192 



SYNOPTIC RADIO METEOROLOGY 



-200 -100 100 



DISTANCE IN KILOMETERS 



Figure 5.15. Idealized cold front in A units. 



A units were used in subsequent cross-section analyses in order to 

 throw frontal discontinuities and air mass differences into sharp relief. 

 The Potential Refractivity Chart of figure 5.17 facilitates the rapid con- 

 version of A^ to A. This simplification eliminates the necessity of using 

 exponential tables for each individual calculation of A and thus lends con- 

 siderable ease to the preparation of charts of the new parameter. 



The reader probably has already observed that the A^'o and A correc- 

 tions do substantially the same thing. Their primary distinction is that 

 ^ is a nonlinear "add-on" correction while A^o is a multiplicative one. 

 The disparity between A^o and A is tabulated in table 5.1. 



These figures are obtained by taking the zero values of A^ (for example, 

 300), subtracting the add-on correction for, say, 3 km (300 — 109 = 181), 

 and reducing this number, A'' = 181, to zero elevation by the A^o reduction. 

 No = N exp (3/7). The 313 exponential atmosphere is adopted for a 

 single reference atmosphere. The large discrepancies of table 5.1 may be 

 avoided for practical applications by choosing a model near to the mean 

 value of A^ of the site under study. 



The A unit has the additional advantage that, while it is a convenient 

 method for height reduction, the ray bending is also readily recoverable 



