An attempt to reconstruct winter temperatures over the past 300 years from snowfall ratios based on diary weather records

In Japan, diaries that record daily weather conditions since the 18th century have been kept in various parts of the country. The winter weather distribution in the Japanese archipelago is characterized by a north-south extension of the mountain ranges, with the Sea of Japan side having more snow days and the Pacific side having more sunny days. On the other hand, when temperatures are higher in winter, precipitation is more likely to be rain, and when temperatures are lower, it is more likely to be snow. Therefore, by obtaining a linear regression equation based on the correlation between the ratio of the number of snowfall days to the total number of precipitation days in winter and the average temperature for the period when meteorological observation records are available (since 1879), it is possible to reconstruct long-term temperature variations from the snowfall ratio obtained from diary weather records in historical periods. Based on this principle, we attempted to reconstruct winter temperatures in Nagasaki since 1700. Fortunately, Nagasaki has records of early meteorological observations by Dutch medical doctors from 1840 to 1860, before the start of official meteorological observations, which can be used to verify the reconstructed temperatures. The results show that winter temperatures in Nagasaki tended to be lower in the period from about 1700 to 1840, but warmed temporarily in the 1850s and 1860s, and then remained lower again until about 1950. The temperature reconstructions for 1840-1860, when the reconstructed and observed temperatures overlap, are almost coincident, demonstrating that the winter temperature reconstructions based on snowfall ratios are mostly accurate.