256 RADIO WAVE PROPAGATION EXPERIMENTS 
which often produces an elevated S-shaped duct and 
frequently is superimposed upon a sea breeze. Figure 
21, plotted from data taken in the vicinity of the 
New Guinea coust,!® indicates the oceurrence of an 
elevated S-shaped curve, below which is found simple 
surface trapping or the usual trade wind surface in- 
version. While the existence of conditions favorable 
to such eftects off the coasts of Japan may be assumed, 
on account of the lofty mountains, there are no 
data available on this point. The only information on 
trapping in the Japanese area has been obtained from 
a study of soundings made in 1928 at Tateno,’® on the 
east coast of Honshu, some fifty miles from Tokyo. 
The data, plotted in Figure 22, shows three cases of 
an M inversion. The numerical quantities are given 
in Table 3. 
TABLE 3. Aerological soundings at Tateno, Honshu, in 
1928, together with computed duct widths and the magnitude 
of the M deficit. 
Elevation Elevation Duct Magnitude 
of duct of duct width of M 
Date top (ft) base (ft) (ft) deficit 
May 1 1,900 1,300 600 —6 
August 23 1,000 650 350 —5 
September 29 1,250 950 300 -2 
Nore on ATTU AND THE ALEUTIANS??? 
Fixed echoes have been obtained at abnormally 
long ranges (100 to 150 miles). The “Battle of the 
Pips” was an illustration of pronounced superrefrac- 
tion. The latter has been observed on at least four 
occasions by operators of airborne radars returning 
to Attu from missions over the Kuriles, when VHF 
radar beacons have returned signals to planes at 5,000 
ft and 300 to 350 miles, or about three times the 
horizon line distance. Meteorological data on duct 
conditions during the cycle of the Aleutian seasons 
are needed (up to 1,000 ft). 
APPENDIX 
StaNDARD ATMOSPHERE 
Definition. The National Advisory Committee on 
Aeronautics [NACA] defines the “standard atmos- 
phere” as that which exhibits: : 
1. A sea level pressure of 1,013 mb (= 760 mm of 
mercury = 29.92 in. of mercury). 
2. A sea level temperature of 15 C (= 59 F) which 
decreases at a rate of 6.5 C per km (= 3.47 F per 1,000 
ft) in the lower atmosphere; and in addition the 
moisture content may be specified as follows: 
3. A relative humidity of 60 per cent, which corre- 
sponds to a water vapor pressure of approximately 10 
mb at sea level and to a rate of decrease in the lower 
atmosphere of about 1 mb per 1,000 ft. 
Properties. The following table for the standard 
atmosphere indicates the variation with height of (a) 
temperature, (b) pressure, (c) water vapor pressure 
for 60 per cent relative humidity, (d) a quantity con- _ 
taining the index of refraction, n, and (e) the modi- 
fied refractive index, MW. 
Water vapor 
Temper- Pres- pressure for 
Altitude ature sure 60% RH 
Meters Feet C F (mb) (mb) (n—1) x 108 M 
0 0 15.0 59.0 1,013 10.2 322 322 
150 492.1140 57.2 995 9.6 316 339 
300 984.3 13.0 55.4 977 9.0 309 357 
500 1,640.4 11.7 58.1 955 8.3 300 379 
1,000 3,280.8 8.5 47.3 894 6.7 281 438 
1,500 4,921.2 5.2 41.4 845 5.3 266 501 
%apIo Mrrrorotocy TErms 
Indea of Refraction. This can be defined for: any 
particular medium as the ratio of the velocity of 
electromagnetic waves in a vacuum to their velocity 
in the medium. The relationship indicating the 
amount of bending or change in direction that occurs 
as electromagnetic radiation crosses a boundary be- 
tween two media with different refractive indices is 
given by Snell’s law: 
7 COS a1 = Ne COS ae 
in which 2, is the refractive index of the first medium, 
nz that of the second medium, @, the angle which the 
ray in leaving the first medium makes with the boun- 
dary, and @, the angle which the ray in penetrating 
the second medium makes with the boundary. In the 
case of the atmosphere (one medium with a variable 
refractive index) this expression can be modified to 
relate the gradual bending of a ray to the manner in 
‘which the refractive index varies. The value of the 
refractive index, n, at any particular point in the 
atmosphere can be determined from measurements of 
pressure, temperature, and humidity by substitution 
in the formula, 
n=1+ = (» + = 10-6 
or, expressed differently, 
79 4800e 
— 65 es etree 
(n— 1) 10° = 5 (» + ) 
in which the temperature, 7’, is measured in °Ix, and 
the atmospheric pressure, p, and vapor pressure, e, 
in millibars. 
Modified Refractive Indea. It is considered more 
convenient, in problems of radio propagation, to de- 
fine a slightly different quantity If, which is related 
to the index of refraction by 
= (n+2- 1) 10° , 
a 
in which 2 is the index of refraction, a is the radius 
of the earth (21 X 10° ft) and fi is the height above 
the surface of the earth (measured in the same units 
as a). In terms of pressure, temperature, humidity, 
and height, M is given by 
m= B(o+ Sm) +4 10°, 
