1276 
because winds aloft are usually fairly uniform. Deter- 
mination of the direction and speed of the cells is gener- 
ally quite simple, especially if the cells are fairly well 
defined so that their positions may be determined at 
ten or fifteen minute intervals. 
It has been observed in the northeastern United 
States that warm-sector precipitation, besides being 
broken into small convective cells, usually consists of 
rather large, isolated areas. That is, a precipitation 
area of roughly elliptical shape, perhaps 100 miles in 
extent, will be broken up into small-size cells of 1-5 
miles diameter. The large areas follow each other 
through the region covered by the radar sweep, with 
almost no isolated showers occurring between them or 
on the fringes of the main groups. 
2. Thunderstorms. The appearance of a thunderstorm 
echo signal on PPI and RHI scopes is similar to that 
shown in Fig. 6. These storms, because of their great 
height, are detectable at greater distances than any 
other type of precipitation. Well-developed storms have 
been detected at distances over 300 miles, although for 
a radar less than 100 ft above the surrounding coun- 
try, the usual maximum range is nearer 200 miles. 
Radar played an important role in the extensive re- 
search program on thunderstorms in Florida and Ohio 
[60-62]. 
Using radar it is possible to determine when a given 
convective cell reaches the thunderstorm stage. Light- 
ning gives a characteristic echo signal on A and R 
scopes as shown in Fig. 14 [89]. The echo signal from 
Fie. 14.—Photograph of R scope showing lightning echo 
signal. Low intensity echo signal from precipitation to. right 
and left of lightning echo signal which persisted for about 
¥4 sec. (1510 EST, 7/20/49, S-band radar, azimuth 320°, range 
50-58 miles, 2-mile markers.) (W@.I.T. Weather Radar Research.) 
lightning is transient, usually lasting less than 14 sec- 
ond, but is easy to detect visually on the scopes. Re- 
flection or scattermg evidently takes place from the 
dielectric gradient established by the stroke, but the 
mechanism is somewhat uncertain. It should be 
made clear that it is an actual radar echo signal which 
is detected, not a radio-static signal emitted by the 
stroke itself. This is proven by the fact that the light- 
ning indication always occurs on the scope at the range 
and azimuth of the storm echo signal. 
RADIOMETEOROLOGY 
The best presentation of thunderstorm echo signals 
is found on the RHI scope, as shown in Fig. 15. Again, 
the reader is cautioned to keep in mind the 10:1 vertical 
Fre. 15.—Cross section along an azimuth through a thun- 
derstorm. Height of top about 20,000 ft. Note multiple towers 
at the top, and weak echo signal about 15,000 ft. (1645 EST, 
7/31/47, X-band radar, azimuth 350°, range 60 miles, 10-mile 
markers.) (M.I.T. Weather Radar Research.) 
expansion of this particular scope. Since this is a fairly 
representative example of the well-developed mid-lati- 
tude thunderstorm, 1t may be concluded that the pre- 
cipitation core of such thunderstorms is approximately 
cylindrical in shape, and is about as wide as it is high. 
- When analyzed by means of accelerated time-scale 
moving pictures, the RHI-scope echo signals of thunder- 
stcrms show discrete volumes of hydrometeors which 
may move upwards as well as downwards. These prob- 
ably correspond to the heavy bursts of rain or hail 
which are occasionally experienced at the surface dur- 
ing intense thunderstorms. Precipitation strong enough 
to give intense echo signals at appreciable distances 
from the radar has occasionally been detected at heights 
exceeding 50,000 ft im mid-latitude thunderstorms. 
Somewhat rarely the precipitation echo signal will form 
in the typical anvil shape which the thundercloud it- 
self assumes. - 
Aircraft have encountered hail im the regions giving 
intense echo signals, but as yet no reliable means exists 
for distinguishing the hail from rain or snow by means 
of radar. A recently developed technique of signal fluc- 
tuation analysis, which is discussed in detail under the 
section on analysis of the storm echo signal, has promise 
of being a powerful method of detecting the more tur- 
bulent regions in thunderstorms. ; 
General Remarks. Because of the comparative re- 
cency of applications of radar as a meteorological ob- 
servation instrument, questions concerning detection 
of some meteorological phenomena must remain un- 
answered for the present. Eventually it may be ex- 
