The climate in the central city is different from that in the suburbs, which is manifested in the following aspects.

The impact of human activities on climate is most obvious in cities. According to the simultaneous observation data of meteorological stations in the urban area and its surrounding suburbs, compared with the suburbs, the urban climate has five island effects such as "heat island", "dry island", "wet island", "turbid island" and "rainy island".

Heat island:

On a clear and windless night, when you enter the urban area from the suburbs, the temperature will increase obviously, and the temperature in the central urban area and the factory-intensive area with the largest population density and building density is higher than that in the suburbs. Taking Shanghai as an example, there was a record that the highest temperature difference between urban and suburban areas was 6.8℃ in winter and 4.8℃ in summer. Through the urban area and then to the suburbs, the temperature drops sharply, showing an obvious "urban heat island" on the temperature distribution map.

The reason for the heat island effect is that, except for a few green spaces, most cities are artificially paved roads, and uneven buildings and structures have formed many "urban streets and valleys" with different aspect ratios. During the day, due to the reflection and absorption between walls and between walls and the ground in the street valley, more solar heat energy can be obtained than in the flat suburbs nearby. If the walls and roofs are painted with deeper colors, their reflectivity will be smaller and they will absorb more solar heat. In addition, because building materials such as brick, asphalt and cement board have greater thermal conductivity and thermal capacity, the heat absorbed and stored by urban streets and valleys during the day far exceeds that of suburban floors. The impervious surface of the urban underlying surface is large, and the rainwater quickly loses from the drainage pipe after rainfall. The water available for evaporation is far less than that of suburban farmland green space, and the latent heat consumed by evaporation is also less. The obtained solar heat energy is mainly used to heat the underlying surface, forming an "underlying surface temperature heat island". Then, heat is transferred to the air through turbulent exchange and long-wave radiation. Air is not good at absorbing sunlight and heat, but it is easy to absorb the heat from the underlying surface and raise its temperature.

In addition, because the energy consumption and population density of cities are much higher than those of suburbs, man-made heat and greenhouse gases (such as carbon dioxide, etc.). ) more is discharged into the air than in the suburbs, which in turn contributes to the formation of urban heat islands. The wind speed at night is generally smaller than during the day, and the heat exchange between suburbs is weak. Urban streets and valleys store more heat during the day and the temperature drops more slowly at night, so the urban heat island effect is more significant.

Dry island and wet island:

The influence of cities on atmospheric humidity is complex. Taking Shanghai as an example, during the seven years of 1984- 1990, the average water vapor pressure of the whole city 1 1 station was lower than that of the four stations in the surrounding suburbs during the same period, showing the effect of "urban dry island". In July, the average intensity of dry island (that is, the average water vapor pressure in urban area is lower than that in suburbs) is the largest. (0.56 hectopascals), the difference (absolute value) between 65438 and 10 is the smallest (only 0.02 hectopascals). However, the water vapor pressure difference in suburbs has obvious diurnal variation. If the average values of four observation times (02: 08: 14: 00 and 20: 00: 00) in a day are calculated, it is found that from April to 10: 00 in a year, the average water vapor pressure in the urban area at 02: 00 at night is higher than that in the suburbs at the same time, and an obvious "urban wet island" appears. The average intensity of wet island is the largest in August. Take 1984 as an example. At 02: 00 in August of that year, the water vapor pressure in the urban area was 0.6 hectopascals higher than that in the suburbs, with the maximum difference of 2.0 hectopascals. At 08: 00 in the daytime and 14, the urban area was obviously lower than that in the suburbs, especially in the afternoon 14, and the highest dry island intensity was 65,438+in July and August of that year. This phenomenon of alternating day and night between urban dry islands and urban wet islands 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. Under the irradiation of the sun during the day, the amount of water vapor that the underlying surface enters the lower air through evapotranspiration (including evaporation and plant transpiration) is smaller in urban areas than in suburbs, especially in midsummer, when crops grow intensively in suburbs, the difference in natural evapotranspiration between urban areas and 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, more water vapor is transported from the lower level of the urban area to the upper level of the air than that in the suburbs, which leads to the lower water vapor pressure near the urban area than that 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 residual water vapor in the lower air 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 formed by different condensation amount in suburbs is called condensation wet island. Most of them formed within 65,438+0-4 hours after sunset. After sunrise, the temperature in the suburbs rose and the dew evaporated, which soon became an "urban dry island". When urban dry islands and wet islands appear, they will be accompanied by urban heat islands. Through the comparative analysis of urban and suburban areas where atmospheric water vapor pressure was observed one by one in Shanghai 1984, the author also found that the formation of urban wet islands in Shanghai included frost wet islands, fog wet islands, rain wet islands and snow wet islands in addition to the above-mentioned condensation wet islands, and they all appeared when the wind was light and accompanied by urban heat islands.

Turbid island:

Solar radiation projected on the earth's surface can be divided into two parts: one part is direct sunlight in the form of parallel rays, which is called direct solar radiation S; The other part is the light scattered by air molecules, suspended particles and cloud particles in all directions when solar radiation passes through the earth's atmosphere, which is called scattered radiation D. Under the same solar radiation intensity, there are more scattered particles in turbid air, and its scattered radiation is stronger than that in dry and clean air, while the direct radiation is greatly weakened. Meteorologists use d/s to express atmospheric turbidity (also called turbidity factor). Urban industrial production, transportation and residential stoves emit more smoke and dust pollutants than suburban areas. Most of these pollutants are condensation nuclei that are good at absorbing water. The vertical turbulence in the city is relatively strong, which is beneficial to the development of low clouds. A large number of observation data prove that there are more low clouds in the urban area than in the nearby suburbs, which makes the scattered radiation in the urban area stronger than that in the suburbs, the direct radiation weaker than that in the suburbs, and the atmospheric turbidity significantly greater than that in the suburbs. Taking Shanghai as an example, according to the radiation data in recent 27 years, the average turbidity D/S at Shanghai Station is 1. 17. Compared with ten suburban stations in the same period, the average turbidity D/S is 15.8%. On the map of Shanghai's turbidity distribution, the urban area shows an obvious turbidity island, which is similar in many foreign cities.

Rain island:

There are many international debates about the influence of cities on precipitation. The United States once set up a dense rainfall observation network in the central plain of St. Louis, Missouri and its nearby suburbs, and used advanced technology for five years of observation and research, which confirmed that there was indeed a "rain island effect" in the city and its downwind. Based on the data of more than 70 rainfall observation stations in Shanghai/KLOC-0, combined with the weather conditions, we have done a lot of case analysis and classified statistics. It is found that the influence of Shanghai cities on precipitation is more obvious in flood season (May-September). On the distribution map of precipitation in flood season in Shanghai in recent 30 years, the precipitation in urban areas is obviously higher than that in suburbs, showing an obvious urban rain island. In non-flood season (10 to April of the following year) and.

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

To sum up, it can be seen that the "five islands" effect in urban climate is the unconscious influence of human beings on local climate in the process of urbanization. Studying its law is not only helpful for urban weather and climate prediction; Moreover, we can consciously improve urban climate conditions through certain human measures, such as strengthening urban greening, adjusting energy structure, rationally planning urban construction, and controlling urban air pollution. To make it develop in a direction conducive to residents' life and production.