to climate warming and sea level rise. According to the most reliable observation data, the global temperature rose from 1885 to 100, an increase of 0.6-0.9℃. The trend of global warming is also around 0.8℃. After 1985, the global surface temperature continued to rise, which most scholars believe is caused by greenhouse gas emissions. The figure lists the warming effects caused by greenhouse gas emissions in three different situations, and the calculation results of climate model also show that this warming is greater in the polar regions than in the equator, and greater in winter than in summer. As the global temperature rises, the temperature of seawater will also rise, which will expand the seawater and lead to the rise of sea level. In addition, due to the intense warming in polar regions, when the concentration of CO2 in the atmosphere doubles, polar ice will melt, the ice boundary will shrink towards the polar regions, and the melting water will lead to the rise of sea level. The actual observation data prove that the global sea level rose by10-12cm from 1880 to 1980. According to the calculation, if the emission of greenhouse gases is controlled at the emission standard of 1985, the global sea level will rise at a speed of 5.5cm/ 10a, 20cm in 2030 and 34cm in 2050. If emissions are not controlled, the sea level will rise by 10 in 2030.

The increase of greenhouse gases has a certain impact on precipitation and global ecosystem. According to the calculation of climate model, the CO2 content in the atmosphere will double, and the annual total precipitation will increase by 7- 1 1% globally, but the change ratio is different at different latitudes.

The destruction of ozone layer by greenhouse gases has great influence on ecology and human health. The reduction of ozone increases the ultraviolet radiation in the solar radiation reaching the ground. If the total ozone in the atmosphere decreases by 1%, the ultraviolet radiation reaching the ground will increase by 2%. This ultraviolet radiation will destroy DNA, change genetic information, destroy protein, kill single-celled marine plankton with water depth of 10m, reduce fish production, destroy forests, reduce crop yield and quality, weaken human immunity, damage eyes and increase diseases such as skin cancer.

In addition, the gas emitted by human activities contains a lot of sulfide, nitride and artificial dust, which will cause air pollution and form "acid rain" under certain conditions, causing serious losses to forests, fish, crops and buildings. The rapid increase of dust in the atmosphere will weaken solar radiation and affect temperature, cloud cover (hygroscopic nuclei in dust) and precipitation.

Second, change the underlying surface characteristics and climate effects.

Human activities can change the natural properties of the underlying surface in many ways. At present, the most prominent is the destruction of forests, slopes and arid areas of vegetation and marine oil pollution.

Forest is a special underlying surface, which not only affects the content of CO2 in the atmosphere, but also forms a unique forest climate, and can affect the climatic conditions of a considerable area nearby. Forest canopy can absorb a large amount of incident solar radiation to promote photosynthesis and transpiration, which makes its own temperature increase little. During the day, the ground under the forest is blocked by the canopy, and the solar radiation does not penetrate much, and the temperature will not rise sharply. At night, due to the protection of the canopy, the effective radiation is not strong, so the temperature is not easy to decrease. Therefore, the daily (annual) temperature difference in the forest is smaller than that in the bare land outside the forest, and the continental degree of temperature is obviously weakened.

The forest canopy can intercept precipitation, and the loose humus layer and litter layer under the forest can store water, reducing the surface runoff after rainfall, so the forest can be called "green water storage reservoir". Rain slowly seeps into the soil, increasing soil moisture and water available for evaporation. Coupled with the transpiration of forest, the absolute humidity and relative humidity in forest are higher than those in bare land outside forest.

Forests can increase rainfall. When the airflow flows through the canopy, due to the obstruction and friction of the forest, it will force the airflow to rise, resulting in enhanced turbulence. In addition, the air humidity in forest areas is high and the condensation height is low, so there are more opportunities for precipitation in forest areas, and the rainfall is more than that in open areas. According to the measured data, the air humidity in forest areas is 15-25% higher than that in non-forest areas, and the annual precipitation can be increased by 6- 10%.

Forests can reduce wind speed. When the wind blows to the forest, the wind speed changes on the windward side of the forest, which is about 100m away from the forest. When passing through the forest, the wind speed decreases rapidly. If the wind carries sediment, it will make the quicksand sink and gradually fix. After passing through the forest, the wind speed still decreases within a certain distance of the leeward side of the forest. In arid areas, forests can reduce the attack of dry wind, prevent wind and fix sand. In windy coastal areas, forests can resist sea breeze and protect farmland. Secretions from forest roots can promote microbial growth and improve soil structure. Forest-covered areas have humid climate, good soil and water conservation and a virtuous cycle of ecological balance, which is called "green ocean".

According to textual research, the world forest once occupied 2/3 of the earth's land area in history, but with the increase of population, the development of agriculture, animal husbandry and industry, the construction of cities and roads and the destruction of war, the world forest area gradually decreased, reaching 46% in the19th century and 37% in the early 20th century. At present, the global forest coverage area averages about 22%. China was also covered with dense forests in ancient times. Later, due to population reproduction, farmland expansion and frequent wars in Ming and Qing dynasties, the national forest coverage rate has dropped to 1949, which has dropped to 8.6%. Since the founding of the People's Republic of China, the Party and the government have organized large-scale afforestation, with an afforestation area of 460 million mu. However, due to the weak foundation, deforestation is quite serious. At present, the forest coverage area is only 12%, ranking165,438+06 among 60 countries in the world.

Due to the destruction of large areas of forests, the climate has become dry, sandstorms have intensified, soil erosion and climate deterioration have occurred. On the contrary, after liberation, China established various shelterbelts in the western northeast, eastern Henan, northwestern Hebei and Shandong coastal areas, which played an important role in transforming natural and climatic conditions.

In arid and semi-arid areas, there used to be grasses and shrubs with strong drought tolerance. They could survive in arid areas and protect the soil there. However, due to the increase of population, there are more immigrants in arid and semi-arid areas, where they expand agriculture and animal husbandry and dig xerophytes as fuel (especially plants on sloping fields), which greatly destroys the local natural vegetation such as grasslands and shrubs. Rainwater on sloping land has fast convergence and velocity, and has strong scouring effect on soil. After losing the protection and blocking of natural vegetation, it will cause serious soil erosion. On the flat land, once the drought comes, farmland crops cannot grow, and the loose land after reclamation has no vegetation protection and is easily eroded by the wind. As a result, the fertile soil on the surface was blown away, but the sand remained, causing desertification. There is a similar situation in animal husbandry, which exceeds the carrying capacity of grassland. In dry years, grass is sparse, the ground is trampled by livestock, and serious wind erosion will occur, resulting in desertification. In desertified land, the climate is worse, which is manifested in: after rain, runoff increases, soil erosion intensifies, and water decreases, which makes local soil and atmosphere dry, surface reflectivity increases, which destroys the original heat balance, precipitation decreases, climate continental degree increases, surface fertility decreases, sandstorm disasters increase greatly, and the climate becomes drier, which is even more unfavorable for plant growth.

According to the estimation of the United Nations Environment Programme, at present, 60,000 square kilometers of land is lost every year in the world due to desertification, and another 2 1 10,000 square kilometers of land is decreasing, which has no economic value in agriculture and animal husbandry. Desertification is also a threat to China. There were 6.5438+0.2 million km2 of desertified land in the historical period of northern China, and the desertified area has increased year by year in recent decades. Therefore, we must consciously take active measures to protect local natural vegetation, carry out large-scale irrigation, carry out artificial afforestation, plant drought-tolerant vegetation, and prevent climate deterioration according to local conditions.

Marine oil pollution is another important aspect that human activities change the nature of the underlying surface. It is estimated that more than 654.38+billion tons of oil are transported to consumption places by sea every year. Due to improper transportation or tanker accidents, more than 6,543,800 tons of oil flows into the ocean every year, and in addition, waste oil generated in industrial processes is discharged into the ocean. It is estimated that the amount of oil injected into the ocean every year is 200-100000 tons.

Some waste oil poured into the sea forms an oil film on the sea surface, which inhibits the evaporation of seawater and dries the ocean air. At the same time, it reduces the transfer of latent heat on the sea surface, which leads to the increase of daily and annual changes of seawater temperature, which makes the ocean lose its function of regulating temperature and produces the "marine desertification effect". The influence of waste oil film on relatively closed sea surfaces, such as the Mediterranean Sea, the Baltic Sea and the Sea of Japan, is more significant than that of the vast Pacific Ocean and the Atlantic Ocean.

In addition, in order to meet the needs of production and transportation, human beings have changed the nature of the underlying surface by filling lakes, digging canals and building large reservoirs, which has also had a major impact on the climate. For example, after the completion of Xin 'anjiang Reservoir 1960, the nearby Chun 'an County is cooler in summer and warmer in winter, and the annual temperature becomes smaller, the first frost is delayed, the final frost is advanced, and the frost-free period is extended by about 20 days on average.

Third, man-made heat and man-made water vapor emissions.

With the development of industry, transportation and urbanization, the world energy consumption has increased rapidly. 1970, the world's energy consumption is equivalent to burning 7.5 billion tons of coal and releasing 25×10/0J of heat. Among them, industrial production and motor vehicle transportation emit a lot of waste heat, and residents' stoves and air conditioners, as well as the metabolism of people and livestock, also release some heat. These "artificial heats" directly warm the atmosphere like stoves. At present, if the artificial heat is averaged over the whole continent; It is equal to releasing 0.05W of heat per square meter of land. Numerically speaking, it is insignificant compared with the average net radiant heat obtained by the whole earth from the sun, but the local warming effect is quite significant because the release of man-made heat is concentrated in some densely populated and developed cities. As shown in the figure, in high-latitude cities such as fairbanks and Moscow, the annual average anthropogenic heat (QF) emission is greater than the net solar radiation; Mid-latitude cities, such as Montreal and Manhattan, have higher annual average anthropogenic heat QF emissions than Rg due to their huge per capita energy consumption. Especially in Montreal, due to the huge energy consumption of air conditioning heating in winter, its artificial heat is equivalent to more than 1 1 times of the net solar radiation. However, in tropical Hong Kong and equatorial Singapore, compared with net solar radiation, man-made heat emissions are very small.

When burning a lot of fossil fuels (natural gas, gasoline, fuel oil and coal, etc.). ), in addition to waste heat emission, there is a certain amount of "artificial water vapor" released into the air. According to METROMEX, the artificial water vapor produced by combustion in St. Louis is 65,438+00.8× 65,438+008 g/h, while the local natural evapotranspiration in summer is 6.7× 10. Obviously, the amount of artificial water vapor is much smaller than that of natural evapotranspiration, but it has a certain effect on the increase of local low cloud cover. It is estimated that the current world energy consumption is increasing by about 5.5% every year. If this rate continues to increase, by the year 2000, the world energy consumption will be five times higher than that of 1970, that is, the annual energy consumption will be 37.5 billion tons of coal. The man-made heat and water vapor it emits are mainly concentrated in cities, and its influence on urban climate will show its importance more and more.

Table 2 Man-made Heat Emissions in Different Cities

Besides CO2, there is a lot of water vapor in the exhaust gas emitted by high-altitude jet planes. According to research, in recent years, the water vapor in the stratosphere (500 hectopascals) has obviously increased. For example, its water vapor content at 1970 is 2× 10-3mL/L, rising to 3× 10. The thermal effect of water vapor is similar to that of carbon dioxide, which has a greenhouse effect on the surface. According to the calculation, if the stratospheric water vapor volume increases by five times, the surface temperature will increase by 2℃, while the stratospheric temperature will decrease by 10℃. The increase of water vapor in the upper air will also lead to the increase of cirrus clouds in the upper air. It is estimated that the number of cirrus clouds has increased by 5- 10% on the North America-Atlantic-Europe route where most jets fly. Clouds have a great influence on the solar radiation and infrared radiation of the earth-atmosphere system, and play an important role in the formation and change of climate.

Fourth, the urban climate

The city is the center of human activities, where the population is dense and the underlying surface changes the most. Industry, commerce and transportation are frequent, consuming the most energy, and emitting a lot of greenhouse gases, "artificial heat", "artificial water vapor", dust and pollutants into the atmosphere. Therefore, the impact of human activities on climate is most prominent in cities. Urban climate is a special local climate under the background of regional climate and the influence of human activities after urbanization. In the early 1980s, Lanzburg, an American scholar, made a comparative summary of the climatic factors in cities and suburbs, as shown in Table 3.

From a large number of observation facts, the characteristics of urban climate can be summarized as "five islands" effect (turbid island, hot island, dry island, wet island and rainy island) and the decrease and change of wind speed.

(A) Urban Turbidity Island Effect

There are four main manifestations of urban turbid island effect. First of all, there are more pollutants in the urban atmosphere than in the suburbs. As far as condensation nuclei are concerned, the average concentration of condensation nuclei in the ocean is 940 grains/cm3, and the absolute maximum is 39,800 grains/cm3. In the air of big cities, the average value is147,000 grains /cm3, which is 56 times that of the ocean, and the absolute maximum value is 4,000,000 grains /cm3, which is more than 0.00 times that of the ocean. Taking Shanghai as an example, according to the monitoring results in recent five years (1986— 1990), the average concentrations of SO2 and nitrogen oxides in urban areas are 8.7 times and 2.4 times higher than those in suburban counties, respectively.

Secondly, there are many condensation nuclei in the urban atmosphere, and the low-level thermal turbulence and mechanical turbulence are relatively strong, so the number of cloudy days with low cloud cover and based on low cloud cover (the number of days with low cloud cover ≥8) is far more than that in the suburbs. According to the statistics of Shanghai in recent ten years (1980- 1989), the average low cloud cover in urban areas is 4.0, and that in suburbs is 2.9. The number of cloudy days (low cloud cover ≥8) in urban areas is 60 days, while the average sunny days (low cloud cover ≤2) in suburban areas are only 3 1 day, urban areas are 132 days and suburban areas are 178 days. Munich, Budapest, new york and other European and American cities have also observed similar phenomena.

Thirdly, in the urban atmosphere, due to more pollutants, lower cloud cover and less sunshine hours, the direct solar radiation is greatly weakened, while there are more scattered particles, and the scattered solar radiation is stronger than the dry and clean air. In the regional distribution of atmospheric turbidity (also known as turbidity factor) expressed by D/S, the urban area is obviously larger than the suburban area. According to the statistical calculation of the observation data in Shanghai in recent 27 years (1959— 1985), the turbidity factor in Shanghai urban area is higher than that in suburbs in the same period 15.8%. On the distribution map of turbidity factors in Shanghai, the urban area presents obvious turbidity islands. Many foreign cities have similar phenomena.

Fourthly, the urban turbidity island effect is also manifested in the fact that the visibility of urban areas is less than that of suburbs. This is because there are many particulate pollutants in the urban atmosphere, which can scatter and absorb light and reduce visibility. When the concentration of NO2 in urban air is extremely high, the sky will turn brown. In such a sky background, it is difficult to distinguish the distance of the target, resulting in line-of-sight obstacles. In addition, in cities, the primary pollutants in automobile exhaust-nitrogen oxides and hydrocarbons-will form light blue smog under strong sunlight, which is called photochemical smog, which leads to the deterioration of urban visibility. This phenomenon is found in Los Angeles, Tokyo, China and other cities.

(B) Urban heat island effect

According to a large number of observation facts, the temperature of a city is often higher than that of its surrounding suburbs. Especially when the weather is clear and windless, the difference between urban temperature Tu and suburban temperature Tr △ Tu-r (also called heat island intensity) is larger. For example, Shanghai,1sunny at 20: 00 on October 22nd, 1984, wind speed 1.8m/s, suburban temperature 13℃ or so. As soon as we entered the urban area, the temperature suddenly rose, the isotherms were dense and the temperature gradient was steep. The temperature in the old city is above 17℃, like a "densely populated area and factory area in the city with the highest temperature, which becomes the" peak "in the heat island (also known as the heat island center). The temperature in downtown 62 is as high as 18.6℃, which is 5.6℃ higher than that in Chuansha and Jiading in the suburbs and 6.5℃ higher than that in Songjiang in the outer suburbs. Similar strong heat islands can appear in Shanghai all year round, especially in clear and windless weather in autumn and winter.

Due to the frequent existence of heat island effect, the monthly average temperature and annual average temperature in big cities are often higher than those in nearby suburbs.

(C) urban dry island and wet island effect

It is pointed out in Table 2 that the relative humidity in cities is lower than that in suburbs, which has obvious dry island effect.

Yes, this is a common feature of urban climate. The influence of cities on atmospheric water vapor pressure is more complicated. Taking Shanghai as an example, according to the average values of water vapor pressure eu and relative humidity RHu in urban area 1 1 station (1984- 1990) and the average values of water vapor pressure er and relative negative values in four suburban stations around the same period, the water vapor pressure and relative humidity in suburbs have obvious daily changes. According to the actual measurement, the absolute diurnal variation of △RHu-r is different. If we take four observation moments in a day (02, 08, 14, 20), we can divide them into four points.

But in the suburbs, it is higher than er (Table 6), resulting in "urban wet island". In the warm season from April to 165438+ 10, there is an obvious phenomenon that dry islands and wet islands alternate day and night, especially in August.

Table 5 Monthly Average Water Vapor Pressure and Its Relative Value in Shanghai

Comparison of Humidity (%) in Suburbs (1984 ——1990)

The formation of the above phenomenon is closely related to the underlying surface factors and weather conditions. During the day, under the irradiation of the sun, the amount of water vapor that the underlying surface evapotranspiration into the lower air is smaller in the urban area (the green area is small, and the amount of water vapor available for evaporation is less) than in the suburbs. Especially in the midsummer season, crops in suburbs grow intensively, and the difference of natural evapotranspiration between suburbs is even greater. Due to the rough underlying surface (dense buildings and uneven height) and heat island effect, the mechanical turbulence and thermal turbulence in urban areas are stronger than those in suburbs. Through the vertical exchange of turbulence, the amount of water vapor transported from the lower level of the urban area to the upper level is more than that in the suburbs, which both lead to the lower surface water vapor pressure in the urban area than in the suburbs, forming an "urban dry island". At night, the wind speed decreases, the air stratification is stable, the temperature in the suburbs drops rapidly, the saturated water vapor pressure decreases, a large amount of water vapor condenses into dew on the surface, the amount of water vapor remaining at low altitude is small, and the water vapor pressure drops rapidly. Due to the heat island effect, the condensation in the urban area is much less than that in the suburbs, the turbulence at night is weak, and the water vapor exchange with the upper air is small. The water vapor pressure near the urban area is higher than that in the suburbs, forming an "urban wet island". This kind of urban wet island is called "condensation wet island" because of the different condensation amount in the suburbs, which is mostly formed within a few hours after sunset and maintained at night. When urban dry island and urban wet island appear, they are inevitably accompanied by urban heat island, because urban dry island is one of the reasons for the formation of urban heat island (the city consumes less heat), and the formation of urban wet island must first have the existence of urban heat island.

The average water vapor pressure in urban areas is lower than that in suburbs, and the relative humidity is lower than that in suburbs due to the heat island effect. Take Shanghai as an example. In recent seven years (1984— 1990), the annual average relative humidity in Shanghai is less than 74% in the downtown area and more than 80% in the suburbs, showing an obvious urban dry island (pictured). According to the general survey, even when the distribution of water vapor pressure presents an urban wet island, the distribution of relative humidity in urban areas is still smaller than that in surrounding suburbs.

In foreign countries, the research on urban dry island and wet island is famous in Leicester, England, Edmonton, Canada, Chicago and St. Louis, USA. The formation of urban wet island is mostly attributed to the difference of condensation quantity in suburbs. A few people talk about the formation of urban wet island because the melting speed of snow in urban areas is faster than that in suburbs. When there is snow in suburbs, the water vapor pressure in the air increases due to the melting ratio of snow water in urban areas, thus forming urban wet island. According to the author's comparative analysis of atmospheric water vapor pressure in 1984 in Shanghai, it is also found that the formation of urban wet islands in Shanghai includes frost wet islands, foggy wet islands, rainy wet islands and snowy wet islands. This will only happen when the wind is light and accompanied by urban heat island.

(d) urban rain island effect

There are many controversies about the influence of cities on precipitation in the world. 197 1— 1975, the United States established a dense rainfall observation network in St. Louis, Missouri and its nearby suburbs, and carried out a five-year meteorological observation experiment (METROMEX) in big cities by using advanced technology, which confirmed that the city and its downwind really had the effect of "rain island" to promote precipitation. There are a lot of observation and research data in this field. Taking Shanghai as an example, according to the data of more than 70 rainfall observation stations in this area/kloc-0, combined with the weather conditions. Through the analysis and classified statistics of several cases, it is found that the influence of Shanghai cities on precipitation is more obvious in the flood season (May-September). On the distribution map of Shanghai's precipitation in flood season in recent 30 years (1960— 1989), the precipitation in the urban area is obviously higher than that in the suburbs, showing an obvious rainy island in the urban area. This phenomenon is not found in the non-flood season (10 to April of the following year) and the annual average precipitation distribution map (schematic diagram).

The conditions for the formation of urban rain island are as follows: under the large-scale weather situation in which the atmospheric circulation is weak, which is beneficial to precipitation in urban areas, the convergence of local airflow caused by urban heat island circulation rises, which is beneficial to the development of convective rain; (2) The roughness of urban underlying surface can block the slow-moving rainfall system, making it move more slowly and prolonging the rainfall time in urban areas; (3) There are many condensed nuclei in urban air, with different chemical composition and particle size. When there are many large nuclei (such as nitrate), it can promote warm cloud precipitation. The influence of the above factors will "induce" the landing point of the maximum intensity of heavy rain to be located in the urban area and its downwind direction, forming a rain island.

Cities not only affect the distribution of precipitation, but also because there are a lot of SO2 and NO2 in the atmosphere. Under a series of complex chemical reactions, sulfuric acid and nitric acid are formed, which fall into "acid rain" through the formation process and scouring process of rain, which is very harmful.

(5) The average wind speed in the city is small, the local difference is large, and there is heat island circulation.

The roughness of urban underlying surface can reduce the average wind speed, which can be proved from the following two aspects: ① the comparison of wind speed before and after the same place in the historical process of its urban development; ② Comparison of wind speed between urban and suburban areas in the same period. There are a lot of measured data in big cities at home and abroad, and Shanghai is still taken as an example. Shanghai Meteorological Observatory began to record wind speed from 1884, and it has been 100 years since then. In the past hundred years, Shanghai's urban development has been very rapid, with the urban population increasing by more than 34 times, the building density also increasing rapidly, and the annual average wind speed has obviously decreased year by year. Table 7.

Table 7 Annual average wind speed of Shanghai Meteorological Observatory over the years (m/s )( 1984— 1990)

It can be seen from Table 8. 12 that no matter what height the anemometer is installed at, the wind speed measured at the same height decreases with the development of Shanghai. Above the ground 12m, the average wind speed in the last five years (1986- 1990) is 34.2% lower than that in more than 90 years ago (1894- 1900). As can be seen from Figure 8.25, the average wind speed (2.5m/s) in the downtown area of Shanghai in recent 10 years is 32.4% lower than that in Nanhui (3.7m/s) in the outer suburbs.

On a large scale, in the weather situation with extremely small pressure gradient, especially on clear nights, due to the existence of urban heat island, a weak low-pressure center is formed in the urban area, and an updraft appears. The air near the ground in the suburbs flows into the city from all directions, and the wind direction converges with the center of the heat island.

The air rising from the center of the heat island flows to the suburbs at a certain height, and sinks in the suburbs, forming a slow heat island circulation, also known as the urban wind system, which is conducive to the accumulation of pollutants in the urban area to form dust cover, and to form low clouds and local convective rain in the urban area. The existence of urban heat island circulation has been observed in Shanghai, Peking and other cities in China.

In addition, due to the different street trends, widths, heights, building types and orientations in the city, the solar radiation energy obtained by different places is obviously different, and local geothermal circulation will occur when the prevailing wind is weak or windless. When the prevailing wind blows through rows of uneven buildings, different ascending and descending airflow is generated due to the barrier effect, resulting in whirl and flow around, which makes the local variation of the wind more complicated.

(Selected from Meteorology and Climatology by Zhou Shuzhen et al. )