ATMOSPHERIC. STRATIFICATION AND REFRACTION 



199 



parts in a million and may be computed from the 

 following formula: 



.. 79p lie , 3.8 X 10 6 e , 1A , 



in which n = index of refraction at height h above 



ground ; 



p = barometric pressure of the atmosphere 



in millibars at height h. (1 mm Hg 



pressure = 1.334 mb); 



e = partial pressure of the water vapor in 



millibars (order of 1 per cent of p) ; 

 T = absolute temperature (°C + 273) at 

 height h . 



In equation (16) the term lle/T is very small in 

 comparison with 'the other terms and may, without 

 serious error, be neglected. This simplification has 

 been used in obtaining the values in the last two 

 columns of Table 1 and in designing the nomogram, 

 Figure 19. 



Workers in the field may prefer to use mixing ratio 

 (practically equal to specific humidity) in place of 

 the water vapor pressure. The relation is given by 



e = 0.00161 ps or 



621e 



(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 = 



. h 1 



n -\ 1 



a 



79p mm 



-I" rp ~l rp2 



O 5 10 15 20 



TEMPERATURE IN DEGREES 



25 30 35 



0.157/i (h in meters). 



Figure 18. Relation of n 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 



