1266 
shown to be quite accurate. About three months after 
he had completed the essential parts of his calculations, 
the first radar-detected storm (a thunderstorm from 
which hail was observed to fall) was reported on Feb- 
ruary 20, 1941, in England. The operating wave length 
of the radar used was approximately 10 cm and the 
storm was followed out to sea to a distance of seven 
miles. At about the same time a like phenomenon was 
also observed at the Radiation Laboratories of the 
Massachusetts Institute of Technology in the United 
States. 
Because of the secrecy enveloping the entire radar 
program, it was not until many months later that the 
potentialities of this discovery were realized by meteor- 
ologists; but even then, again because of security 
regulations, few were given detailed information. Conse- 
quently, exploitation of this new observational tech- 
nique was greatly hindered. Also radars were generally 
inaccessible, those useful for storm-detection purposes 
were limited in number, and most of these were assigned 
to very high priority work for aircraft detection. 
After World War II, several agencies in different 
countries initiated research programs in the field of 
radar storm detection. A large part of the information 
contained in this article is the result of this research. 
The rapid progress is due to the wealth of surplus radar 
and electronics equipment, aircraft, and the intense 
interest in the subject by persons skilled in the fields 
of meteorology, mathematics, physics, aviation, and 
electronics. 
THEORY AND FACTORS OF RADAR STORM 
DETECTION 
‘The theory of radar storm detection defines the 
_ process by which precipitation is detected by radar. A 
large number of factors must be taken into considera- 
tion, and determination of the value of some of them is 
an extremely difficult problem due to their nature or 
variability. A direct approach to the problem often em- 
ployed is that of definition of the power received from a 
given or defined volume of the storm. It may be shown 
[8; 65, pp. 23-29] that the power received at the 
antenna due to the echo from precipitation is 
Lo (CREE pes DB 
i = atop | x8 ) on (A), (1) 
power received (w), 
power transmitted (w), 
aperture of the parabola (m2), 
¢ = beam width, vertical (degrees), 
@ = beam width, horizontal (degrees), 
h = pulse length (m), 
oN 
n 
aU 
lu 
I il 
= wave length (em), 
= reflectivity of the storm region per unit 
volume (cm7?), 
k = attenuation factor, 
R = distance of the storm (km). 
The term (P,A2¢6h/\*) is dependent upon the type 
of radar; the term (7\*) concerns the nature of the 
precipitation; and the term (k/R?) takes into account 
RADIOMETEOROLOGY 
the effect of intervening space between the radar and 
the region of the storm for which the power of the 
echo is to be determined. 
Equation (1) may be stated in slightly different form 
to facilitate computation: 
—16 P.@r *pohkeny 
i? : 
where G = gain of the antenna and parabola (pure 
number), and y = fraction of the beam filled by the 
storm. 
Equation (2) states in convenient form all factors 
entering the problem, and these factors may be inserted 
in the equation in commonly used units. The new term 
G may be determined from A and i. The term yj is 
employed to remove the restriction that the beam must 
be entirely filled by the storm, thus making the equation 
general. : 
A clear understanding of the meaning and significance 
of these various terms is mandatory if correct interpre- 
tations are to be made of radar storm presentations. It 
is well beyond the scope of this article to give this 
subject the treatment it deserves, but brief discussions 
concerning each of the terms in equation (1) will be 
given, with references to enable more thorough study. 
Factors Pertaining to the Radar System. Transmitted 
and Recewed Power, P; and P,. It should be readily 
apparent that the amount of power transmitted will 
affect the power of the echo signal; the stronger the 
former, the greater the latter. Maximum power output 
of a radar system is normally limited by the character- 
istics of its magnetron (a special type of vacuum tube 
used to convert direct-current power into radio energy 
of very high frequency). The great peak power of 
pulsed radar systems, which may be in terms of mega- 
watts, is explained by the fact that the magnetron is 
operating only a few tenths of one per cent of the time 
[49, pp. 344-348]. This is determined by the ratio be- 
tween the pulse repetition frequency and the duration 
of the radiated pulse of energy. 
The lower limit of power which the receiver can de- 
tect is limited by the receiver’s sensitivity. In micro- 
wave regions this sensitivity limit is not set by atmos- 
pheric static, but rather by the electron or thermal 
‘noise’? which originates in the various receiver com- 
ponents [49, pp. 28-41]. If the power for the signal 
which may just be distinguished from “noise” is used 
in equation (2), the equation may be solved for range, 
which will then become the maximum range at which 
a given storm may be detected. 
Antenna Gain, G, and Aperture of the Parabola, A. The 
antenna aperture defines the projected area of the 
parabola (used to focus the radio energy) normal to the 
axis of the beam (see Fig. 1). Hither A or the antenna 
gain G, whichever is more convenient, may be used in 
these formulas, since they are related by the semi- 
empirical formula 
P, = 6.1 X 10 (2) 
_ 4rAf 
Gene (3) 
