* * * The same characteristics of urban climate

Urban climate is not only influenced by regional climate background, but also reflects the role of human activities after urbanization. Therefore, the urban climate in different climate regions is not the same, but there are also some * * * urban climate characteristics, and the * * * urban climate characteristics are as follows:

1. Due to the special nature of urban underlying surface; Fuel in the air produces more carbon dioxide; Coupled with artificial heat sources and other factors, the urban temperature is significantly higher than that in the suburbs, which is the so-called "urban heat island effect." Studies by many scholars at home and abroad show that the intensity of urban heat island is greater at night than during the day, and the temperature difference in suburbs increases rapidly after sunset, and decreases obviously after sunrise.

2. In cities, because the underlying surfaces are mostly buildings and impervious roads, the evaporation and transpiration are small, so the average absolute humidity and relative humidity of urban air are small. However, due to the diurnal variation of thermal characteristics of urban underlying surface, turbulent exchange of boundary layer and human factors, the daily amplitude of urban absolute humidity is greater than that of suburbs, and the urban absolute humidity is lower than that of suburbs during the day, forming a "dry island" and the urban absolute humidity is higher than that of suburbs at night, forming a "wet island".

3. Because there are a lot of dust and other hygroscopic nuclei in the urban air, even if the water vapor in the air is not saturated and the relative humidity is only 70~80%, there will be fog in the city, so the fog in the city is more than that in the suburbs. In some cities, under the strong sunlight, the exhaust gas from automobile exhaust will also form a kind of light blue smoke with ozone aldehyde and peroxyacetyl nitrate (PAN) as the main components, which is called "photochemical smog" and is harmful to human body.

4. Due to the urban heat island effect, the air in the center of the city is constantly heated, and the relatively cold air in the surrounding suburbs gathers to supplement the urban area, while the rising air in the center of the urban heat island diverges and sinks to the surrounding suburbs at a certain height to make up for the low-altitude vacancy in the suburbs, thus forming a local circulation called urban heat island circulation. This kind of circulation is most obvious under the static weather conditions of static partial cloudy and extremely weak background wind field. It should be pointed out that although the urban heat island effect is greater at night than during the day, the atmospheric stratification in the suburbs is stable at night and the updraft layer is not strong, while the atmospheric stratification in the suburbs is unstable during the day and rises rapidly after flowing into the urban area, so the urban heat island circulation is strong during the day and the suburban wind is paroxysmal at night.

5. Because there is updraft in the center of urban heat island and there are many condensation nuclei such as dust in the air, there are more clouds than suburbs, and the precipitation in the city and its downwind is more than other areas. In addition, because cities use a lot of energy, a lot of sulfur dioxide and nitrogen oxides are discharged into the atmosphere. Under a series of complex chemical reactions, they form sulfuric acid and nitric acid, which fall into acid rain through the formation and scouring process of rain. Acid rain will lead to poor soil, slow down the growth of forests, inhibit the activities of microorganisms, pose a threat to the survival of fish and irritate people's throats and eyes ... Therefore, preventing acid rain is an urgent task.