Secular changes of temperature and amount of snow cover for cities

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Long-term data of the number of days with snow cover (snow days) were collected and analyzed, in order to clarify the urban influence upon the amount of snow cover. In Japan, there were 800 stations which observed the amount of snow cover. Among these, 435 stations are selected, which can be traced back to 1931 and continued up to the present.Cluster anaysis is applied to 17 stations in Hokkaido and 43 stations in Tohoku Destrict which supply the homogeneous snow data, to divide the four or five regions from the viewpoint of the year-to year variations. The secular changes of the regional mean vlue are regarded as natural climatic changes, and the difference between the urban and the rural stations is regarded as the urban influences. Results obtained are as follows :1) The number of snow days in Sapporo is 6 days less than the regional mean value.2) The number of snow days in Sendai is 8 days less than the regional mean value which is remarkably increasing.3) Decreasing tendencies of the number of snow days is observed even the cities with a population of 200,000.4) The urban influences are occurred with a time lag, according to a city size.