NOCTURNAL HEAT ISLAND OF SMALL TOWN, ITS MANIFESTATION AND MECHANISM

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A large number of investigations have been theoretically and observationally given on the problems of urban heat island. Some of them have intended to obtain the informations about the heat island of small towns (Tamiya, 1968 etc.) and that around a large shopping center. It is expected that the results from these studies 'on small towns or the like could be applicable to the explanation of the heat island of large cities. Judging from these studies, the heat island of small towns is thought to be formed under the condition of clear calm night with strong surface inversion. Most of the studies were, however, based on a few observations. The aim of this paper is to clarify meteorological conditions by which the heat island of small town is formed, based on forty examples of observations at about five hours after sunset by automobile with thermistor thermometer carried out from July 1978 to February 1979. Then the mechanism of the phenomenonn is examined. Observations Three small towns are selected for the observation in. Tsukuba Academic New Town. They are Ohzone, Takezono and Namiki with population of 1, 800, 2, 000 and 3, 500, respectively, in 1978. Ohzone is a small rural settlement which has been existed before the construction of Tsukuba Academic New Town, and consists of mostly one or two storied wooden houses. The latter two have been constructed for officials of the New Town and consist of multi-storied concrete block buildings. These three are located on a flat upland with a heigh of 25-30m a, s. 1. near the Higashi-Oodori Boulevard (Fig. 1). Observation points and the route are shown in Fig. 1. Most of the points outside the small towns are surrounded by field, green zone and vacancy. Observations were started from the point 15 and ended at the same point running through 1-7-Ohzone-12-25Takezono-28-32-Namiki-34-35. It took about 30 minutes to finish each observation. Therefore, time correction was not applied. The date of the temperature observations are given on Table 1. Duplicate observations were carried out at the points outside the towns. Two observed values must differ because of the difference of the lane and time during the traverses. Discrimination among them is, however, difficult in practice. Means of the differences between two values and their standard deviations are given in Fig. 2, which shows that the mean values at all the points are smaller than 0.4°C, and that no systematic differences are identified. Meteorological data representing the surrounding air conditions are acquired from the Environmental Research Center (ERC) of the University of Tsukuba (Fig. 1). The values of elements used for analysis are also given on Table 1. Some features of the heat island A case of distinct heat island is shown in Fig. 3 (Oct. 16. 1978). In this night, under the condition of clear and weak wind (1. 2 m/s, NW) the inversion reached 6.0°C in the bottom 28.5m layer. On the contrary, in case of indistinct heat island (Oct. 29. 1978, Fig. 4) the sky was covered with cloud and the precipitation of 20mm was recorded during the day. In spite of weak wind (1.0 m/s) inversion intensity was only 0.9°C in the same bottom layer. Temperature deviation at each point from the mean temperature of the route is calculated for every observations. The average values of 40 observations are also indicated with standard deviations (Fig. 5). Northern part of the route is colder. The coldest is the point 6. Mean values of Ohzone, Takezono and Namiki are 1°C (Northern side) -0.5°C (Southern side), 1°C and 0.5°C, respectively. For the further examinations, heat island intensity is defined here as the difference between mean temperature of the points inside the small town and that of specified points outside (for Ohzone, points 3-6 and 14-19; for Takezono, points 14-19 and 29-31; for Namiki, points 29-31, 34 and 35).
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NOCTURNAL HEAT ISLAND OF SMALL TOWN, ITS MANIFESTATION AND MECHANISM

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