RADAR STORM OBSERVATION 
order that the true characteristics of the storm may 
also be determined from the analysis of the echo signal 
presentation. 
Radarscope Photography. Photography plays an im- 
portant part in analysis of the storm echo signal be- 
cause it is the only fully developed, practical method 
of obtaining accurate and permanent records of the 
scope displays [40]. Radarscope photography is not 
difficult; standard films, lenses, apertures, and shutter 
settings are quite sufficient to produce excellent results. 
In some respects, photographs of PPI and RHI scopes 
are more satisfactory for analysis than direct ispection 
of the scopes, because storm echo signals on all parts of 
the scope photograph may be studied in relation to each 
other. On the actual scope only a comparatively small 
portion is available for study at any instant, unless the 
antenna is scanning quite rapidly. 
Only the sweep trace registers on the negative; the 
afterglow or persistence of the screen, while visible to 
the eye for many seconds, does not appreciably affect 
the film. For a complete PPI photograph the sweep 
must revolve through 360° during the time the shutter 
is open in order to enable the entire display to be photo- 
graphed. With high-speed films (either panchromatic or 
orthochromatic), aperture openings of the order of {/6.3 
are required to obtain photographs with scope-intensity 
settings comfortable to the unprotected eye. 
The most dramatic application of photography to 
radar storm detection is obtained through cinemato- 
graphic techniques. Using standard or special motion- 
picture cameras, one frame of the film is exposed for 
each successive and complete 360° scan of the antenna. 
This scanning process may take from 5 to 30 seconds 
to complete, depending upon the design of the radar. 
When the film is projected at the normal rate of about 
16 frames per second a time-scale contraction of several 
hundred results. Consequently twenty-four hours of 
radar storm presentations may be viewed in five or ten 
minutes. 
With this technique the movement and development 
of precipitation echo signals are beautifully demon- 
strated. It becomes possible to evaluate accurately such 
factors as cell life and growth, which may otherwise be 
studied only by means of laborious and time-consuming 
plots. This photographic technique has been applied to 
RHI as well as PPI scopes, with equally interesting re- 
sults. By this method it has been observed that large 
numbers of hydrometeors are carried upwards, some- 
times with high velocities. This is especially common in 
the early stages of development of active convective 
cells. 
Another photographie technique which seems to be 
especially applicable to radar storm observation is pho- 
tography of PPI and RHI scopes with Polaroid Land 
Camera film. Using this film, it is possible, without the 
use of complicated equipment, to have a finished print 
about one minute after exposure. Tests have shown that 
these prints are of sufficient contrast to permit accurate 
measurement of the movement and development of the 
precipitation-area echo signals. This technique should 
1279 
prove of great assistance to forecasters employing radar 
for storm observation, since it provides an accurate 
permanent record of the changes taking place. 
Control of Polarization. The energy radiated by the 
radar is normally linearly polarized either in the hori- 
zontal or vertical plane. Only that portion of the re- 
ceived echo energy which is polarized in the same 
plane is accepted by the receiving antenna. Small rain- 
drops have a unique property which distinguishes them 
from all other targets, that of perfect rotational sym- 
metry with the line of sight [49, pp. 84-85]. As a result 
of this symmetry, the intensity and phase of the scat- 
tered radiation are not functions of the plane of polari- 
zation of the incident radiation. 
To make use of this property, the linear polarized 
energy is altered to circular polarization by reorienta- 
tion of dielectric slugs inserted in the antenna wave 
guide, or by “quarter-wave plates” placed in front of 
the parabolas. The circularly polarized energy back- 
scattered by spherical raindrops will be reconverted to 
linear polarization by the plate or slug, but im a direc- 
tion perpendicular to the original plane of polarization. 
Therefore it will not be accepted by the radar, and is 
not presented on the scopes. That portion of the cir- 
cularly polarized radiation which is scattered by asym- 
metrical targets will undergo a change of the sense of 
rotation of the vector representing the field of radia- 
tion. This back-scattered energy has no special preferred 
plane of polarization upon reconversion from circular 
to linear polarization; hence some of the energy will be 
polarized im the proper plane and will be accepted at 
the antenna. 
Experimental tests have verified the usefulness of the 
theory outlined above for the detection of ground tar- 
gets in the presence of rain [1]. As yet, it is not known 
conclusively whether the technique can be used to dif- 
ferentiate rain from snow particles. It should be pointed 
out that while the determination of the absolute power 
of an echo signal is at present quite difficult, measure- 
ment of the relative difference in signal strength of two 
echoes may be accomplished with relative ease and ac- 
curacy by means of the pulse integrator. If the back- 
scattered energy from a region containing both rain 
and snow is passed through a quarter-wave plate or 
dielectric slug and the percentage decrease in echo-sig- 
nal strength of the two regions is unequal, a positive 
method of identifying the type of hydrometeors by 
radar is possible. 
THE FUTURE OF RADAR STORM DETECTION 
A discussion of the future of radar storm detection 
may logically be divided into three topics: possible 
future uses for forecasting, possible future equipment 
development, and possible use for research in physical 
meteorology. The three are somewhat interdependent, 
of course. 
Use in Forecasting. The value of radar for weather 
observation can be greatly enhanced by increasing the 
range through suitable location of the systems. This 
can be done in two ways: (1) by placing radar sets on 
