The possibility of studying the precipitations recorded in a large number of meteorological stations in Piemonte led us to analyse what has happened in the last century. A comparison between periods of 10-20 years, using an arbitrary index of rainfall, shows an increase in intense events in the last 20 years though we can observe a general reduction, in absolute value, of the quantity of rain. It is shown that, in the last 30 years, there has been a change in the granulometric distribution of Suspended Particulate Matter (SPM) which, in standard conditions, might become Condensation Nuclei (CN). This might have infl uenced condensation phenomenon, causing a greater frequency of intense events.