The formation and causes of temperature difference between urban and rural areas

Anthropogenic heat is not the only heat source of urban heat islands, because the performance of urban and rural surfaces in absorbing and storing solar heat is quite different. For example, the reflectivity of sunlight heat on urban underlying surfaces is smaller than that in rural areas (generally 10% to 30% smaller), and the concrete, bricks, stones and steel underlying urban surfaces have large heat capacity and high thermal conductivity, which means they can be stored in large quantities. Abundant solar heat during the day. In addition, the urban underlying surface is densely built, and the "sky dome visibility" in streets and courtyards is much smaller than in open suburbs. The long-wave radiation heat from the ground is reflected multiple times between walls and floors, thus greatly reducing the heat loss from the ground to space. Both of these reasons cause the temperature to cool slowly after sunset, making urban areas particularly hot in the evening and first half of the night in summer. In addition to the cyclic changes such as annual changes, weekly changes, and daily changes, the urban-rural temperature difference also has non-periodic changes. This is mainly caused by changes in wind speed and cloud cover. Wind speed is extremely important for heat island intensity. Because strong winds not only cause up-and-down convection, blowing hot urban air out of the city, but also directly transport cool fresh air from the suburbs into the city. An article studied the relationship between the urban-rural temperature difference and wind speed in four cities of different sizes in South Korea, and found that wind speed can significantly 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 heat island disappearance, then the heat island in Seoul with a population of 8.4 million begins to disappear at 11.1 m/s, and the heat island in cities with a population of 130,000 to 150,000 such as Guangyang begins to disappear at 4-5 m/s. Disappeared, the heat island of Salido with a population of 60,000 no longer exists at 3.9 meters/second. On cloudy or cloudy days, both urban and rural daytime sunlight shortwave radiation heat income and ground longwave radiation heat expenditure decrease, thus reducing the temperature difference between urban and rural areas.

The heat sources that cause urban "thermal pollution" mainly include fixed heat sources and mobile heat sources. For example, modern buildings made of concrete, bricks and stones absorb a lot of heat but dissipate heat slowly, turning the urban area into a "heat storage device"; roads paved with cement and asphalt are more like "heat-absorbing panels". It is often 2-10℃ higher than the surface temperature of grassland. Although the rapidly increasing number of air conditioners can create a comfortable "microclimate", each air conditioner is like a piece of "radiator", keeping the surrounding air at a high temperature of above 40°C. Even computers that are rapidly spreading are dissipating a lot of heat to the surroundings... Narrow streets, smaller and smaller courtyards, and living rooms hinder the heat exchange between the sky and the earth, and the ground cools very slowly. In addition to numerous fixed heat sources, motor vehicles, people coming and going, etc. are mobile heat sources.