TROPOSPHERIC PROPAGATION AND RADIO METEOROLOGY 143 
Tas.E 1. Standard atmosphere with 60 per cent relative humidity. 
NACA standard atmosphere 
Dry air Dry air 
Altitude, Temp, pressure, index, 
meters Cc mb (n — 1)10° 
0 15.0 1013 278 
150 14.0 995 274 
300 13.0 977 270 
500 11.7 955 265 
1000 8.5 894 251 
1500 5.2 845 240 
Moist standard atmosphere 
e(mb) Moist 
for air index, M= 
60% RH (n — 1)10° (n + h/a — 1)10° 
10.2 825 325 
9.6 318 342 
9.0 312 359 
8.3 304 382 
6.7 283 440 
5.3 266 501 
the water vapor pressure. The relation is given by 
POI! pe ores = at (17) 
where s is in grams of water per kilogram of air. 
The variation of n with temperature and relative 
humidity for an air pressure of 1,000 mb is illustrated 
in Figure 18. It is seen that the refractive index de- 
pends on humidity more critically than on tempera 
ture. The dependence on humidity is greater at the 
higher temperatures where a given relative humidity 
represents a larger amount of water vapor. 
In practice it is customary to use the modified 
refractive index given by 
m= (n+2—1) 108 OP +. 
+ 0.157h (h in meters) 
3.8 X 10% 
= 
pee El 
_ evs 
ee 
8 
380 
o ne 
2360 
if ae 
320 
Ee as 
S==S= 
SS 
300 r—~— 
015 10 “5 Ze) 5 Le) 1S 20 25 300-35 
TEMPERATURE IN DEGREES C 
Ficure 18. Relation of x to temperature and relative 
humidity. 
In order to compute M directly from temperature, 
relative humidity, and height data, the nomogram 
(Figure 19) has been constructed. Detailed instruc- 
tions for its use are given. 
The National Advisory Committee on Aeronautics 
[NACA] standard atmosphere commonly used in 
aeronautics assumes a sea level pressure of 1,013 mb 
( = 760 mm Hg) and a sea level temperature of 15 C 
decreasing at a rate of 6.5 C per kilometer in the 
lower atmosphere. The NACA standard atmosphere 
is not concerned with the moisture content. In the 
actual atmosphere the. moisture may vary between 
extremely wide limits, but as a typical value a rela- 
tive humidity of 60 per cent may be assumed as the 
standard condition. This corresponds to a water 
vapor pressure of approximately 10 mb at sea level 
and a rate of decrease of water vapor pressure in the 
lower levels of about 1 mb per 1,000 ft. At higher 
levels the rate of decrease of the water vapor pres- 
sure is less rapid. These conditions are represented in 
Table 1 for the atmosphere up to 1,500 m. 
Both the dry and the moist standard atmosphere 
exhibit a very nearly linear increase of M with 
‘height. According to equation (12), 
M—M,= = - 108° = 0.157 ; h in meters . 
By using this formula in conjunction with Table 1 it 
is easily shown that k = % for the dry standard 
atmosphere, and k = % for the standard atmos- 
phere with a 60 per cent relative humidity. This 
value of & is the one commonly adopted in coverage 
diagrams corrected for standard refraction. 
Because of the great variability of the moisture 
content of the atmosphere with season, geographical 
location, etc., a moist standard atmosphere has a 
limited physical significance. The standard should 
rather be defined in terms of a fixed linear slope of 
the refractive index, and for this purpose the value 
k =% has been chosen. 
The Measurement of Refractive Index 
The lower atmosphere frequently is stratified by 
nonstandard distributions of temperature and hu- 
midity which vary rapidly and irregularly as func- 
tions of the height. The refractive index is then no 
longer linear but has a more complicated dependence 
on height, determined from equation (16). The strati- 
fication which is of particular importance in tropo- 
spheric propagation is found in the lower part of the 
atmosphere, that is, below about 4,000 to 5,000 ft 
and frequently in the lowest few hundred feet above 
ground. 
Since the variation in the atmospheric pressure 
