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Sunshine duration variability and trends in Italy from homogenized instrumental time series (1936–2013)

A data set of quality checked daily sunshine duration measurements was collected from 104 Italian sites over the 1936 to 2013 period. Monthly mean values were homogenized, projected onto a grid, and subjected to principle component analysis, which identified two significantly different regions: North and South. Sunshine duration temporal evolution is presented, and possible reasons for differences between the two regions are discussed in the light of a comparison with the trends found in observations of total cloud cover and with results from two neighboring regions: the Alps and Spain. In addition, trends for irradiance records, estimated from sunshine duration records using the Ångström‐Prescott formula, are presented too. The major feature of the trends, an increase in sunshine duration from the mid‐1980s, was common to both northern and southern Italy; the decrease in the preceding 30 year period was not, as northern Italy had a lower rate of decrease than southern Italy. The few records available during the earliest period of the data set indicate that sunshine duration in Italy increased from the mid‐1930s to the mid‐1950s. The further steps needed to identify and quantify the mechanisms giving rise to the observed trends and to the reported regional differences in dimming and brightening are outlined.