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

Casazza, M.

Piano, A.

Earth-prints Repository

235
ANNALS OF GEOPHYSICS, VOL. 46, N. 2, April 2003
Intense events of rainfalls in Piemonte
Marco Casazza and Angelo Piano
Dipartimento di Fisica Generale «A. Avogadro», Università di Torino, Italy
Abstract
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.
Mailing address: Dr. Angelo Piano, Dipartimento di 
Fisica Generale «A. Avogadro»,  Università di Torino, Via 
P. Giuria 1, 10125 Torino, Italy; e-mail: piano@to.infn.it
Key  words  precipitation – intense events – inorganic 
particulate matter
1.  Introduction
Recent natural events of great intensity and 
their frequency have aroused the attention of 
atmospheric physicists (Diodato, 1999) who, 
individually, have suggested different hypotheses 
to explain what has happened.
Luckily, precipitations have been studied for a 
long time in Italy for three main reasons: the fi rst 
is linked to Fr. Angelo Secchi of Reggio Emilia, 
who lived in the 19th century and promoted, in 
1886, the creation of a National Meteorological 
Service; the second is due to the easiness of 
measuring rainfalls; the third is the danger of 
this phenomenon, that persuaded many people 
to accept fl oods as unavoidable events.
We have a great quantity of data relative to our 
Region in the last century. Our «Dipartimento di 
Fisica Generale» has two meteorological stations 
in Torino. The aim of this work is to obtain an index 
of pluviometric intensity to characterise different 
periods and, particularly, the last 20 years.
2.  Pluviometry in Piemonte
In our Region, rain distribution shows a 
greater regularity in the plains. The absolute 
maxima are located in the interface between the 
mountains and the plains, with a decrease going 
to inner mountain regions. The mountainous 
areas having poor rainfalls are, in fact, the 
inner western alpine valleys: Val Susa, Varaita 
and Maira (with mean precipitations per year 
not exceeding 900 mm). The same tendency 
is more evident in the northern valleys (in Val 
d’Ossola we go from mean values of 2300 mm 
to 1200 mm). This phenomenon is due to the 
obstruction created by the fi rst relieves close to 
the plain that stops a portion of air masses from 
the southern quadrant.
In the interface with the plains, we can fi nd 
the places with greatest precipitations. The most 
important area goes from Lago Maggiore to Valli 
di Lanzo (with a yearly maximum of 2350 mm 
in Cicogna (VB)). The second one is in the SE 
of the Region, close to the borders Piemonte, 
Liguria, Lombardia and Emilia Romagna 
(Scrivia and Curone basins). The third area 
is close to Maritime Alps, from the southern 
plain of the «Provincia» of Cuneo and from the 
Langhe to the mountains between Valle Gesso 
and Tanaro. The last one is referred to as the 
Prealps between Val Chisone and Val Pellice. In 
this cases, the abundance of rainfalls is due to 
236
Marco Casazza and Angelo Piano
the air masses coming from the plain, which, 
rising, discharge their humidity. In the second 
and third cases, instead, precipitations are linked 
with the presence of the sea; in fact, having winds 
from S-SE (from Genova), humid currents cross 
this area. The lowest precipitation levels are in 
the plain areas close to the Apennines. The best 
condition for having rain in our Region is to 
have air currents from S or SE; the leeward 
area of the Maritime Alps receives the air poor 
in humidity, so it is characterised by a small 
amount of rainfalls. All the other plain areas are 
crossed by more humid air masses. That is why 
the northern plains are the rainiest ones.
3.  Number of days of rain and mean   
     intensity
The number of days of rain in a year goes 
from a minimum, typical of the Mediterranean 
region, of 50 to a maximum, of a continental type, 
of 140. The areas having fewer events (less than 
70 per year) are the Monferrato and a portion of 
Langhe. In the plain regions we can go, usually, 
from 70 to 90 days while, in the relieves, we do 
not exceed 90 days, with a small area of Toce 
basin, where we exceed 110 days per year.
The values of daily precipitations, obtained 
dividing the mean annual value by the number of 
rainy days, go from 8 to 24 mm/day. Usually the 
mean intensity value in our Region goes from 10 
to 15 mm/day. Areas with lower intensities (less 
than 10 mm/day) are located around Alessandria 
and the inner Alpine Valleys. The greatest ones, 
instead, are in the areas characterised by abundant 
yearly rain; this is due more to the intensity than 
to the number of rainy days. That is why it is 
important to fi nd an index that is not infl uenced 
by local phenomena.
4.  Choices
Because of the distribution in our Region 
and because of the validation of data, we have 
privileged, the measurement points considered 
in the Annali Idrologici. Among all the pos-
sible stations available, for different reasons 
(absence of historical data, lack of long periods 
of registration or poor accuracy of historical 
records), we had to reject many of them. We 
did a further reduction of the number of stations 
considered by grouping (after a comparison of 
experimental data) some stations (like Torino and 
Moncalieri) to obtain a uniform distribution in 
the Region. Furthermore, studying long periods, 
the fi nal choice of the stations (shown in fi g. 1) 
seemed expressive for our purposes.
The analysed periods are 1931-1950 and 
1980-2000. The fi rst one was chosen because 
of the quantity of experimental data (many 
stations started their activity in this period) and 
their importance, related to the small immission 
(in the period 1931-1938) of pollutants in the 
atmosphere. Until 1950, in fact, in Torino the 
heating systems were working using coal or 
wood, but they were fewer than now. The second 
fact is that the number of cars or similar means 
of transport was limited (it was not uncommon 
to use animal traction). It should be possible, in 
Fig.  1.  Chart of the stations of «Uffi cio Idrografi co» 
chosen to create our rainfall indexes.
237
Intense events of rainfalls in Piemonte
that way, to see the possible effects of pollutants 
or, anyway, of a different thermal situation of the 
atmosphere.
5.  Rainfall index
In order to fi nd extreme events, we decided 
not to study the mean quantity of rain in a period, 
because it is possible to increase it having a small 
quantity of rain every day (without fl oods). So, we 
decided to analyse the accumulated precipitation, 
in millimetres, for a period of 24 and 48 h. We did 
not consider greater periods of time, because it 
is rare (in the chosen periods, extreme events of 
more than two days of precipitations, as the one 
of 13-16 October 2000, have been, in total, less 
than 10) to have great precipitations on the third 
day. Besides, we considered, as intense events, 
the ones originated by a large quantity of water 
in a short time.
We created an Absolute Intensity Index (AII) 
for 24 or 48 h, given by 
           .
To see only the extreme events, we fi xed a lower 
limit under which the index was considered equal 
to 0. The threshold limit was chosen to be 6% 
(for 24 h) and 7% (for 48 h) of the mean of the 
annual precipitation quantity along the period 
considered for each single station. This choice 
offers at least two advantages: we obtained an 
optimum number of events for each period and, 
furthermore, we found an index not linked with 
local climatological factors. In fact, the threshold 
values, moving from one station to a different 
one, cancel the differences that exist among them. 
We can point out, using the AII, the increase in 
millimetres of water fallen in each single event, 
even if it is diffi cult to see how this could be 
meaningful during the comparison among the 
different stations. That is why we created a Nor-
malised Intensity Index (NII), referred to the 
mean annual value of the period considered, cal-
culated as mean of annual means
     .
In this case, if in the last 20 years we have had 
less rain, the value of NII increases giving sig-
nifi cance to the single events. In this case too, 
the thresholds chosen are equal to those of 
the AII. 
6.  Conclusions
To see what has happened in the 40 years 
considered and not having all the data of the 
fi rst 20 years (we had all the data just for the 
period 1931-1941), we have divided the periods 
considered into groups of 11 years, superposing 
them if necessary. In order not to have any 
problems with the threshold limits in the different 
periods, we decided to use a new one (equal to 
the highest of them), that did not create any 
problem using the NII, because we are referring 
to percentages. 
Figures 2 and 4 show the number of events 
with AII with values different from 0 for a period, 
respectively, of 24 h and of 48 h. Furthermore, 
fi gs. 3 and 5 highlight the number of events with 
NII different from 0 for periods of 24 h and 48 
h respectively. Our analysis shows an increase, 
in the last period considered, in the number of 
events, particularly intense ones. For nearly all 
the stations, in fact, mainly in the period 1990-
2000, we can see an increase in intense events, 
though it rains less than before. It is important 
to remember here that, during the fl ood of 15-16 
October 2000, that provoked so much damage 
and so many deaths, we had in many areas more 
than 500 mm of water in 3 days.
Considering the causes of those types of 
rainfalls, the interest in the GEOFIT group of 
«Università degli Studi di Torino» is focused 
on the study the infl uence of Suspended Par-
ticulate Matter (SPM) in the events of intense 
precipitations (Bacci et al., 1983). We can say, 
using the data of 30 years of experimental meas-
ures, that the granulometric distribution of inor-
ganic solid SPM with diameter of less than 10 mm
has changed. 
AII mm rain
24h1
=
AII mm rain
48h2
=
NII AII
mean annual value for the period
=
238
Marco Casazza and Angelo Piano
Fig.  2.  Number of events of 24 h with AII different 
from 0.
Fig.  3.  Number of events of 24 h with NII different 
from 0.
Fig.  4.  Number of events of 48 h with AII different 
from 0.
Fig.  5.  Number of events of 48 h with NII different 
from 0.
239
Intense events of rainfalls in Piemonte
Table  I.  Cut-off diameters for each single stage of 
non-viable Andersen Cascade Impactor. 
Stage  Cutt-off diameter
           (mm)
0           u11
 1          7-11
 2         4.7-7
 3       3.3-4.7
 4       2.1-3.3
 5       1.1-2.1
 6      0.65-1.1
 7     0.43-0.65
Fig.  6.  Granulometric breakdown of particulate matter in 1978, 1985, and 2000.
Figure 6 shows the results of 3 representative 
measures of the years 1978, 1985, and 2000, 
mean of several measures done in the same 
period of the year with stable weather conditions. 
The instruments used for those measures are 8 
stages  non-viable Andersen Cascade Impactors 
(Andersen, 1958; Vaughan, 1989), whose cut-
off diameters are shown in table I. This type of 
instrument gives us the possibility to study the 
granulometric repartition of SPM between 0.43 
mm and 10mm.
It is possible to see that the breakdown for 
the year 2000 (confi rmed by many measures in 
2001) is more fl attened than before and it is quite 
common in outdoor measurements. 
We think that this might have infl uenced the 
production of raindrops in such a way to obtain 
an increase in intense rain events, considering 
other causes too. Among them, it is important to 
consider the energy involved in the condensation 
process and the humidity of the air in different 
climatological conditions.
In addition, we are again studying the che-
mical and physical characterization of solid 
inorganic particulate matter in urban and rural 
areas to understand the degree of infl uence of 
this in the condensation of water vapour in the 
atmosphere.
REFERENCES
ANDERSEN, A. A. (1958): A new sampler for the collection, 
sizing and enumeration of viable particles, J. Bacteriol.,
76, 471-484.
Annali Idrologici: Ministero dei Lavori Pubblici, Servizio 
Idrografi co (Istituto Poligrafi co dello Stato, Roma).
BACCI, P., A. LONGHETTO, G. MARCAZZAN, A. PIANO, F. 
PRODI, C. SABBIONI and A. VENTURA (1983): Aerosol 
240
Marco Casazza and Angelo Piano
characterization in a Po Valley site, in Proceeding of the 
Gesellschaft für Aerosolforschung Meeting, J. Aerosol 
Sci., 14 (3), 222-225.
DIODATO, N. (1999): Ricostruzione storica di eventi natura-
li estremi a carattere idrometeorologico nel Sannio 
Beneventano dal Medioevo al 1998, Boll. Geofi s., 22,
5-39.
VAUGHAN, N.P. (1989): The Andersen impactor: calibration, 
wall losses and numerical simulation, J. Aerosol Sci.,
20 (1), 67-90.


Intense events of rainfalls in Piemonte

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