THE MICROBIOLOGY OF THE ATMOSPHERE 



when the air mass moves into a new region. Air masses are classified, 

 therefore, according to their origin, and we may have, for example: 

 polar maritime, polar continental, tropical maritime, and tropical con- 

 tinental types, as well as air masses of indeterminate origin (Belasco, 1952). 

 The different air masses interest us, not only because they bring different 

 kinds of weather but also because they might conceivably bring an air- 

 spora characteristic of their place of origin. 



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