Asking about "Urban Heat Island"

For everyone who studies and works in Beijing, the sultry summer is always the most uncomfortable season of the year. Perhaps you will find it strange that Beijing, with its high latitude, is always very hot in summer. After years of research, experts found that the "heat island effect" is one of the culprits of Beijing's persistent high temperature.

What kind of physical phenomenon is "heat island effect"? How has it profoundly changed the environment and temperature in Beijing? Professor Liu Shuhua, from the Department of Atmospheric Science of Peking University Institute of Physics, who has deeply studied the relationship between the "heat island effect" and Beijing's climate, told reporters about the mystery of the "heat island effect".

The surface composition of modern cities makes energy absorption slower, and the whole city is like a "heat island".

It turns out that in this modern metropolis where we live, the surface is composed of cement, concrete and asphalt roads, while the surface in the suburbs is composed of vegetation and soil. The thermal balance characteristics between them are quite different.

The special surface of the city makes it absorb more sunlight than the soil, and the increasingly popular electrical equipment such as air conditioners, millions of cars and various man-made heat are also excessively absorbed.

Therefore, the heat storage capacity of urban surface is much larger than that of suburbs. In addition, the atmosphere over the city is turbid and the content of greenhouse gases is high, which obviously affects the loss of long-wave radiation on the ground and leads to the greenhouse effect. Therefore, the temperature of the city with dense buildings is obviously higher than that of the surrounding suburbs, making the city like a "heat island".

Liu Jiaoshou told reporters that due to the characteristics of the surface, the surface temperature rises, and the heated air will form a hot air flow. Under the influence of atmospheric balance, the air in suburbs and other urban fringe areas will sink to supplement the rising areas, thus forming a dynamic heat island circulation.

Heat island circulation has a great influence on urban precipitation. Usually, when the temperature rises, the air forms an updraft, which is easy to form convective weather, leading to an increase in urban precipitation intensity. The representative of this weather in Beijing is frequent thunderstorms in the afternoon.

The heat island circulation in Beijing has further worsened air pollution, even in the western suburbs.

It is noteworthy that heat island circulation not only increases precipitation, but also aggravates air pollution, and the pollution generated by cities further affects cities through circulation. Liu Jiaoshou introduced the manifestation of heat island effect under the special terrain of Beijing surrounded by mountains on three sides. During the day, the mountainous area in the northwest of the suburbs with the same height receives more energy than the plain area, and the temperature rises rapidly, and an updraft will form above it, which will drive the plain airflow to form a south wind, thus affecting the mountainous area. At night, the heat loss in mountainous areas with the same height is faster than that in plain areas. Coupled with the man-made heat island effect, it will turn to the north wind after 9: 00 pm, first from the mountainous area, and gradually affect the city.

Therefore, it is the north wind all year round in Beijing, which gradually turns to the south wind after sunrise during the day and then turns to the north wind at night, which has caused the problem of regional climate formation. These pollutants are sent to mountain areas for storage in the daytime in late autumn and winter. When there is no big weather system, the pollutants will be sent back by circulation at night 10. For five consecutive days in late autumn and early winter, Beijing's pollution index reached five.

Liu Jiaoshou pointed out that if the city is surrounded by plains in an ideal state, the development of the city will definitely lead to the enhancement of heat island circulation. Due to the special possibility of Beijing's topography, the temperature difference and air pressure difference between Beijing's urban and suburban areas have caused the formation of heat island circulation. According to the numerical simulation and observation of Hu Xiaoming, Master of Atmospheric Science Department of Peking University Institute of Physics, the ground temperature difference between urban center and suburb is 6℃, and the long-term temperature difference is 1-2℃. The temperature difference between different urban areas in Beijing is related to the economic development and population in this area. Generally speaking, the temperature in the west is higher than that in the east. The average temperature in Haidian in July is 14, which is about 1℃ higher than that in Chaoyang.

To reduce the heat island effect, it is necessary to reduce the ground area with large heat capacity, and it is even more indispensable to increase greening.

He was asked whether the heat island effect played a key role in various factors leading to the high temperature disaster in Beijing. Liu Jiaoshou said that from the perspective of global warming, it has only increased by about 0.4℃ in ten years, so as far as Beijing is concerned, the impact of global temperature change is negligible. The high temperature disaster in Beijing is mainly the result of the superposition of high temperature year and Beijing heat island effect in the high and low temperature natural cycle. Therefore, it is urgent to take effective measures to control the heat island effect.

Liu Jiaoshou emphasized that in order to slow down the urban heat island effect, cities must first reduce the ground area of urban heat capacity. At present, the general buildings in Beijing are all cement structures and the pavement is asphalt pavement, which absorbs a lot of heat. Among them, the black reflectivity of asphalt road is very low, so the heat absorbed by asphalt road is much higher than that of light color in the same area.

If the roofs in urban areas are all green, or a rest place, such as a swimming pool, can not only reduce the increase of heat, but also increase entertainment facilities. In Tokyo, Japan, many roofs are used as tennis courts, swimming pools or gardens to effectively reduce the occurrence of heat island effect, and these roofs are rarely used. Research shows that if the vegetation area in Beijing increases by 30%, the temperature will drop by 2-3℃. At present, buildings in Beijing account for more than 30% of the ground, and most old roofs can be effectively afforested, so it is entirely possible to increase vegetation by 30%.

For pavement, we should consciously promote the honeycomb structure, because the use of honeycomb floor can effectively change the heat balance of the ground, naturally open up the circulation of water and heat on the ground, and reduce the surface temperature by 3-4℃ during the evaporation of water on the ground. Removing fences, changing building orientation and improving ground ventilation effect are effective means to reduce heat island effect.

Liu Jiaoshou has studied urban residential areas and found that there are tens of thousands of people living in more than 50 buildings, which are densely arranged. In addition, the area of green space has decreased. Because the building is a cement body that absorbs a lot of heat, the heat island effect is very obvious, and the building area is reduced by increasing the height. If ten buildings are built, most of the original buildings will become green spaces, which will increase the ground ventilation, optimize the surface heat balance and dynamic effect, and the heat island effect will decrease rapidly.

He told reporters that ventilation is also closely related to the urban heat island effect. In coastal cities, if the streets are perpendicular to the coastline, the ventilation of sea breeze can be fully utilized, and whether the city is conducive to cleaning or reducing the heat island effect will be greatly improved. If urban buildings are parallel to the coastline, it will have a great influence on urban heat dissipation.

In view of the fact that there are too many physical walls in Beijing, he suggested that the high-rise walls of some units be demolished and built into fences to increase air circulation. Moreover, in order to adapt to the characteristics of the north-south wind direction in Beijing, attention should be paid to adjusting the direction when building buildings in the future, and it is not appropriate to use all east-west board buildings. If the building structure can ensure good lighting in winter and good ventilation in summer, the heat generated during heating and cooling can be reduced.