Temperature observations in Bologna, Italy, from 1715 to 1815: a comparison with other contemporary series and an overview of three centuries of changing climate

The observations taken in Bologna, Italy, from 1715 to 1815, three times a day, with a number of thermometers (i.e., Stancari air thermometers, Little Florentine thermometer, Florentine stick thermometer and a number of Réaumur spirit and mercury thermometers) some of them operating in parallel, have been recovered and analysed. The early thermometers had unknown scales and temperature units, with deviations due to the bulb shape or the thermometric liquid, but it is possible to interpret them after comparison between parallel readings. Historical sources and the analysis of the data fingerprints and their variability permit recognition of where instruments were located and who the observers were. It is also possible to relate the indoor climate of historical buildings to the outdoor one, and transform indoor readings as they were taken outdoors, expressed in Celsius. The Bologna series has been compared with the contemporary observations in Padua, Venice and Milano. The climate analysis shows that the temperature fluctuated but with an increasing trend. The 1730-1770 decades constituted the coldest period and 1980--today the warmest one. The eighteenth century was generally cold and had an impressive frequency of extremely severe winters that exceeded the rest of the series. The whole dataset (i.e. 1715-2015) of daily temperatures has been included to allow further use for scientific purposes. Finally, the paper provides a methodological example of procedures to recover and analyse early instrumental series.