Morphology of Thunderstorms and Hailstorms as Affected 
by Vertical Wind Shear 
Cuester W. Newton 
Department of Meteorology, Chicago, Illinois 
Abstract—Within convective clouds imbedded in a current with pronounced vertical 
shear, horizontal velocities are considerably different (1 to 10 m/sec or more) from 
environment winds, because of intense vertical transfer of momentum inside the cloud. 
Pressure fields induced by relative motions tend to promote new cloud growth on the 
downshear flank of a large convective system containing both updrafts and down- 
drafts. Physical analysis of the forces, with the aid of experimental analogy, indicates 
that the induced pressure field is quantitatively capable of triggering convection and 
that it significantly augments vertical accelerations in the newly formed cells. Possible 
influences on growth and distribution of large hail relative to the main body of a 
storm, are discussed with examples. 
INTRODUCTION 
Although theories for hail formation differ in 
important details, it is generally accepted that 
the vigor of updrafts (or successions of ‘bubbles’) 
is a major factor in hailstone growth, since this 
determines the possible effective length of the 
path swept out by a hailstone [Ludlam, 195s}. 
It is thus natural that the degree of instability 
serves as an excellent predictor for maximum hail 
size. 
Air-mass structure, however, does not by itself 
determine the character of the convection. In air 
masses having much the same thermodynamic 
structure, small and chaotically-distributed, or 
very large organized thunderstorm systems may 
oceur. Particularly in the latter, the distribution 
of severe convective phenomena tends to be 
strongly asymmetrical. 
In the outline for the Woods Hole Conference, 
H. Weickmann posed the questions: ‘““‘What do 
we know of hail formation; is it due to a very 
intensive or persistent updraft?” and “What is 
the significance of the zones of high winds (jet 
streams) which appear to be a typical feature of 
hailstorms?” This paper is designed to suggest 
how these two questions are related, for certain 
types of convective storms. 
This analysis will concern only the effects of 
vertical shear on the structure of a cloud system, 
without regard to antecedent conditions for its 
formation. The latter, as is well known, is strongly 
influenced by the temperature and moisture ad- 
vection and by large-scale weak but. persistent 
vertical motions [Ffulks, 1951], which are most 
pronounced in the jet-stream region. 
339 
INTERACTIONS BETWEEN IN-CLOUD 
AND AMBIENT WINDS 
As background for comments on the hail ques- 
tion, it is necessary to review some results of an 
earlier investigation [Newton and Newton, 1959] 
which treats, in greater detail, the interactions 
between convective cloud systems and the wind 
fields in which they are imbedded. 
Rainfall and radar observations show that or- 
unized convective systems, even relatively solid 
te 
g 
squall lines, are composed of distinct agglomera- 
tions of thunderstorm cells, having average 
widths in the range 15 to 50 km. Such an aggre- 
gate, henceforth called a ‘rainstorm,’ is the entity 
to be discussed here. 
In a typical organized convective situation 
where storms are found in the warm sector or 
ahead of a middle-latitude cyclone, the wind veers 
with height. Figure 1 shows a rainstorm in such a 
wind field, V, and V, being the environment 
winds in upper and lower levels. Large rainstorms 
are made up of alternate up- and downdrafts 
which exchange horizontal momentum between 
upper and lower levels. Complete mixing would 
result in mean in-cloud velocities as shown by 
the dashed arrows in the figure. 
As indicated by the double-shafted arrows, 
relative motions would exist between ambient 
winds and in-cloud air. Evidently the right flank 
(with respect to motion of existing cloud) is most 
favored for new cloud growth, due to relative in- 
flow of moist air on that flank in lower levels. 
Several authors [Desai and Mal, 1938; Hum- 
phreys, 1940, pp. 353 and 3859; Newton, 1950; 
U.S. Weather Bureau, 1949] have suggested that 
