Formation and causes of temperature difference between urban and rural areas

Man-made heat is not the only heat source of urban heat island, because there is no small difference between urban and rural areas in absorbing and storing solar heat. For example, the reflectivity of urban underlying surface to solar heat is smaller than that of rural areas (generally less than 10%-30%), and the concrete, brick, stone and steel of urban underlying surface have large heat capacity and high thermal conductivity, which stores a lot of solar heat during the day. In addition, the buildings under the city are dense, and the sky visibility of streets and courtyards is much smaller than that of empty suburbs. The long-wave radiation heat of the ground is reflected between the wall and the ground for many times, thus greatly reducing the heat lost from the ground to the space. Both of these reasons lead to the slower temperature drop after the sun goes down, which makes the city particularly hot from mid-night to midnight in summer. In addition to the above-mentioned periodic changes such as annual, weekly and daily changes, the temperature difference between urban and rural areas also varies aperiodically. This is mainly caused by changes in wind speed and cloud cover. Wind speed is very important to the strength of heat island. Because the strong wind not only causes convection up and down, but also blows the hot air from the city to the outside of the city and directly transports the cold fresh air from the suburbs to the city. Some articles have studied the relationship between urban-rural temperature difference and wind speed in four cities of different scales in South Korea, and concluded that wind speed can obviously reduce the urban-rural temperature difference. Moreover, if the temperature difference between urban and rural areas is less than 0.5℃ as an indicator of the disappearance of the heat island, the Seoul heat island with a population of 8.4 million will begin to disappear at 1 1. 1 m/s, and the urban heat island with a population of Yang Guang, etc.13-150,000 will disappear at 4-5 m/s. In cloudy or cloudy weather, the income of short-wave radiation heat and the expenditure of long-wave radiation heat in urban and rural areas decrease during the day, which also reduces the temperature difference between urban and rural areas.

The heat sources that lead to urban "thermal pollution" mainly include fixed heat sources and flowing heat sources. For example, modern buildings piled up with concrete, bricks and stones absorb more heat but emit it slowly, making the urban area a "heat reservoir"; The pavement paved with cement asphalt is more like a "heat absorption plate", which is often 2 ~ 10℃ higher than the grassland surface temperature. Although the rapid increase of air conditioners can create a comfortable "microclimate", one air conditioner after another is like a piece of "radiator", which keeps the surrounding air at a high temperature above 40℃ all the time. Even the rapidly spreading computers are emitting a lot of heat to the surroundings ... Narrow streets, smaller courtyards and living rooms hinder the heat exchange between heaven and earth, and the ground temperature drops very slowly. In addition to numerous fixed heat sources, both motor vehicles and people are mobile heat sources.