Why is it so hot in Guangzhou in summer?

Generally, big cities are hotter than suburbs because of the stealing heat effect.

The comprehensive effects of urban population density, heat exhaustion of factories and vehicles, energy release of residents' daily life, urban building structure and underlying surface characteristics are the main reasons. The intensity of heat island has obvious diurnal and seasonal variations. The diurnal variation is strong at night and weak during the day, and the maximum value appears on a clear and windless night. The maximum heat island intensity observed in Shanghai is above 6℃. Seasonal distribution is also related to urban characteristics and climatic conditions. Beijing is the strongest in winter, the weakest in summer and the middle in spring and autumn. Shanghai, Guangzhou 10 is the strongest. The annual average temperature difference between urban and rural areas is about 65438 0℃, such as 0.7 ~ 65438 0.0℃ in Beijing, 0.5 ~ 65438 0.4℃ in Shanghai and 0.5 ~ 65438 0.5℃ in Los Angeles. Urban heat island can affect the temperature stratification in the surface layer and reach a certain height. Unstable stratification is dominant in cities during the day, and inversion occurs in rural areas at night. The existence of horizontal temperature difference makes the warm air in the city rise and spread to a certain height, while the nearby rural airflow sinks and converges with the city along the ground, forming a heat island circulation, which is called "country wind", and this flow field is especially obvious at night. Urban heat island also affects urban air humidity, cloud cover and precipitation to some extent. The effects on plants are early germination and flowering, delayed defoliation and dormancy.

Urban heat island effect is one of the typical characteristics of urban climate. It is a phenomenon that the temperature in cities is higher than that in suburbs. On the one hand, the formation of urban heat island is the heat emitted by people in modern big cities; On the other hand, the buildings in the city are dense, and the asphalt and cement pavements have higher specific heat capacity (can absorb more heat) than the soil and vegetation in the suburbs, while the reflectivity is small, which makes the city absorb and store more solar energy than the suburbs during the day, and the temperature in the city is still higher than that in the suburbs at night. Urban heat island is centered on the city center, where a strong warm air flow rises, while relatively cold air sinks over the suburbs, thus forming suburban circulation. Under the action of this local circulation, various pollutants in the air gather over the city. Without strong cold air, urban air pollution will be aggravated, and human living environment will be destroyed, leading to various diseases and even death.

Heat island is a comprehensive phenomenon of microclimate change caused by people changing the urban surface, and it is one of the most obvious characteristics of urban climate. Due to the acceleration of urbanization, dense urban buildings, asphalt roads and cement pavements have greater heat capacity and heat absorption rate than the soil and vegetation in the suburbs, which makes the urban area store more heat and radiate it to the surrounding areas and the atmosphere, resulting in the temperature in the urban area being generally higher than that in the surrounding suburbs at the same time. The high-temperature urban area is surrounded by low-temperature suburbs, just like an island in the sea of Wang Yang. This phenomenon is called urban heat island effect.

Climate is the external cause of urban heat island effect, while urbanization is the internal cause of urban heat island formation. It is generally believed that there are three reasons for the heat island effect:

First, the surface properties of cities and suburbs are different, and the thermal properties are quite different. The urban area has low reflectivity, more heat absorption, less evaporation heat consumption, fast heat conduction and slow radiation heat loss, just the opposite in the suburbs;

Second, the man-made heat emitted by urban areas is greater than that in suburbs;

Third, the concentration of air pollutants in urban areas is high, and there are many aerosol particles, which plays a role in heat preservation to a certain extent.

Air pollution plays a very complicated and special role in urban heat island effect. The concentration of air pollutants from industrial production, transportation and daily life is particularly high in urban areas. It covers the whole city like a thick blanket. During the day, it greatly weakens the direct radiation of the sun, slows down the temperature rise in urban areas, and sometimes produces the "island" effect of the city. At night, it will greatly reduce the heat loss caused by effective long-wave radiation on the urban surface, play a role in heat preservation, make the city "cool" slower than the suburbs, and form a night heat island phenomenon.