Problems and Methods of Rainfall Investigation 
Address of the Honorary Chairman of the Conference 
Tor BERGERON 
Meteorological Institute, Uppsala, Sweden 
Mr. Chairman, Ladies, and Gentlemen: 
It is a great honor and pleasure for me to have 
the opportunity of taking part in this Confer- 
ence on Precipitation, a field that has always 
been of primary interest to me. I want to thank 
the American Geophysical Union most fervently 
for the confidence they have shown by making 
me Honorary Chairman, thereby enabling me to 
come to this interesting Conference. 
I state the Conference is to be very important, 
since its theme, as we know, is of utmost impor- 
tance to mankind, and because the papers and 
abstracts already available show that a mighty 
attack on the named problem is imminent. The 
names of the authors and the fact that the pro- 
gram allows time for proper discussion of each 
paper seems to ensure, if this plan is followed, 
a very good result of our joint attack. 
In 1875 the great physicist and physiologist 
H. von HeLtMHo.tTz gave a lecture on “Cyclones 
and Thunderstorms.” He felt that the impos- 
sibility of predicting the beginning and end of 
rain touched a sore spot in the minds of physi- 
cists. He continues thus: 
“Under the same sky in which the stars pursue 
their orbits, as symbols of the unchangeable law- 
fulness of Nature, we see the clouds towering, 
the rain pouring, and the winds changing, as 
symbols—as it were—of just the opposite ex- 
treme, the most capricious of all processes in 
Nature, impossible to bring inside the fence of its 
Laws.” 
However, at Helmholtz’s time, nearly a hun- 
dred years ago, one only vaguely realized the 
vital role of these phenomena to mankind. Now, 
we know how the clouds and their precipitation 
determine unambiguously the life conditions of 
the better part of the so-called biosphere, the 
sphere of living beings. We know, for instance, 
that the photosynthesis, which is controlled by 
actinic radiation, and on land by the fresh-water 
supply from precipitating clouds, is by far the 
biggest industry on our planet. It outrivals by 
several orders of magnitude any kind of arti- 
ficial production of energy on our Earth. We 
dread the catastrophe of soil erosion, caused both 
by the excess and lack of rain, as you know very 
well in the United States. We badly need the 
hydroelectric power, and we now fear the sink- 
ing ground-water level; not to speak of other 
nearby effects of rain or drought. 
But today, our colleagues the physicists have, 
on the whole, very successfully repressed that 
sore point from their conscience, leaving us 
meteorologists almost alone with these vast, in- 
tricate and vital problems, which we evidently 
still have not mastered. (I need only to refer to 
the last 24-hour rain that we have all experienced 
here, or perhaps in Boston. It may have been 
forecast to some extent, probably as some show- 
ers; but it was certainly not forecast in this 
amount and of this kind, that is, as a 24-hour 
abundant rain.) Intense or steady precipitation 
will generally be the real vital and fatal type, of 
course. Therefore, it seems reasonable that we 
now devote our attention to mechanisms produc- 
ing such precipitation, even if the weak or inter- 
mittent type also may be of considerable interest. 
We may hope to find the most clean-cut and 
typical mechanisms, the most conspicuous fac- 
tors, behind the intense weather phenomena. 
This will help the scientific treatment and solu- 
tion of these meteorological problems, which 
otherwise often are too complex. These typical 
cases will also, if treated artificially, point to the 
best methods of getting artificial precipitation, 
or to prevent certain kinds of precipitation, as 
for instance hail. 
Abundant precipitation will only be produced 
by what one might call sustained mechanisms; 
that is, mechanisms producing more or less 
orderly lifting of air, bringing new moisture and 
new releasing particles (appropriate nuclei, etc.) 
continually through a certain limited space of 
the lower troposphere. These limited spaces will 
either be fixed geographically, or they will re- 
main stationary with respect to the leading air- 
flow, as is the case with frontal rain and rain 
of convective character. In any case the main 
