What are the impacts of autumn climate change on cities?

The total radiation is weakened and the sunshine is reduced. Due to air pollution, dust and atmospheric turbidity increase, so the solar radiation reaching the ground is greatly reduced. According to comparative observation, the total radiation in urban areas is about 10-20% less than that in suburbs. In winter, the solar altitude angle is weakened especially when it is small, and sometimes it can even be reduced to 50%. At the same time, the sunshine hours are gradually decreasing. Generally, the sunshine is 5- 15% less than that in rural environment. It is observed that the annual sunshine hours in Tokyo have decreased by about 70-80 hours in the past 50 years.

(2) There is high temperature in the city, and there is a heat island effect. The city seems to be a warm island. The urban temperature is high, the suburban temperature is low, and the isotherm is circular. We call it the "heat island effect". It is observed that the urban "heat island effect" can generally make the annual average temperature in urban areas 0.5-65438 0.0℃ higher than that in suburbs.

But in different seasons and different weather conditions, the temperature difference between urban and suburban areas is also different. For example, under the clear weather condition of 1979 in winter, the comparative observation in Shanghai found that the temperature in the city center was 6℃ higher than that in the suburbs.

The formation of heat island effect is related to the heat preservation effect of pollutants over the city, the reduction of heat consumption by ground evaporation, the low wind speed, the reduction of heat horizontal transportation, the release of artificial heat and the heat exchange with organisms.

(3) The occurrence of heat island circulation is accompanied by heat island effect, resulting in heat island circulation. Especially in summer, the air flow in the city center rises and spreads around after reaching a certain height, while the air in the suburbs flows to the city center on the ground.

(4) Low humidity, fog and increased precipitation. Because the urban area has good drainage, dry ground and small evaporation, the absolute humidity is lower than that in the suburbs, and the difference is generally below 1 millibar. The relative humidity is lower because of the high temperature in the urban area. According to European observation, the annual average relative humidity difference between cities and suburbs is 4-6%.

Because there are many condensation nuclei over the city, the number of foggy days has obviously increased. From the end of 19 to the beginning of the 20th century, the number of foggy days in Tokyo increased by 20-30 days on average, and that in Shanghai increased by about 15 days. Compared with suburbs, the number of foggy days in urban areas is 100% more than that in suburbs, and it is 30% more in summer.

Urban precipitation has also increased. Observations show that the annual precipitation in cities is 5- 10% more than that in suburbs. In addition, the number of light rain days (0. 1- 1.0mm/ day) also increased significantly. According to the observation in Tokyo, the city center is about 20 days longer than the suburbs.