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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. Therefore, it is necessary to strengthen research and take measures to consciously plan and control various human activities that affect the environment and climate, so as to make them develop in a direction conducive to improving climate conditions.

(a) 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). According to conclusive observation, the content of these gases in the atmosphere has increased sharply in recent decades, while ozone O3 in the stratosphere. The total amount has dropped sharply. As mentioned above, these gases have obvious greenhouse effect, and there are two strong absorption bands at the wavelengths of 9500 nm (micron) and 12500- 17000μm, which are the absorption bands of O3 and CO2 respectively. Especially the absorption band of CO2, absorbs about 70-90% of infrared long-wave radiation. The long-wave radiation from the earth-atmosphere system is mainly concentrated in the wavelength range of 7000- 13000μm, which is called the atmospheric window. These gases, such as CH4, N2O and CFCS, all have their absorption bands in this atmospheric window, and the increase of these greenhouse gas concentrations in the atmosphere will inevitably play an important role in climate variability.

Before industrialization, the atmospheric CO2 concentration was stable at (280 10) × 10-3 ml/L for a long time, but it rose rapidly in recent decades, reaching 345× 10-3ml/L in 1990, and after the 1990s. Figure 8. 14 (omitted) shows the year-to-year change of the measured values of Mauna Loa volcano (1959- 1993) in the United States. The sharp increase of carbon dioxide concentration in the atmosphere is mainly caused by burning a lot of fossil fuels and cutting down a lot of forests. According to research, part of CO2 (about 50%) discharged into the atmosphere is absorbed by the ocean, and the other part is absorbed by the forest to become solid organisms and stored in nature. However, due to the massive destruction of forests at present, forests not only reduce the absorption of CO2 in the atmosphere, but also increase the amount of CO2 discharged into the atmosphere due to the burning and decay of destroyed forests. At present, there are many different estimates for the future increase of carbon dioxide. For example, according to the current CO2 emission level, the atmospheric CO2 concentration will be 4.25× 10-3mL/L in 2025, which is 1.55 times that before industrialization.

Methane (CH4 biogas) is another important greenhouse gas. It is mainly discharged into the atmosphere through the burning of rice fields, ruminants, swamps and organisms. From 200 years ago to 1 10000 years ago, CH4 content was stable at 0.75-0.80× 10-3mL/L, and increased rapidly in recent years. CH4 content increased to 1.25× 10-3mL/L at 1950 and to 1.72× 10-3mL/L at 1990. According to the observation records of 23 fixed-point land observation stations around the world and 14 ship observation stations in the Pacific Ocean at different latitudes, Dlugokencky et al. estimated the annual variation value of CH4 mixing ratio (m) in the atmosphere in the past 10 year, as shown in Figure 8. 15 (omitted). According to the current growth rate, the CH4 content in the atmosphere will reach 2.0× 10-3mL/L in 2000, 2.34×10-3 ml/L in 2030 and 2050 respectively.

The emission of nitrous oxide (N2O) into the atmosphere is related to the increase of farmland area and the application of nitrogen fertilizer. Supersonic flight in the stratosphere will also produce N2O. Before industrialization, the content of N2O in the atmosphere was about 2.85× 10-3 ml/L, and 1985 and 1990 increased to 3.05× 10-3mL/L and 3.10×/respectively. Considering the future emissions, it is predicted that the content of N2O in the atmosphere may increase to 3.50×10-3-4.50×10-3 ml/L by 2030. N2O can not only cause global warming, but also cause stratospheric ozone dissociation and destroy the ozone layer through photochemical action.

Chlorofluorocarbons (CFCS) are the main raw materials for refrigeration industry (such as refrigerators), sprays and foaming agents. Some compounds in this family, such as freon 1 1 (CC 12f, CFC 1 1) and freon 12 (CC 12f2, CFC/kloc-. In recent years, it has been considered as the main factor to destroy stratospheric ozone, so limiting the production of CFC 1 1 and CFC 12 has become a prominent international problem.

Before the development of refrigeration industry, there was no such gas component in the atmosphere. CFC 1 1 started to have industrial emissions at 1945, and CFC 12 existed 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. Figure 8. 16 (omitted) shows the changes of CFC 12 in recent decades, and its future content changes depend on future restrictions.

According to special observation and calculation, the annual increment of the concentration of major greenhouse gases in the atmosphere and the decay time in the atmosphere are shown in Table 8.7 (omitted). 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. However, they have strong warming effect, large annual increment, long attenuation time in the atmosphere and great influence.

Ozone (O3) is also a greenhouse gas, which is produced by natural factors (ultraviolet rays in solar radiation react with oxygen molecules in the upper atmosphere), but it can be destroyed by gases emitted by human activities, such as chlorofluorocarbons, haloalkyl compounds, N2O, CH4, CO, etc., among which CFC 1 1 and CFC 12 are the main ones. Fig. 8. 17 (omitted) is the interannual variation ratio of zonal mean total ozone anomaly in each climatic zone (196- 1985). As can be seen from the figure, since the early 1980s, the amount of ozone has decreased sharply, with the lowest value of-15% in Antarctica and-15% in the Arctic. 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 A research report published by the World Meteorological Organization shows that the ozone over three quarters of Antarctica's land and the nearby sea surface has decreased by more than 65% compared with 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 at present, the global temperature has increased from 1885 to 1985, with an increase of 0.6-0.9℃. Figure 8+00 (omitted) points out the actual temperature change from 1860 to 1985 (the difference between the global annual average temperature 1985), indicating that the global warming trend 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 by 10- 12cm in a century 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 latitude changes are different. Generally speaking, the precipitation in high latitudes is increased due to warming, while in mid latitudes, it becomes dry due to the northward movement of subtropical arid areas after warming, and the precipitation in subtropical areas is increased, and in low latitudes, it is increased due to the strengthening of warming convection.

As far as the global ecosystem is concerned, climate warming caused by human activities will lead to partial thawing of the tundra frozen at high latitudes, and the northern boundary of the forest will develop towards the polar regions. It will dry up in the mid-latitude, and some forests and biomes that like humidity and warmth will gradually be replaced by biomes that are currently heard in the subtropical zone. According to the forecast, after doubling CO2, the global desert will expand by 3%, the forest area will decrease by 1 1%, and the grassland will expand by 1 1%, which is caused by land drought in mid-latitude areas.

The destruction of ozone layer in greenhouse gases has great influence on the main state 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 ribonucleic acid (DNA), change genetic information, destroy protein, kill unicellular 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, nitrogen-containing compounds 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.

(b) Changing the underlying surface characteristics and climate impact.

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, it will be forced to rise due to the obstruction and friction of the forest, 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 more rainfall than 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 the invasion of sea breeze and protect farmland. The secretion of forest roots can promote the growth of microorganisms and improve the 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, reduces precipitation, strengthens the continental degree of climate, reduces surface fertility, greatly increases sandstorm disasters, and makes the climate more arid, which is not conducive to 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.

(3) Emissions of man-made heat and steam

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- 10J 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 extinguishing the furnace. 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 Table 8.8, 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 amount of 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.

* See Zhou Shuzhen and Shu Tong. Urban climatology. Beijing: Meteorological Press.1997; 197

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.

(4) 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 shadow 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 8.9.

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.

See landsberg, urban climate. Academic Press. 198 1.

(1) 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 is 147000 grains /cm3, which is 56 times that of the ocean, and the absolute maximum value is 400000 grains /cm3, which is more than 0 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 NO2 in urban areas are 8.7 times and 2.4 times that 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 number of sunny days (low cloud cover ≤2) is only 3 1 day. On the contrary, the average number of cloudy days in urban areas is 132 days, and that in suburbs is 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 observation data in Shanghai in recent 27 years (1959- 1985), the turbidity factor in Shanghai urban area is higher than that in the suburbs in the same period 15.8%. On the distribution map of turbidity factors in Shanghai, the urban area presents obvious turbidity islands (Figure 8. 19, omitted). 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, due to the proportion of nitrogen oxides and hydrocarbons, the primary pollutants in the exhaust gas emitted by automobiles in cities, light blue smoke will be formed after photochemical reaction under strong sunlight, which will lead to the deterioration of urban visibility. This phenomenon is found in Los Angeles, Tokyo, China and other cities.

(1) Underlying surface factors:

1. The impervious area of the underlying surface is large: except for a small amount of green space, most of the cities are artificially paved roads, square buildings and structures, and the impervious area of the underlying surface is much larger than that of suburban green space. After the rain, the rainwater quickly loses from the drainage pipe, so there is less water available for evaporation than in the suburbs. In the energy balance, the contribution of the net radiation Qn to the latent heat QE of evapotranspiration is much smaller than that in the suburbs, while the sensible heat QH used to heat the underlying surface and transport it into the air is more than that in the suburbs. This makes the urban underlying surface temperature higher than that of the suburbs, forming an "urban underlying surface temperature heat island", and thus makes the urban temperature higher than that of the suburbs through turbulent exchange and long-wave radiation.

2. Thermal properties of underlying surface: thermal conductivity K and thermal capacity C of urban underlying surface.

The heat storage capacity of the surface is obviously higher than that of the suburbs. There is more heat storage during the day, and the ground temperature drops slower than that in the suburbs at night. Through the heat exchange between ground and air, the temperature in the urban area is higher than that in the suburbs.

3. Geometry of underlying surface: The uneven level, a building in the city, has formed many "urban street valleys" with different aspect ratios. During the day, due to the reflection and absorption between walls and between walls and the ground, more solar radiation energy can be obtained than that in the suburbs under the same other conditions. If the walls and roofs are painted with deeper colors, their reflectivity will be smaller and they will absorb more solar energy. Because the building materials of walls, roofs and floors have higher thermal conductivity and thermal capacity, "urban streets and valleys" absorb and store much more heat energy during the day.

Secondly, in the "urban street valley", the visibility of the sky (smy view fector, abbreviated as SVF) is smaller than that of the empty suburbs (Figure 8.2 1, omitted). In the exchange of long-wave radiation energy at the bottom of the street, the long-wave inverse radiation value is not only the inverse radiation from the atmosphere, but also the long-wave radiation from the walls and eaves. Therefore, its long-wave net radiation heat loss is smaller than that of suburban wilderness, and the wind speed in urban streets and valleys is relatively small, so the heat is not easy to dissipate, which leads to its higher temperature than that in suburbs.

(2) Man-made heat and greenhouse gases

1. Man-made heat: In mid-high latitude cities, especially in winter, a large amount of man-made heat emitted by cities is an important factor in the formation of heat islands. In many cities, the intensity of heat island in winter is greater than that in warm season, and the intensity of heat island from Monday to Friday is greater than that in weekends, which is affected by this.

2. Greenhouse gases: Due to the large energy consumption, the greenhouse gases such as CO2 emitted into the atmosphere by cities are far more than those in suburbs, and its humidification effect is obvious.

(3) Weather and meteorological conditions

1. In the stable weather situation with small pressure gradient, it is beneficial to the formation of urban heat island. When a strong cold front crosses the border, there is no heat island phenomenon.

2. When the wind speed is high and the air stratification is unstable, the horizontal and vertical air mixing between suburbs is strong, and the temperature difference between urban and suburban areas is not obvious. Generally speaking, the night wind speed is small, the air stability is increased and the heat island is enhanced.

3. When there are no clouds on clear days, there are obvious differences in reflectivity and long-wave radiation between suburbs, which is beneficial to the formation of heat island.

(2) 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 (Figure 8.20, omitted), with dense isotherms and steep temperature gradient. The temperature in the old city was 65,438. The temperature in densely populated areas and factory areas in the city is the highest, 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.

The heat island effect can be observed in large and small cities all over the world, regardless of their latitude, land and sea location and topographic relief. Its heat island intensity is closely related to city size, population density, energy consumption and building density.

There are many factors for the formation of urban heat island (see Table 8. 10 for details), among which the underlying surface factor, man-made heat and greenhouse gas emission are two aspects affected by human activities. In the same city, under different weather situations and meteorological conditions, the heat island effect is sometimes very obvious (there is no wind on sunny days), and the intensity of the heat island can reach 6℃- 10℃, sometimes it is weak or not obvious (strong wind, extremely unstable). 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.

(3) urban dry island and wet island effect

It is pointed out in Table 8.8 that the relative humidity in cities is lower than that in suburbs, which has obvious dry island effect, which is a common feature in urban climate. The influence of cities on atmospheric water vapor pressure is complicated. Take Shanghai as an example. According to the average value of water vapor pressure eu and relative humidity RHu in urban area 1 1 station (1984- 1990) in recent 7 years, it is compared with the average value of water vapor pressure er and relative humidity RHr in four suburban stations around the same period (see table 8.65438+).

The relative humidity has obvious diurnal variation. According to the actual measurement, although the absolute value of △RHu-r has changed, it is all negative. The "urban dry island effect" is presented all day. The diurnal variation of △eu-r is different. If we calculate the average of four observation times (02, 08, 14, 20: 00) in a day, we find that in most months of the year, it is 02: 00 at night.

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. Figures 8.22 and 8.23 (omitted) give examples of1August 1984 13 (urban dry island) and 02: 00 (urban wet island) alternating day and night. This phenomenon often occurs in warm seasons in many cities in Europe and America.

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. Figure 8.22 is an example of a dewing wet island. After sunrise, the temperature in the suburbs rises and the dew evaporates. Soon, the water vapor pressure in the suburbs is higher than that in the urban areas, and it has become an urban dry island. 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.