292 
10* T 
n 
lor 
i 
"E 
£ TG cm DISH 
5 
¢ | IN 1957 
E 1 
n 
(>) 
w 
E 
2 
$ 
10" l 
3 \ 
152 cm DISH 
IN 1958 
10" 
l) 
\ 
10 fie ee 1 
A | 10 100 
RANGE (KM) 
Fic. 1—Plot of computed Zmin , minimum de- 
tectable cloud-reflectivity factor for 3.2-cm radar, 
versus target range for APQ-13A sets used in 1957 
and 1958 
1000 | 
To cm. DISH 
IN 1957 
do nin (microns) 
al 
152 cm DISH 
IN 1958 
ees a 
| 
{ 10 100 
RANGE (KM) 
Fic. 2—Estimates of minimum detectable 
median-volume drop diameter versus target range 
for APQ-13A radar sets used in 1957 and 1958 on 
Mt. Withington, New Mexico 
In 1958 the radar sensitivity was determined 
by use of 10-cm spherical metallic reflectors sup- 
ported on captive balloons 6 or more km dis- 
tant. From these measurements, estimates were 
obtained for the minimum detectable cloud re- 
flectivity and for the minimum detectable me- 
dian-volume cloud-drop diameter as a function 
MOORE AND VONNEGUT 
of distance. Plots of these estimates are shown 
in Figures 1 and 2. The pertinent data on the 
APQ-13A radars used are given in Table 1. 
The curves of Figure 1 were derived from cali- 
brations of the radar with metallic reflectors 
carried on captive balloons. Assuming that the 
path to the minimum detectable target lies 
through cloud, we applied appropriate correc- 
tions for cloud-droplet attenuation. These curves 
are based on the relationships summarized in 
Mason [1957]. The lower limit on the usable 
range is imposed by the time required for re- 
covery of the TR tube after the transmitter 
pulse. The curves of Figure 2 were estimated 
from the data in Figure 1, by use of relationships 
derived by Atlas [1954]. For this we assumed a 
cloud liquid-water content of 0.5 gm m™*. We 
feel that these estimates of minimum drop size 
detectable are of the right magnitude, for with 
the 152-em-diameter parabolic antenna, we re- 
peatedly obtained properly shaped echoes from 
nonprecipitating Stratocumulus clouds. 
Other instrumentation—We increased the sen- 
sitivity of the detection of the initial electrifica- 
tion by supplementing measurements made on 
the summit with potential-gradient measure- 
ments made within and above the cloud. Radio- 
sondes modified to measure the vertical compo- 
nent of the potential gradient were suspended in 
the cloud from a tethered balloon. Most of the 
radiosonde field measurements were made with 
apparatus employing three radioactive probes. 
(These will be described in detail elsewhere.) To 
supplement the data obtained with this appara- 
tus, several tethered balloon flights were also 
made with a newly designed passive electric field 
meter devised by Roy Hendrick of Cornell Aero- 
nautical Laboratory. The operation of this field 
meter was difficult because of the conditions 
within the cloud. Nevertheless, we obtained sey- 
eral comparative measurements with this instru- 
ment and the potential-gradient measuring ra- 
diosondes employing radioactive probes. Since 
the measurements with the two instruments were 
in substantial agreement, we gained some con- 
fidence in the radioactive probes when used 
alone. 
Measurements directly above the top of the 
cloud were made with a P-88 aircraft equipped 
for measuring the component of the potential 
gradient normal to the plane of the aircraft 
wings. (The participation of James Cook, owner 
and pilot of the airplane, was made _ possible 
through the co-operation of the United States 
