SOME CONSIDERATIONS ON THE CAUSE OF CITY TEMPERATURE AT KUMAGAYA CITY

In the previous paper, the author analyzed the detailed distribution of temperature in and around Kumagaya City, situated in the central part of Kantô Plain, concerning various weather conditions (Geographical Review of Japan, Vol. 37, No. 5, pp. 243-254). Relationship between city temperature and meteorological factors suggests that the cause of city temperature of a local city in Japan is seen in the difference of radiation balance between urban areas and surroundings and especially construction materials of a city plays an important role in deciding city temperature.<br> Further considerations on the cause of city temperature were attempted in this paper. The comprehensive book about city climate by A. Kratzer shows that the following three factors are important as the causes of city temperature: artificial heat produced in urban areas, pollution of city air and effect of construction materials of a city.<br> In the case of this paper, however, it seems that the amount of artificial heat is relatively small and pollution of city air is unimportant, because the city size is small and there are very few factories in the city which consume a large amount of coal. Especially in summer season, these two factors are negligible.<br> Difference of radiation balanced between the urban area and the surroundings affects on the city temperature in two ways. Those are the physical properties of constrution materials of the city concerning heat balance and the unevennss of the surface coverage of the city influencing heat transfer. Physical nature of construction materials of the city decides surface temperature of the earth. Nocturnal cooling of surface temperature of the earth is represented by Brunt's formula in accuracy of first approximation. The surface of the earth (T) is given by <br>_??_<br> wher T₀; surface temperature of the earth at the time of sunset, R; nocturnal radiation from the surface of the earth, ρ; density of ground, c; specific heat of ground, κ; specific conductivity of heat of ground. Assuming that the nocturnal radiation from the ground to the air (R) does not vary with time, T is decided by 1/cρ√_κ.<br> This formula was applied to the surface temperature of various construction materials of the city and then evaluation of nocturnal cooling was attempted as follows. Firstly, areas of roads, houses, grounds and so on in each 50m-meshed square were measured by using the map of 1/3000 scale and the relative portion of their respective areas for each section was calculated. Secondly, 1/cρ√_κ in each section was computd. Values of 1/cρ√_κ of each material are given in Table 1. (Concrete, asphalt, roofing the and soil are listed from above to below.) Then, this value in each section was converted into value of 1/cρ√_κ, in 200 mmeshed setion. The result is shown in Figure 2.<br> The close relationship between temperature in the urban area and the value of 1/cρ√_κ is shown in Figure 3. This figure shows that both physical properties of construction materials and unevenness of the city play important roles in deciding city temperature.<br> Unevenness of surface coverage in an urban area results in the decrease of nocturnal radiation and wind velocity. Consequently, decrease of noctvrnal radiation accelerates decreasing of nocturnal cooling in an urban area and drop of wind speed in an urban area promotes decreasing of heat tranfer from below to above in city air. As a result, temperature in an urban area becomes higher than in the surroundings.