In the recent years a tremendous increase in economic, environmental and human losses caused by weather hazards, especially floods and droughts, has raised the alarm over the possibility that the recent increases in extraordinary atmospheric events were are due to a changing climate.
The analyses till now performed on daily series show a positive trend in the mean amount per wet day (i.e. precipitation intensity) for some areas and a tendency toward higher frequencies of heavy and extreme rainfalls in the last decades. Besides, there are some indications that the overall areas of the world affected by either drought and/or excessive wetness have increased.
In spite of the profound impact that changes in precipitation distribution would have on both human society and the natural environment, at present time results are available only for a few areas and often rather short periods (after the War World II). The lack of information in precipitation- related statistics is mainly due to the fact that analyses over wide spatial and temporal scales are hampered by great difficulties in setting up large data-setsdata sets of high quality daily precipitation series. A further problem concerning extreme events depends on statistics: these events are in fact rare and trends have generally to be estimated on the basis of a few cases per each station, causing significance to became a very critical factor.
In this context, our objective was to give a complete view of the evolution of the precipitation distribution in Italy over the last 120 years, with particular interest inregard to both heavy and extreme events and dry spells. For this purpose it was necessary to create a database of some tens of historical (at least 120 years) daily precipitation series uniformyuniformly distributed over the Italian territory. The series were validated, homogenized and completed, as concerning missing data not available on in yearbooks concerns, with statistical methods. The series analysis pointed out indicated that, besides a decrease in southern Italy and no significant trend in northern Italy in total precipitation amount, the most interesting result is was a strong and highly statistically significant decrease of in the number of wet days all over Italy (about 12% over the period 1880-2002). This trend is was not uniform allover throughout the precipitation distribution, but it had an opposite patternshows opposite behavior for low intensity events and heavy precipitation: being the former first decreasingdecreased, while the latter last increasingincreased. This iswas more pronounced in the northern part of Italy, while, in the southern part, where the negative trend of the number of wet days iswas stronger, there arewere no significant variations in heavy events. This research has been was funded by MIUR
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