Noteworthy Keyword
【Heat island prevention measures】

Introduction

Hot weather is magnified in big cities, an increasingly common norm that is known as the heat island phenomenon. Heat that flows into a city results from 1) solar heat and 2) exhaust heat generated in a city (combustion, electricity and automobiles). 3) Furthermore, heat dissipation obstacles generated in cities overlap, creating a thermal storage effect and contributing to heat island formation. Although many people think that global warming results in heat islands, its contribution is numerically only about 1/3.



1. Contribution to warming by CO2

Half of the CO2 generated by burning fossil fuels remains in the atmosphere, and the concentration gradually increases with distance from the Earth's surface. CO2 is transparent to visible light and can not prevent the Earth from incident sunlight. However, CO2 is not transparent to infrared rays of heat emission at night and traps heat on the surface of the Earth. According to the IPCC Fourth Assessment Report, the average temperature on the surface of the Earth has increased 0.74 degrees C over the past 100 years.



At the end of 21st century, global average temperature is expected to rise 2.4-6.4 degrees C in the case of unrestrained economic growth and 1.1-2.9 degrees C if certain reduction measures are undertaken.

2. The present state of summer temperatures of Osaka

The summer temperatures of Osaka Prefecture in August 2000 rose dramatically compared to the measured tempreature of the same time period in 1980. The numerically-extracted values of the city indicated that the rise in temperatures for 100 years is +2.1 degrees C, which is approximately three times the global average for the same period. We should not take the cause of the heat island phenomenon as the extension of the global warming. In fact, the temperatures of Osaka rose about twice as high as the average temperature in Japan. It suggests that regional circumstances greatly influence the rise in temperature of Osaka. The annual number of hours of sweltering nights (>30 degrees C) in Osaka rose as follows: 380 hours in 1981 to 638 hours in 2000 (This number of hours shows about 1.5 times as long as that in the suburban areas).

The cause of high temperatures of Osaka

As the causes generally pointed out, thermal storage of solar heat, poor ventilation, little amount of green space and water surface, and significant automobile use (expansion of exhaust heat, asphalt or concrete pavement and asphalt parking lot) bring about an increase in heat storage.

3. Heat island prevention measures

The following two measures to prevent heat islands are examined: A) Reduction of man-made exhaust heat by control of heat from factories, offices and automobiles; B) Suppression of thermal storage in buildings and roads by promotion of heat emission, expansion of green space and open water surface. Osaka Prefectural Head Office and City Hall set a goal in 2004 and drafted heat island prevention measures. As a model case, small-scale cool spots were created, opened, and exhibited. Greening on the roofs of Osaka Prefectural Head Office and City Hall, man-made rivers and replacement of the blacktop parking lot with a lawn are representative. As a practical action, sprinkling asphalt or concrete pavement with water during high temperatures creates a cooling effect. Sprinkling pavements tested in Hirakata City, Takatsuki City and Moriguchi City in 2005 resulted in a maximum decrease of 8 degrees C. However, because sprinkling water is washed away and evaporated, permeability and water holding properties are vital to pavement materials. Pavements with high water holding properties (porosity over 20%) can retain a low temperature for longer.

New type of water sprinkling

Recently, it has been found that titanium oxide photocatalyst produces superhydrophilic coating, allowing a water screen to be spread over a wall. It has also been proved that water sprinkling on the glass wall can have a heat reducing effect.

Furthermore, an air-conditioner misting microfine water-droplets has been developed. Trials have begun with the air-conditioner named “Dry Mist,” which produces water droplets that completely evaporate before reaching the ground.


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