日本と韓国の諸都市における都市規模とヒートアイランド強度 City Size and Urban Heat Island Intensity for Japanese and Korean Cities
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The urban heat island intensity (difference between the highest urban temperature and the lowest rural temperature, <i>Δ</i>T<sub>u-r</sub>) increases generally with urban population. An apparent linear relationship between the maximum heat island intensity and urban population can be seen for North American and European cities by Oke (1973). There was a bend in the regression line at the point of around 300, 000 inhabitants for Japanese cities by Fukuoka (1983). The similar relationship was also observed for Korean cities by Park (1986).<br> Figure 1 shows the relationships between the maximum heat island intensity and urban population for Japanese and Korean cities. The regression equations for Japanese and Korean cties can be given as follows.<br> Japanese cities;<br> ΔT<sub>u-r(max)</sub>=1.21 log<i>P</i>-3.92 (r<sup>2</sup>=0.70;populatio<300, 000)<br> ΔT<sub>u-r(max)</sub>=4.01 log<i>P</i>-19.09 (r<sup>2</sup>=0.87;populatio>300, 000)<br> Korean cities;<br> ΔT<sub>u-r(max)</sub>=1.19 log<i>P</i>-4.73 (r<sup>2</sup>=0.97;populatio<300, 000)<br> ΔT<sub>u-r(max)</sub>=3.74 log<i>P</i>-18.44 (r<sup>2</sup>=0.98;populatio>300, 000)<br> The data on North American and European cities were obtained from Oke's work (1973, 1981). Based on those data, regression equations of North American and European cities can be represented as follows.<br> North American cities;<br> ΔT<sub>u-r(max)</sub>=2.96 log<i>P</i>-6.46 (r<sup>2</sup>=0.95)<br> European cities;<br> ΔT<sub>u-r(max)</sub>=1.92 log<i>P</i>-3.41 (r<sup>2</sup>=0.69)<br> where <i>P</i> is urban population. These two equations were created as part of the present study and are different from Oke's earlier equations (1973).<br> Since the correlation coefficients are very high, it can be concluded that the maximum heat island intensity is closely related to urban population. The slopes of the regression lines were smaller for Japanese and Korean cities than those in North American and European cities for population below 300, 000, but greater for larger population. The possible explanation can be partially given by the difference of urban structure, urban activities, etc., between larger cities (over 300, 000 population) and smaller cities (less than 300, 000). Here, the author has given an attention to the correlation of urban structure and the heat isalnd intensity. The sky view factor (i. e., ratio of buildingheight (H) width of urban canyons (street: W)) to and ratio of impermeable surface (i.e., ratio of building and road area to total city area) chosen as indices of urban structure.<br> Figure 2 shows the relationships between the maximum heat island intensity and sky view factor for Japanese, Korean, North American, and European settlements. As high correlation coefficients (Japanese cities, -0.83; Korean cities, -0.93; North American cities, -0.96; European cities; -0.82) were observed, it is clear that sky view factor is closely correlated to the heat island intensity.<br> A decrease of sky view factor results in an increase of absorbing short-wave radiation for daytime and a decrease of outgoing long-wave radiation for nighttime thus causing a warming of the urban area (Kobayashi, 1979; Barring and Mattsson, 1985). And a decrease of wind speed by building in urban canopy layer results in a decrease of sensible heat flux from the ground thus causing a warming of the urban area (Nishizawa, 1958; Nunez and Oke, 1976).<br> Figure 3 shows the high correlation between the maximum heat island intensity and ratio of impermeable surface coverage for Japanese and Korean cities (correlation coefficient: Japanese cities, 0.88; Korean cites, 0.95). It is noticeable that the prevailing impermeable materials play an important role in increasing the heat island intensity.