EFFECTS OF VERTICAL WIND ON STORMS 
their boundaries. This is consistent with the fact 
that a few large rainstorms tend to predominate 
(e.g., in squall lines) when the vertical shear is 
strong. 
On the lower end of the scale, small trade 
Cumuli,as shown by Malkus [1949], acquire meas- 
urable but not great velocities relative to ambi- 
ent winds. Induced pressures at their boundaries 
are small and probably have no appreciable effect 
on cloud growth. For this reason, clouds of the 
scale treated by Malkus do not propagate in the 
manner described above. Rather, they appear to 
grow on the upshear side because [Scorer and 
Ludlam, 1953] successive towers rising from the 
same base appear upwind from older towers which 
have been carried off by the wind. Ackerman 
[1956] has shown observationally that shear in- 
hibits production of rain in tropical Cumuli, but 
that rain may occur with proportionately larger 
shear when the buoyancy is increased. 
VERTICAL SHEAR AND Hat 
The above discussion leads to the following con- 
clusions possibly applicable to hail occurrence: 
(a) With strong vertical shear and pronounced 
veering of wind with height, growth of new con- 
vection is most favored on the right flank of a 
rainstorm. Since young cells have most intense 
vertical motions [Byers and Braham, 1948], large 
hail should tend to occur predominantly on that 
flank. The hail track should have restricted width 
compared with the track of the rainstorm as a 
whole. 
(b) The vertical pressure gradient induced by 
cloud-environment interactions favors, in the 
newly growing cells, upward accelerations 
stronger than provided by buoyancy forces asso- 
ciated with temperature anomaly alone. 
(c) An additional factor favoring localized 
strong vertical motions with large hail growth, is 
that new updrafts growing on the downshear 
flank are sheltered by the main body of the rain- 
storm itself (cf. Fig. 1), from the decreased buoy- 
ancy which would result from entrainment of dry 
air in upper levels. 
(d) The above effects should be most evident 
in vigorous rainstorms of large horizontal extent, 
and probably not noticeable in small storms. 
EXAMPLES 
Heavy hail damage occurs in a variety of cir- 
cumstances, often in warm sectors or ahead of 
warm fronts, but more commonly behind cold 
345, 
fronts, in the geographical regions of most fre- 
quent occurrence [Harrison and Beckwith, 1951; 
Douglas and Hitschfeld, 1958]. Since we have 
been unable to find any detailed descriptions of 
large storms behind cold fronts, attention will be 
confined to two warm-sector situations where the 
shear was of the type shown in Figure 1. 
There is little evidence of asymmetry of the 
kind described above, in the Alberta hailstorms 
described in great detail by Douglas and Hitsch- 
feld {1958}. On the whole, those cases involved 
storms of small diameter, mostly in weak shear 
(or with vector shear nearly along the mean wind 
direction). In one example wherein there was sig- 
nificant veering of wind with height, the principal 
hail fall occurred with several cells forming suc- 
cessively toward right of the individual cell paths. 
Figure 3 shows a set of observations collected 
by Harrison {1952], giving the precipitation dis- 
tribution in the neighborhood of an incipient 
tornado (stippled track) at Fort Wayne, Indiana, 
near 19h30m CST on April 28, 1951. As shown 
by Figure 4, the heavy hail was concentrated on 
the south edge of a large rainstorm. This storm 
was located a short distance ahead of the cold 
@ R+, no hail 
4 Light hail 
4 Mod. hail 
a Heavy hail 
A 
Pickle shaped, 5"long 
Baseball size 
fe) 1 2 3 4 5 
al 1 EEE 
MILES 
Fig. 3—Hail and rain distribution, Fort Wayne, 
Indiana, near 19h30m CST April 28, 1951 [Harri- 
son, 1952] 
