The earliest temperature record in Paris, 1658–1660, by Ismaël Boulliau, and a comparison with the contemporary series of the Medici Network (1654–1670) in Florence

The earliest temperature series in Paris, from May 1658 to September 1660, taken by Boulliau is made available at daily resolution after a careful work of homogenization, correction and calculation of the average values. New results are achieved concerning the Little Florentine Thermometer, building location, thermometer exposure, observation methodologies followed by Boulliau and weather in the mid-seventeenth century. Two methods are used and compared to calculate the daily average from readings taken at random sampling times. The first one is based on the reading needing the smallest correction to be transformed into a daily average; the second considers all the readings of the day and makes a bulk average of the individual results. The series is compared with the temperature record by the Grand Duke Ferdinand II in Florence, which was the primary station of the Medici Network (1654–70). In addition, the comparison of the earliest temperature series in Paris and Florence with their respective 1961–1990 reference period gives a clear image of the change of weather conditions in Europe in the middle of the seventeenth century. Key features were a strong variability for warm-air and cold-air outbreaks, severe winters and cold summers in Paris, not affecting Florence.