PREFACE xl 
of a water film on the crystal surface which facilitates sticking of two crystals to one 
another after they have collided. We may conclude, then, that aggregation of snow 
crystals is also enhanced inside a water cloud at low temperatures, which is in agree- 
ment with the observation that the structure of the largest snow flakes is always a 
mixture of crystals and frozen or unforzen cloud or drizzle droplets. In a water cloud, 
of course, the attraction between ice and water particles caused by the vapor-pres- 
sure difference according to Vierhout’s theory improves aggregation between the 
snow crystal and the cloud droplets. After the capture of a cloud droplet, Nakaya 
explained (p. 270) that this may either freeze to the ice surface or evaporate on to 
it in a strange process that resembles the floating of little water drops over a clean 
water surface which was first described by O. Reynolds (Papers on Mechanical and 
Physical Subjects, 1869-1882). 
The growth by accretion in the ice phase was also the subject of a theoretical paper 
given by Douglas. He investigated the growth of spherical particles by sublimation 
and accretion. It would be most interesting if similar investigations could be carried 
out for simple geometric forms as they actually occur, for instance, stars, prisms, 
several ‘stars’ interlocked with each other, etc. As in falling, air will pass through the 
stars, they may collect many more cloud droplets than a spherical particle and there- 
fore attain a considerable greater growth rate than was found by these authors. 
(6) What do we know of hail formation; is it due to a very intensive or very per- 
sistent updraft? 
Very little definite evidence is available on this subject. When one compares the 
characteristics of hail clouds with simple thunderstorms using radar data as was 
done by Donaldson, Chmela, and Shackford, evidence points to the significance of 
very powerful updrafts. This is also expressed in Beckwith’s report on severe hail 
damage to aireraft in altitudes at or near 40,000 ft. The extent and pattern of hail 
damage, derived from crop insurance records, has been successfully used by Stout, 
Blackmer, and Wilk for a study of path and duration of hail storms, but the data 
are not detailed enough to yield the life history of a single cell. 
(7) What is the significance of the zones of high winds (jet streams) which appear 
to be a typical feature of hailstorms? 
This subject was discussed in a paper by Dessens which was followed by a stimu- 
lating discussion. No doubt the problem is still with us to search for a physical ex- 
planation if the relationship between hailstroms and jet streams can be verified. 
Data given by Beckwith indicate that only 18% of hailstroms occurred simultane- 
ously with the jet stream. No criterion exists so far how to judge the significance 
of this figure. In this search, investigations as reported by Hitschfeld will be of im- 
mense value as they will help us to analyze the true trajectories of particles which 
are potential hailstones. Of particular interest was here the discovery that the storm 
column remains upright in even a strong wind shear in contrary to the typical bend- 
ing over of trade wind Cumulus clouds. The influence of vertical wind shear on 
thunderstorm systems has been discussed in an interesting theoretical paper by 
Newton who found that small clouds may be destroyed whereas large convective 
systems may be maintained by vertical shear. Also, Anderson discusses theoretically 
the formation of self-sustaining storms, while Cunningham showed beautiful pic- 
tures of a hailstorm which had been taken from high-flying aireraft. 
This completes the discussion of the specific problems which we had raised in the 
letter of invitation, but we call the readers’ attention also to the stimulating papers 
given by Essenwanger, Volz, Vonnegut, and Moore. These papers give valuable 
