RADAR STORM OBSERVATION 
pected that descriptions of radar detection of such phe- 
nomena as tornadoes, waterspouts, dust storms, and 
other atmospheric anomalies will be available. Forest 
fires sometimes appear as areas similar to weak rain- 
echo signals on PPI scopes. The lack of vertical develop- 
ment, coupled with other meteorological information, 
is usually sufficient to provide correct identification. It 
is unknown at present whether the temperature grad- 
ients or the cinders and ash carried aloft by convective 
currents cause the echoes. 
The small droplet size in fogs and drizzle make these 
hydrometeors poor targets for radar detection at 3- and 
10-cem wave lengths. However, detection by radars op- 
erating on wave lengths near 1 cm (K-band) is common. 
Because of the scarcity of K-band radar, which has not 
been developed much beyond the laboratory stage, 
little information is available concerning radar fog and 
drizzle detection. This equipment may eventually prove 
valuable for reporting the rate of approach or develop- 
ment of coastal fogs. 
The use of radar for determining meteorological con- 
ditions in the troposphere has been suggested by 
Friend [27]. An interesting application of K-band radar 
to meteorology has recently been developed by the 
Weather Radar Section of the United States Army 
Signal Corps Engineering Laboratories [7, 28]. This 
highly specialized radar system was designed with a 
vertically directed beam for the purpose of cloud base 
and top detection. The radar is equipped with an A 
scope, but in order to provide a permanent record the 
video signal is fed into a special slow-rate facsimile re- 
corder in such a manner that a vertical time cross 
section of the clouds is plotted.’ 
Radar provides an ideal means of studying the results 
produced by seeding of clouds with COs, silver iodide, 
etc. If the seeding is done by aircraft, the position of 
the plane is known, and if particles of precipitable size 
form, such vital information as time of formation, 
height, and position are readily available from the 
radar scope indications [56]. 
Another type of echo which has been ascribed to 
atmospheric conditions [23] has been given the pictur- 
esque designation of “angel.” “Angels” are most com- 
monly seen with the cloud-detection radar described 
above and are strong echo signals with no visible cause. 
They occur most frequently in the lower levels of the 
atmosphere, preferring heights near inversions. They 
have been ascribed to insects too small to be seen, 
strong temperature and moisture gradients, and regions 
of high ionization. Their cause is not yet known. 
ANALYSIS OF THE RADAR STORM-ECHO 
SIGNAL 
Besides visual inspection of the scopes, there are 
other very powerful methods of studying the echo sig- 
nals resulting from storms. Most of this work is still in 
3. For records showing typical results, see Figs. 8 and 9 in 
the article, ‘‘Instruments and Techniques for Meteorological 
Measurements” by M. Ference, Jr., pp. 1207-1222 in this Com- 
pendium. 
1277 
the preliminary stages, but enough has been accom- 
plished to warrant description of present techniques. 
The Pulse Integrator. Echo-signal intensity may be 
measured at the radar in at least three different ways: 
(1) by the intensity of the spot on a PPI or RHI 
scope; (2) by the amount of vertical deflection on an 
A or R scope [82]; and (3) by the echo-signal voltage 
returned to the radar before it is presented on the 
scopes. The first method must be rejected at present 
because of fundamental difficulties as well as technical 
problems. The second method is commonly used, but 
some doubt exists as to the accuracy of this method 
and the possibility of observer bias. The third method 
employs a device designated as the ‘‘pulse integrator.” 
The pulse integrator [70] measures electronically the 
average strength of the pulsating, fluctuating storm 
echo signals over a short interval of time. This interval, 
which determines the number of pulses averaged, may 
be varied to suit conditions, but from two to four sec- 
onds has been found satisfactory. The output of the 
pulse integrator may be fed into any of several types 
of visual meters or recorders so that a permanent rec- 
ord is made. The precipitation-echo pulse integrator 
records are calibrated by feeding signals of known 
strength into the antenna and comparing the readings. 
This method of echo-signal intensity measurement is 
entirely free of observer bias and variability, but results 
obtained so far have not been satisfactorily correlated 
with rainfall intensity measurements in an absolute 
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Fie. 16.—Pulse integrator records plotted against precipi- 
tation intensity. Echo-signal power records from X-band and 
S-band radars were converted to units of reflectivity and 
matched with the precipitation rate as measured with a modi- 
fied Fergusson rain gage. Note the time-scale shift of about 
5 min, used to improve the match between the records. This 
is explained by the time which elapses before a particularly 
heavy ‘“‘burst”’ of rain, falling from a point well up in the 
beam, reaches the surface. (M.I.T. Weather Radar Research.) 
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