Urban Thermal PollutionUrban Thermal Pollution and Urban Heat Island

The strongest heat islands in the world are in large and medium-sized cities in mid- and high latitudes, such as Vancouver, Canada, with 11°C (July 4, 1972), and Berlin, Germany, with 13.3°C. Fairbanks, the capital of Alaska, USA, located near the Arctic Circle, once reached 14°C. The proportion of man-made heat sources in cities. According to surveys of some large cities in the United States, stationary heat sources such as factories, household stoves, air conditioners, and heating account for about 3/4, while mobile heat sources such as cars, motorcycles, and trams account for about 1/4. The metabolic calories of humans and livestock are generally less than 1%. The temperature difference between urban and rural areas generally increases with increasing latitude. Because the ratio of man-made heat and solar radiation heat balance (short-wave heat income from the sun minus heat expenditure from long-wave radiation from the ground) increases rapidly from the equator to high latitudes. For example, Singapore and Hong Kong at the equator and the tropics are only 3% to 4%, while Fairbanks near the Arctic Circle is as high as 105%, which means that man-made heat is already more than the balance of solar heat! Throughout the year, The amount of solar radiation heat and balance is the smallest in winter. In addition, there is heating heat in mid- to high-latitudes in winter, so the heat island effect is strongest in winter. By the same token, the heat island is stronger at night than during the day, especially within 3-5 hours after sunset. Because the cooling rate in urban areas is much slower than that in rural areas.

However, the winter situation in cities at mid- and high-latitudes is different, because the city also has a large amount of artificial heat in the morning and evening heating periods. For example, Calgary, Canada, has two gas consumption peaks (for heating and cooking) near 08:00 in the morning and 20:00 in the evening in winter, so the urban heat island intensity also has a peak at 09:00 and 21:00. In addition, the temperature difference between urban and rural areas also varies weekly. This is because many countries have uniformly stipulated that Sunday is a rest day, most factories are closed, and the traffic of motor vehicles on the streets is much less than usual. For example, the five-year average temperature difference between urban and rural areas in New Haven, Connecticut, USA, from 1939 to 1943, was as high as 1.2°C from Monday to Saturday, while on Sunday it was only 0.6°C. After the United States designated Saturday as a rest day, Baltimore, Maryland, observed an average temperature difference of 0.82°C between urban and rural areas from Monday to Friday in winter, while the average temperature difference between Saturday and Sunday was only 0.30°C. However, man-made 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, so 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 8.4 million people begins to disappear at 11.1 m/s, and cities with 130,000 to 150,000 people such as Guangyang have heat islands 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. For example, four comparative observations were conducted in Shanghai in 1984. When the wind speed was roughly the same, the temperature difference between urban and rural areas on two sunny days (May 8 and October 20) was 2.5°C and 2.2°C respectively. On cloudy and cloudy days (May 28 and November 28), the temperature difference between urban and rural areas is only 0.4℃ and 0.7℃ respectively. Therefore, the reason why Shanghai considers October to November as the month with the largest temperature difference between urban and rural areas in the whole year is because they are the seasons with the least cloud cover in Shanghai.

The existence of urban heat islands shortens winter in urban areas and reduces frost and snow. Sometimes it even snows in the suburbs and rains in the city (such as Shanghai on January 17-18, 1996). Urban heat islands also reduce heating energy consumption in urban areas in winter. However, the high temperatures caused by urban heat islands in mid- and low-latitude cities in summer not only reduce people's work efficiency, but also increase the number of heat strokes and deaths. For example, in St. Louis, the United States, from July 9 to July 14, 1966, the maximum temperature was 38.1°C to 41.1°C, which was 5.0°C to 7.5°C higher than before and after the heat wave. At this time, the number of deaths in the urban area increased sharply from the original normal 35 people/day to 152 people/day. In July 1980, heat waves hit St. Louis and Kansas City again. Death rates in the commercial districts of the two cities increased by 57% and 64% respectively, while the nearby suburbs only increased by about 10%. Someone studied the city of Los Angeles in the United States and pointed out that because the temperature difference between urban and rural areas increased by 2.8°C, the city consumed an additional 1 billion watts of electric energy for air conditioning cooling, which is equivalent to approximately US$150,000 per hour. Based on this, it is estimated that the heat island effect in the United States costs millions of dollars more in air-conditioning electricity per hour in summer.

In addition, high temperatures in summer will aggravate urban water supply shortages, frequent fires, and intensify photochemical smog disasters, etc. Therefore, urban heat island itself is also a kind of pollution, called thermal pollution.