222 



SYNOPTIC RADIO METEOROLOGY 



continental air mass throughout the occurrence of the storm. The essen- 

 tial feature here, as in figure 5.52, is the absence of detail of A structure 

 due to the presence of a uniform air mass over this station. Compare 

 this figure with the epoch chart for Oklahoma City (fig. 5.62), where the 

 structure of the idealized model is clearly reflected by the prefrontal A 

 unit high, strong gradient across the frontal zone, and the A unit low 

 behind the front. This rather fortuitous agreement is felt to be due to 

 the strategic location of Oklahoma City with respect to the motion of 

 contrasting air masses about the polar front. That is, this epoch chart 

 represents a point of confluence of virtually unmodified polar continental 

 and tropical maritime air. 



//f //f //f //f I If //f I If ijf ijf ijf ijf ijf ijf ijf ijfijf ///- // 



a5 



LEGEND 

 A ISOPLETH 

 COLD FRONT 



J \ L 



J \ L 



-2000 



-1500 



-500 



-200 



DISTANCE FROM FRONT , km 

 Figure 5.59. Epoch chart, Rapid City, S. Dak., in A units. 



