Abstract
In northern Italy, and especially in the Po Valley, instability phenomena such as hail and thunder storms are mainly associated with the penetration of Atlantic disturbances, especially cold fronts, or pre-frontal squall lines in the mid-seasons, and with the thermal convection of hot and moist air in the summer. Changes of their seasonal distribution may be considered as an index of anomalies in the synoptic pressure pattern, which determines the general circulation over Europe. Two different types of meteorological data have been analysed: (1) historical, non-specialist descriptions, taken from chronicles, annals or diaries after AD 1300, when people began to record, in addition to the most outstanding climatic events and natural hazards, minor observations including instability phenomena; (2) regular daily observations made by trained meteorologists after 1740. The historical descriptions show that in the Po Valley the highest thunderstorm frequency is in the summer, between June and August, but in the 1500s the distribution was anomalous. In that century, a broad maximum was found between March and August, and a secondary maximum was found in December. In the early period of observations made by trained meteorological personnel, i.e., 1740–1799, a summer maximum of thunderstorm frequency was found, almost equally distributed from June to August, followed by May and September. In the most recent period, i.e., 1940–1989, the seasonal distribution was very regular for three decades (1940 to 1969) with the mode in June, followed by July and August, and it then changed, with more late peaks. The historical written sources show that the seasonal distribution of hail was more homogeneous than thunderstorms. Hail was especially frequent in the summer, from June to August, except in the 1500s, when the seasonal distribution was practically one month earlier. In the early period of regular meteorological observations, hail was seasonally distributed less regularly than thunderstorms, with more skew and earlier in the year. The maximum frequency was in June, followed by April, May, July and August. In the most recent period, the distribution was still irregular, with marked fluctuations, the distribution being displaced later in the season.