ALTITUDE (KM) 
{O15 MST 
\ KM 
(O\T MST 
Fie. 4—Precipitation echoes formed subsequent to two different episodes of cloud electrification at 
09h 58m MST and 10h 183m MST, August 13, 1957 (these echo photographs have been retouched to re- 
store the images as they appear on the original film) 
ALTITUDE (KM) 
Wie Ye MST 
WS MST 
W23 MST 
Fie. 5—RHI radar sequence showing growth of precipitation echoes on August 16, 1957 (these echo 
photographs have been retouched to restore the images as they appear on the original film) 
erown to the size shown. With the formation of 
rain, the vigorous updraft in the cloud ceased, 
and the electrification relaxed back to  fair- 
weather values. The top of the cloud turned to 
ice and disintegrated while heavy rain fell to the 
eround. A new convective cell was formed over- 
head at 10h 11m MST, and negative charge con- 
centrations again appeared within the base of 
the cloud. A new hollow echo (see Fig. 4) ap- 
peared in this cell at 10h 15m MST. With the 
development of the precipitation echo, the up- 
draft in the cell ceased, and the electrification 
disappeared. Starting at 10h 20m MST, tor- 
rential rain and small hail fell to the mountain; 
no further electrical activity occurred until new 
convection appeared. A new cell 4 km to the 
south produced the first lightning at 10h 41m 
MST. 
Regarding Figure 5, at 1lh 16m MST the 
first echo appeared almost overhead, at a slant 
range of about 2 km and just below the freezing 
level. It had the cross section of an inverted 
hollow cup. The threshold median drop size for 
detection may have been less than 100 microns 
with the high radar sensitivity at this short 
range. The radiosonde echo was to one side of 
the first precipitation echo. The first rain ar- 
rived at the summit as 3-mm drops at 11h 20m 
MST. The rainfall became torrential by 11h 24m 
MST. From several considerations, the initial 
rain was probably formed by a coalescence 
mechanism. Close examination of the original 
film showed that until 11h 45m MST there was 
no ‘bright band’ in the precipitation echo at the 
melting level, where it first appeared on the out- 
skirts of the echo. 
The data of Table 2 and the Appendix were 
obtained when organized electrification in clouds 
was observed to precede closely the detection of 
a precipitation echo overhead [Moore and others, 
1958] followed by a burst of rain falling to the 
summit. 
COMPUTATION OF APPARENT RAINDROP 
CoLLECTION EFFICIENCIES IN 
ELECTRIFIED CLoups 
With these data and the usual simplified rain- 
drop growth model we can compute apparent 
collection efficiency, liquid water content prod- 
ucts [suggested by Atlas, 1955] for these clouds 
assuming that each raindrop is independent of 
the others. According to this model, the growth 
of a falling raindrop by accretion is a function 
of its horizontal cross-sectional area, the dis- 
tance it falls relative to the cloud, the cloud’s 
liquid water content, and the raindrop collec- 
tion efficiency. 
dM = pwaterdV = chLAdz = chAvdt (1) 
where 
