204 



SYNOPTIC RADIO METEOROLOGY 



Figure 5.30. U-hour ANo chart, 0130E 21 Feb. 1952. 



The change is determined on a 24-hour basis in order to remove effects of 

 the diurnal cycle of A^'o. The AA^o charts show a general rise of Nq in the 

 warm sector and a drop in A^^o behind the front amounting, in the warm 

 sector, to 35 to 40 A^ units by 1330 EST on February 20 (fig. 5.29) accom- 

 panied by a 40 to 50 A^ drop behind the front. 



The relative sensitivity of A^o to humidity changes is emphasized by 

 the AA^'o charts. The N drop behind the cold front occurs in a region of 

 increasing pressure and decreasing temperature — a combination that 

 increases the dry term and depresses the wet term. The decrease in the 

 wet term from rapidly dropping dewpoint more than compensates for the 

 increased dry term. As an example, in the 24-hour period ending 0130 

 EST on the 19th, the station pressure at Oklahoma City increased 13 mbars. 

 The dry term increased 12 N units while the wet term dropped 42 A'^ units, 

 giving a net change of minus 30 A'' units. This A^'o rise in the warm sector 

 and the drop behind the cold front is consistent throughout the develop- 

 ment of the polar front wave and appears to be what one would expect 

 for this type of weather system. The present system had about a 35 A^ 

 unit rise and a 40 A?^ unit drop. This general pattern might be expected 

 to occur in all fast-moving cold fronts with varying intensity, depending 

 upon the individual synoptic pattern. In any case, it appears that the 

 Nq pattern is a sufficiently stable and conservative property of the atmos- 

 phere so that it should be possible to develop forecasting rules for A^^o, 

 but not, of course, without the analysis of many more N patterns. 



