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ANNUAL, REPORT SMITHSONIAN INSTITUTION, 19 3 9 



regions, especially at upper levels, although of course they gradually 

 become more and more modified as time goes on by the conditions 

 encountered during their progress. When air masses from widely 

 separated places of origin, and with distinctly different properties, 

 are brought into juxtaposition by the currents that are continually 

 traversing the atmosphere, these bodies of air do not freely mix, but 

 tend to retain their identity and to remain separated from one an- 

 other throughout much of their history by more or less well-defined 

 surfaces of discontinuity or sharp transition zones in temperature, 

 humidity, and velocity; until eventually, after profound modifica- 

 tion by the long continued action of external influences, they are 



Figure 5. — Formation of an extratropical cyclone from a disturbance on the boundary 

 between a cold and a warm current. See figure 6. 



subject to mixture, dissipation, and ultimate disappearance as sepa- 

 rate bodies. It is at the interfaces where different air masses meet, 

 or so-called "frontal surfaces," that the processes involved in weather 

 phenomena are in general most active, although many important 

 phenomena also frequently take place within the body of an air mass. 

 In the analysis of synoptic maps, it is necessary to identify and 

 delimit the separate air masses, assign them to their places in some 

 recognized classification of air masses according to source region and 

 characteristics, trace their movements and progressive modifications 

 from day to day, and determine the associated physical processes and 

 their relations to the attendant weather phenomena. 



With respect to their physical characteristics, air masses may be 

 classified (Willett, 1933, 1936) first into tropical and polar, and each 

 of these types further subdivided into continental and maritime, 



