x PREFACE 
This question was answered through the contributions of W. A. Mordy, M. Nei- 
burger and C. W. Chien, Cl. Rooth, and P. Squires and 8. Twomey. While the au- 
thors of the first three papers showed theoretically and through the application of 
electronic-computer techniques the close relationship between the aerosol spectrum 
and the cloud droplet spectrum, the last-named authors confirmed this through i- 
vestigations of the cloud-droplet spectrum in Cumulus clouds over continents and 
over the ocean. They were furthermore able to show that not so much the presence 
of giant nuclei as this difference in the microstructure of the clouds is the determin- 
ing factor in the formation of warm rain. These papers must be regarded as mile 
stones for our understanding of cloud formation and of the warm-rain mechanism. 
As these papers will no doubt lead to an increased research effort into the nature 
and spectrum of condensation nuclei it was very timely that in the same session Dr. 
Goetz discussed his aerosol spectrograph. It is an ingenuously designed centrifuge 
that yields just that size range of aerosols which is crucial for the condensation 
process. 
(3) Is the evolution of a soft-hail, hail, or warm-rain particle a process which is 
traceable back to a distinct initial particle or to just one process, or is it a process 
whose trail we lose in a great number of eligible particles which have accidentally 
met after having experienced various life histories? 
This question has been answered for the warm-rain particle in the aforementioned 
paper by P. Squires and 8. Twomey. According to these authors the warm-rain drops 
do not go back to a distinct particle (giant nucleus), but to a population of cloud 
drops which is favorable for the evolution of a warm-rain particle through coales- 
cence, or, in other words, for the release of colloidal instability in form of rain. In 
the case of the soft-hail and hail particles R. Singer and R. List showed convincingly 
in beautiful cross sections through hailstones and graupel that both, large droplets 
as well as snow crystals may constitute the initial particle around which coalescence 
proceeds. 
(4) What do we know of Nature’s efficiency of the precipitation processes? 
This problem was illuminated in R. Wexler’s stimulating lecture on this subject 
and in the ensuing discussion. The problem of the efficiency of the natural rain proc- 
ess is of prime importance not only for our understanding of the rain mechanism, 
but more so for our appraisal of artificial precipitation control. It is one of the cen- 
tral problems of cloud physics and of meteorology in general, and it was in order to 
underline its significance that Wexler’s lecture appeared in a central position in the 
program. Bergeron’s Operation Pluvius, Grunow’s, Magono’s and Nakaya’s raindrop 
and snow-crystal analyses, Hallgren and Hosler’s, and List’s laboratory investiga- 
tions will ultimately contribute to its solution as well as the excellent work on the 
physical evaluation of seeding activities in Santa Barbara, California, on which Todd 
reported. Elliott’s discussion of the Wexler paper indicated again the great impor- 
tance of a simultaneous consideration of various scales as the efficiency of the natural 
rain mechanism may come out quite different if the budget of the water vapor is 
considered for a whole cloud system or for a micro-element within the system. 
(5) What do we know of the collection efficiency of snow crystals and flakes which 
seems to play a very important role for the efficiency of both, continuous and con- 
vective, precipitation? 
The collection efficiency of ice spheres for crystals has been discussed in the pa- 
per presented by Hallgren and Hosler. In their observations the importance of the 
temperature interval near the freezing level was emphasized owing to the formation 
