82 BERNICE ACKERMAN 
orc 
80 + 
60k TOP 
OF 
W 
a ECHO 
P40 
O 
Ww 
ne 40) 
Ww 
= be 
Lo | 
uw | 
S) | 
a 8° T 
lJ 
mk 
= col BASE 
2 OF 
2 ECHO 
4ob 
20 
oO n L 
aise an aes o +8 +16 +24 
TEMPERATURE - DEG C 
Fia. 4—Frequency distributions of the temper- 
atures at the tops and bases of initial echoes; 
total sample: 294 first echoes 
quired (or both) are larger than can reasonably 
be expected. 
Since the clouds being discussed extended 5000 
or 6000 ft below the freezing level, and that 
much or more above the freezing level, both 
coalescence and ice-crystal processes could have 
been, and probably were, important in the for- 
mation of the first echoes. It is suggested here 
that the chief difference between the echoes en- 
tirely from particles produced through the Ber- 
geron process and those resulting at least par- 
tially from a condensation-coalescence process 
was in the time-wise efficiency of the two mecha- 
nisms. Where first echoes had low bases the time 
required by the all-water process to produce 
large drops in the lower portions of the cloud 
must have been less than or equal to that re- 
quired by the ice-crystal mechanism to produce 
large drops in the cold reaches of the cloud. 
The data, in fact, contain evidence of inde- 
pendent and practically simultaneous develop- 
ment of echoes in lower and upper portions of 
the cloud. On three occasions the initial appear- 
ances of echo clouds were in two vertical seg- 
ments, the lower one completely below the freez- 
ing level, the upper one completely above it. 
In Figure 5 are shown the frequency distribu- 
tions of the levels of echo formation on the in- 
dividual days. Although there are day-to-day 
differences in the heights at which the tops of 
the initial echoes occurred they are not nearly 
as pronounced as the differences observed for the 
base heights. The total range of temperatures 
at which the echo bases occurred was about the 
same for all days but the ‘characteristic’ base 
height varied from one of the ‘preferred’ levels 
to the other. On July 24 and August 17 there 
was high incidence of first-echo bases at tem- 
peratures between —4 and —8°C; on August 1 
and August 24, there was marked preference for 
first-echo bases to occur at temperatures around 
14°C. The tendency for a ‘preferred’ level is less 
marked on July 27 and August 13, but on both 
days the distributions were skewed, toward the 
colder levels on the former day, toward the 
warmer ones on the latter. 
Only on July 20 did the bases oecur with high 
frequency at both levels. The echoes on this day, 
in particular, tend to support the thesis that 
precipitation may be initiated by different proc- 
esses in different parts of the cloud, and that, 
the source of variability is in the time require- 
ments for the two processes. The formation of 
the echo in the lower parts of the cloud evidently 
lagged only slightly that in the upper portion. 
Over 60% of the echoes with high bases when 
initially detected had bases at the lower ‘pre- 
ferred’ level (+12 to +16°C) three minutes 
later. This represented between 6000 and 10,000 
ft of descent in three minutes, in many cases 
coincident with ascent of the echo tops. 
To date, attempts to find an explanation of 
the variation in the effectiveness of an all-water 
process from cloud to cloud and from day to day 
have been unsuccessful. It is possible to find one 
that fits the observations for two or three of the 
days, but what is required, of course, is an ex- 
planation fitting the observations on all seven 
days. A consistent relationship could not be found 
between height of echo base and time of day or 
time from beginning of convection. Similarly 
there appeared to be no association between ‘cold’ 
and ‘warm’ base first echoes and topography. Nor 
was there any indication that proximity to older 
rain clouds at the time of formation made a dif- 
ference. Locale and time of occurrence do not ap- 
pear to be important factors. 
The day-to-day differences in the character- 
istic height of first echo bases must reflect varia- 
tion in the interval of time required for the ini- 
