1284 
acterizes the scattering function in the microwave re- 
gion. Figure 1 shows the ratio of true scattering to 
Rayleigh scattering for 1.25 cm and 3 cm as computed 
2.5 
2.0 
IN 
TRUE/RAYLEIGH RATIO 
0.5 
(0) 
(0) ! 2 3 4 5 6 
DIAMETER OF RAINDROPS (MM) 
Fic. 1.—Ratio of true scattering to Rayleigh scattering. 
by Haddock (unpublished) from calculations made by 
the National Bureau of Standards. For a wave length 
of 10 cm, Rayleigh scattering holds almost exactly. 
Tapue I. Rapar Cross SEcTION (¢) FoR VARIOUS 
Rain INTENSITIES 
(in 10-* em? per cubic centimeter) 
Rainfall A = 1.25 cm X=3cm 
intensity 
(mm hr™) Ryde Haddock Ryde Haddock 
0.25 0.534 0.570 0.0115 0.0143 
1.25 6.58 6.50 0.116 0.156 
2.5 18.2 19.5 0.337 0.459 
12.5 162 176 4.57 5.59 
25 390 415 14.5 16.8 
50 901 968 46.2 49.1 
100 2000 1920 148 139 
150 3130 2890 289 238 
Table I gives the radar cross section for different 
rain intensities based on the mean drop-size distribution 
of Laws and Parsons [6], as computed separately by 
Ryde and Haddock. In view of the complexity of the 
computations, the disagreement is not surprising. 
Refraction 
In passing through the atmosphere the radio waves 
are subject to refraction and thereby travel a curved 
path. The curvature of the ray is stronger the greater 
the gradient of the refractive index perpendicular to 
its path (the ray tends to curve towards higher re- 
fractive index). In a standard atmosphere of 60 per cent 
relative humidity the curvature of the ray in the lowest 
few kilometers of the atmosphere is about one-quarter 
the curvature of the earth. In tropical regions the curva- 
ture is greater; in Florida during summer it averages 
about 40 per cent of the earth’s curvature. 
In regions where the vapor pressure decreases rapidly 
RADIOMETEOROLOGY 
with elevation through a temperature inversion, such 
as over the trade wind regions of the subtropical high, 
the curvature of the ray is greater than that of the 
earth. In such cases the rays, initially nearly horizontal 
with the earth’s surface, may be said to be “trapped” 
within a narrow layer along the earth and the radar 
detection of objects near the surface may extend to ab- 
normally long ranges. Generally, however, there is no 
precipitation on such occasions and, as a result, this 
“anomalous propagation” is of little importance for 
storm detection. Occasionally during such conditions, 
echoes from objects on the surface are detected by the 
radar and may be mistaken for a rain shower. By care- 
ful examination of the A scope,” the experienced observer 
can easily recognize the characteristic fluctuating signal 
of a rainstorm. 
(KM) 
HEIGHT 
400 
200 300 
RANGE (KM) 
Fic. 2.—Range-height chart for refraction in a standard 
(S) and tropical (7) atmosphere for elevation angles of 0°, 
0.5°, and 1° ; 
fo} 100 
In Fig. 2 a range-height diagram is given for a stand- 
ard and a tropical atmosphere for rays initially at angles 
of elevation of 0°, 0.5°, and 1° above the horizontal. As 
an example of the use of the diagram, it may be seen 
that a radar with a 1° beam can detect precipitation 
only between elevations of about 2 to 6 km at a distance 
of 200 km. If the top of the storm were at 4 km only 
one-half of the beam would be intercepted at that range. 
It is evident that a serious deterrent to the detection 
of precipitation over long ranges is the fact that beyond 
a distance of about 200 km (varying from summer to 
winter) the upper portion of the beam is above most 
storms, and even at shorter ranges much of the beam 
2. On the A scope the abscissa is distance and the ordinate 
is the relative signal strength at the receiver. 
