This is a well-known fact since the 1970s, and heat island intensity is remarkable during winter nights.For example, the annual mean temperature in Tokyo has risen by 3.2 ° C over the last 100 years, mainly because of the increase in the daily minimum temperature of the night by 4.5 ° C. The daily maximum temperature has increased by only 1.5 ° C. Furthermore, the rise in daily maximum temperature in August is only 0.8 ° C in 100 years. This is almost the same as the average global warming. In other words, since the maximum summer temperature in Tokyo is not higher than the surroundings, it is not realistic to reduce the daytime air temperature in summer.
On the other hand, it is also true that the people in the city suffer from the stress of extreme heat. Why does it happen? This is because the ground surface temperature is extremely high. Surfaces exposed to direct sunlight, such as urban roads and buildings, are about 30 ° C higher than the air temperature. A person standing on such a wide ground feels the heat caused by radiant heat in addition to the air temperature. The amount of radiant heat is about 100W. This is a very large amount of heat considering that the power of the disposable body warmer is about 1W. It is obvious that the surface temperature of the city has a great impact on heat stress.
It is very strange that the urban ground surface temperature is higher than in the suburbs. The urban ground surface is covered by materials with large heat capacities such as asphalt or concrete, and the temperature is hard to change. As a result, the temperature in the city remains higher than in the suburb at night. This is the basic mechanism of the heat island. Application of this theory should lead to a cool island of the city at daytime.
The reason why the surface temperature of urban areas are extremely high is that the heat transfer coefficient to the atmosphere is very small compared to the ground surface in the suburbs. The heat transfer coefficient that conducts heat from the surface to the atmosphere is inversely proportional to the square root of the size of the surface, so there is a difference of almost one order of magnitude in heat transfer between a leaf of a few centimeters of plant and an urban structure of several meters. For this reason, the urban surface cannot release the heat received from the sun and the temperature rises despite the large heat capacity.
One solution to this problem is a fractal sunshade. This is an agglomeration of sunshades like small leaves, a Sierpinski tetrahedron with a fractal dimension of 2. By having a fractal structure, it is well-ventilated and prevents the interference of small leaves with a large heat transfer coefficient, effectively transferring heat to the atmosphere. Figures show a thermal and a visible image of a fractal sunshade installed on a concrete urban model. The surface temperature of the fractal shade is low. The temperature of concrete under the shade is lower than the temperature during the day. In addition, since less heat is stored in the daytime, the temperature is lower than the surrounding concrete even at night.
The surface temperature of the sunshade would be further cooled down by using water, but a large amount of water would be required to offset the solar heat. In contrast to the shade itself, the ground under the shade can be easily cooled down by a small amount of water because the heat input is much smaller than the direct sunlight. By covering the ground with a water-retentive material and retaining rainwater greatly contributes to improving the thermal environment under the shade. The fractal sunshade is well ventilated, so the evaporated water is quickly released to the air outside and the humidity does not rise much there, but the cooled ground remains in place. The cooled ground contributes to lowering the radiant heat, and at the same time lowers the air temperature.
The combination of fractal awnings and water-retaining pavements can drastically reduce urban surface temperatures with only rainwater without the use of extra energy or water. This is an effective solution to improve the thermal environment of a city.
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