WEATHER — WOOLARD 



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according as the source region was over land or ocean; still further 

 subdivision may be made on the basis of the geographical location 

 of the source region. For example, in the United States, two types 

 of tropical maritime air are important — tropical Pacific (Tp) and 

 tropical Atlantic (Ta) . When an air mass becomes significantly 

 modified from its original state, it is designated as a transitional 

 mass and is distinguished by prefixing a capital N to its symbol— 

 as, for example, NTa. A wholly satisfactory classification is difficult 



Figure 6. — Idealized estratropical cyclone formed by the process illustrated in figure 5, 

 •with vertical (east-west) cross sections through the northern part (above) and the 

 southern part (below). 



to achieve; and at the present time the tendency is to classify air 

 masses on the basis of whether they are warmer or colder than the 

 surface over which they are flowing, rather than on the geographical 

 basis used in the diagrams accompanying this paper. 



The development of air-mass analysis has greatly stimulated 

 efforts to obtain more adequate data and to devise effective means 

 for their application to the physical analysis of day-to-day weather 

 and the interpretation of synoptic maps. The daily location of the 

 "fronts," or lines on the surface of the earth which bound the different 

 air masses, presents a difficult problem in practice. The contrasts 



