PRECIPITATION AS REVEALED BY RADAR 
8-24-56 
“ EcHo Groups 
81 
ZN 5 
\4 A 
\ 
oe ; 
(OX 14:3-16 ys a) 
3a) aN 
Fig. 3—Precipitation areas on August 24, 1956; presentation as described 
in Fig. 2 
of the first echoes. The conversion from height 
to temperature was made using the regular Tuc- 
son radiosonde. Since the temperature-height 
relation differed little on the seven days, the 
histograms for the height (msl) are very similar 
to those shown in Figure 4. 
The height at which echo clouds first formed 
was highly variable, with ranges of over 40°C 
of temperature (roughly 20,000 ft) m both bases 
and tops. The latter were smoothly distributed 
with two-thirds of the cases falling in the inter- 
val between —4 and —16°C. The modal value of 
—10°C is similar to that reported for the tops 
of initial echoes in clouds in New Mexico [Work- 
man and Reynolds, 1949]. Almost all of the echo 
clouds had tops above the freezing level when 
they were first detected, but, as can be seen from 
the lower section of Figure 4, a large fraction of 
them (nearly 60%) extended below the freezing 
level. 
The height distribution of the bases of first 
echoes (Fig. 4) is quite different from that of the 
tops. The bases occurred with high frequency at 
two levels, one above the freezing level at tem- 
peratures between —4 and —8°C, the other well 
below the freezing level with temperatures be- 
tween +12 and +16°C. Cases for which bases 
were colder than —4°C were no doubt echoes 
from particles produced by the Bergeron ice 
mechanism, but at least portions of the echoes 
haying bases at temperatures above +10°C 
must have been composed of particles grown 
without the involvement of the ice phase. 
The high incidence of first echoes with bases 
at these very warm temperatures indicates that 
an all-water process must be effective in ini- 
tiating precipitation in some of these Arizona 
clouds, if not in the entire depth indicated by 
the first echo, then at least in the lower portion. 
This conclusion is reached by reasoning similar 
to that used by Battan [1953] in describing the 
effectiveness of the all-water mechanism in Ohio; 
namely, that the large drops at the base of the 
echo cannot be explained by downward move- 
ment of large drops from the freezing level be- 
cause the drop sizes or downward flow of air re- 
