PREFACE 1x 
new avenues in precipitation research where snow crystals and hail particles are be- 
ing used as inexpensive aerological sondes of great resolving power. This may lead 
to a degree of understanding of the accompanying storms which otherwise cannot 
be achieved. 
In other papers different scales were connected: the areal microstructure of the 
rate of precipitation (mesoscale) reveals the structure of hurricanes or other rain 
storms (T. Bergeron), and the arrangement of radar echoes points to the structure 
of orographic convection (B. Ackerman) and to the morphology of squall lines or 
fronts (P. Austin). 
Finally, there are the papers and comments which brought the important connec- 
tion to the synoptic scale, as in the study of cloud distributions over the tropical 
oceans (C. Ronne and J. Malkus), in the synoptic analysis of the fine structure of 
convective storms (T. Fujita), or in the analysis of large scale flow patterns which 
lead to rain or drought (J. Namias), or in the very interesting problems which pre- 
cipitation processes present to numerical forecasting (J. Smagorinsky). 
The reader will have noticed that here the significance of cloud physics is illumi- 
nated not just in the light of two seales—microphysics and macrophysics—but in 
three according to Bergeron’s definition (see page 61): (1) microphysics dealing 
with processes related to cloud elements, (2) mesophysics dealing with processes 
ranging from individual clouds to a cloud system, and (3) macrophysics dealing with 
processes of synoptic scale. Such a definition would be, as Bergeron pointed out dur- 
ing the Conference, in better agreement with current use in meteorology of the terms 
micro, meso and macro. 
Since it is virtually impossible to discuss the amount of valuable information and 
inspiration which has come from this Conference, the post-Conference status of some 
key problems which we had mentioned in our letter of invitation shall shortly be 
appraised. These were: 
(1) Can we prove that true sublimation nuclei do not exist and that AgI acts 
only as freezing nucleus? 
B. J. Mason’s paper gave an almost complete answer: he showed that AgI not 
only acts as a freezing nucleus initiating nucleation at water saturation, but also as 
a sublimation nucleus at temperatures below — 12°C. It is not an ideal sublimation 
nucleus but still requires 10-15 % supersaturation with respect to ice. (This result 
was qualitatively anticipated in Anderson’s paper, Re-evaporation Ice Nuclei, which 
he gave at the First Woods Hole Conference.) Mason’s investigations also explain 
why hygroscopic substances cannot act as freezing nuclei but may act as sublima- 
tion nuclei. It appears now that one of the last unsolved questions is how Nature 
performs in bulk water, the act of crystallization at the freezing point proper. 
In addition to this work, investigations on concentration and efficiency of freezing 
nuclei have been reported by H. W. Georgi, D. B. Kline, and 8. J. Birstein. The 
diligent work of these scientists who are hampered by bulky, unconventional, and 
heavy equipment which demands great observing skill leads us into an ever-deepen- 
ing understanding of the microcosmos around us which occupies a key position in 
the life history of raindrops or snowflakes. Their work furthers not only our under- 
standing of the fundamental significance of nuclei for the precipitation process but 
also for an appraisal of the influence of man-made air pollution on precipitation 
processes (see also the paper of J. P. Lodge). 
(2) How much does the aerosol spectrum influence cloud structure and therefore 
also precipitation processes? 
