Chapter 11 — WATCH ROUTINES (CONTINUED) 



209.303 



Figure 11-10. — Basic isobaric patterns. 



Stations in these regions reduce their pres- 

 sure to mean sea level; in so doing, discrep- 

 ancies may occur. It is advisable to htive a 

 topographic chart of the terrain in order to 

 determine whether apparent packing of the iso- 

 bars on a previous map is due to the mountain 

 barrier effect or a steep pressure gradient actu- 

 ally being present. 



Isallobaric Analysis 



An isallobar is a line of equal pressure 

 change. In other words, isallobars are lines 

 of equal pressure tendency and are usually 

 drawn for an interval of 1-mb change. For 

 example, all stations having a pressure tendency 

 plotted as minus 10 would be connected with 

 a line representing a 1-mb change. 



Isallobars drawn for plus values are nor- 

 mally drawn on the chart in dashed blue lines, 



and isallobars for minus values are drawn in 

 dashed red lines. (See fig. 11-11.) 



Isallobars are reliably indicative of the di- 

 rection in which pressure systems have been 

 moving, and consequently are useful in fore- 

 casting the movements. 



Air-Mass Analysis 



The air masses have been studied as the 

 frontal analysis was made, now they must b3 

 classified and labeled. The current properties 

 and the past history of the air mass are the 

 guidelines to classification of the air masses. 

 (See ch. 14.) 



It is cu3tomary to indicate the air-mass 

 classification by placing the appropriate sym- 

 bol on the weather map in the location of the 

 air mass. The symbols used are given in fig- 

 ure 11-9. 



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