PRESENTATION OF A^ DATA 19 



and eo as 



eo = e(Po/p), (1.41) 



where R is the universal gas constant, w the molecular weight of air, and 

 Cjc the specific heat of air at constant pressure. By setting Po = 1000 

 mbar and noting that R/mCp = 0.286, one obtains 



• . = ^(^r + 3.73X.O-|,(l<^r. (1.42) 



Thus, it is seen that <^ is obtained by correcting the dry air term of A'^, 

 77.6p/T, by the factor (lOOO/p)"-^" and the water vapor term by the 

 factor (1000/p)°-^-'^. In well-mixed air, 6 and Co are independent of height 

 and conseciuently is independent of height. If the temperature lapse 

 rate is less than the dry adiabatic rate, the potential temperature increases 

 with height and, conseciuently, the first term of (/> decreases with height. 

 If the lapse of the partial pressure of water vapor is less than than given 

 by (1.41) then eo and the second term of </> increase with height, conversely, 

 if e decreases more rapidly with height than the decrease given by (1.41), 

 eo and the second term of decrease with height. Abundant examples of 

 the vertical distribution of </> under various meteorological conditions are 

 given throughout volume 13 of the Radiation Laboratory Series [32]. 

 The potential refraction modulus has recently been used by Jehn [33] to 

 illustrate the variability of A^ about a classic polar-front wave. 



A conceptually similar approach to that above is to arbitrarily adopt a 

 reduced-to-sea-level value of refract ivity 



A^o = A^. exp (-H/i/7.0) (1.43) 



where h is in kilometers, that effectively removes the station elevation 

 dependence of N s [34] and allows the emphasis of air-mass differences. 



Since the effects of the atmosphere upon the propagation of radio waves 

 are dependent upon either the absolute value of n or its gradient, we may 

 use this as a criterion for the choice of appropriate units to represent the 

 A^ profile. One may recover both n and its gradient from a knowledge of 

 the height structure alone. The corrections are additive in the case of 

 B, M , and A and nmltiplicative for A^'o. The values B, M, and A are in 

 convenient form for refraction calculations since it is the gradient above 

 the surface of the earth that is needed. The value, A'^o, removes the effect 

 of station elevation and t hus is convenient for mapping of surface condi- 

 tions in a fashion similar to sea-level pressure rather than station pressure. 

 One cannot obtain the gradient and absolute value of n from a 0(/O distri- 

 bution, although <l>(h) may be graphically calculated from the familiar 



