323 PROPAGATION THROUGH THE STANDARD ATMOSPHERE 
as * penne gn ante -— — 
conse 
Fiaure 1. 
Transmission along and reflection from the ionosphere occurs primarily -with frequencies below 30 mc. 
At higher frequencies useful transmission is primarily concerned with the nearly horizontal rays in the troposphere; 
higher angle radiation passes through the ionosphere and is lost. 
Standard Atmosphere 
Propagation of radio waves in the troposphere is 
materially influenced by the distributions of tem- 
perature, pressure, and water vapor. The condition 
most nearly approximated in the Temperate Zone has 
been accepted as the so-called standard atmosphere, 
and propagation under this condition has been stud- 
ied and calculations made thereon. 
In the standard atmosphere specified by the 
National Advisory Committee on Aeronautics 
[NACA] the temperature is assumed to decrease with 
altitude at the rate of 6.5 C per kilometer, starting 
from 15 C at sea level, with a sea level dry air 
pressure of 1013.2 millibars, which is equivalent to 
760 mm Hg pressure (see Table 1). 
To simulate the actual atmosphere of the temper- 
ate zones it is necessary further to specify a water 
vapor pressure. The value chosen is 10 millibars at 
TABLE 1. 
NACA standard dry atmosphere 
h p—e 
Altitude t Dry air Index of 
aaa aaa |e ECLA pressure refraction 
Feet Meters Cc millibars (n — 1)108 
0 0 15.0 1,013.2 278 
500 152 14.0 995 274 
1,000 305 13.0 977.1 270 
1,500 457 12.0 960 266 
2,000 610 11.0 942.1 262 
3,000 -915 9.1 908.1 254 
4,000 1,220 7.1 875.1 247 
5,000 1,525 5.1 843 240 
sea level, decreasing with altitude at the rate of 
1 millibar per 1,000 ft up to 10,000 ft. With this 
addition the conditions for a moist standard atmos- 
phere are specified in Table 1. This value of water 
vapor pressure yields a value of relative humidity 
approximating 60 per cent. 
Listed also in Table 1 is the index of refraction n. 
The gradient of this quantity, dn/dh, controls the 
curvature of the rays for a wave moving in the ap- 
proximately horizontal direction; n is given by the 
formula 
79 4800e 
= (ees 
(n — 1)10° = (p + T ), (1). 
where T is the absolute temperature, p and e are 
the total pressure and moisture vapor pressure, re- 
spectively, in millibars, at height h above sea level. 
In the moist standard atmosphere, n decreases lin- 
early with height h ct the approximate rate of 
Properties of the atmosphere. 
Moist standard atmosphere 
e Saturated 
Water vapor water vapor Per cent Index of 
pressure pressure relative refraction 
millibars millibars humidity (n — 1)108 
10 17.1 58.5 318 
9.5 16 59.4 312.4 
9 15 60.0 309 
8.5 14 60.7 304 
8 13.1 61 295.6 
7 11.6 60.3 284 
6 10.1 59.4 273 
5 8.8 57 262 
