A SYNOPTIC ILLUSTRATION 195 



The M unit came into being out of an approach similar to that which 

 led to the development of the B unit. The condition dn/dz = — 1/a (a 

 radio duct) implies an effective earth of infinite radius (effective earth's 

 radius factor, /v = oo, see ch. 1). The M unit is designed so that dM /dz 

 = when k — oo. M units are employed from time to time in radio 

 meteorological analysis. The Canterbury Project [41], for example, used 

 M unit analyses in the study of ranges of over-water radar signals. 



5.4. A Synoptic Illustration 



The specialized field of synoptic radio meteorology attempts a descrip- 

 tion of the variations in atmospheric refraction that arise from large 

 scale weather changes such as the passage of a polar front or the move- 

 ment of an air mass over a particular geographic region. The term air 

 mass is used to describe a portion of the troposphere that has at the surface 

 generally homogeneous properties. Although no air mass is in fact 

 homogeneous, the advantages of the air mass concept as a convenient 

 fiction are evident in the cataloging of meteorological observations for 

 climatic or synoptic purposes. 



The region of interaction between the cold air of the poles and the 

 warm air of the tropics is referred to as the polar front and is generally 

 located between 30 and 60°N. From time to time a section of the polar 

 front is displaced northward by a flow of warm tropical air while an adja- 

 cent section is simultaneously displaced southward by a flow of polar 

 air. The interaction of the flow of polar and tropical air results in the 

 formation of a "wave" that moves along the polar front, often for thou- 

 sands of kilometers. An example of a fully developed polar front wave is 

 shown on figure 5.18(a), in the same manner that it would appear on a 

 daily weather map. Across the Great Plains and eastern seaboard of the 

 United States the polar front wave normally moves along the line AB in 

 figure 5.18(a). An idealized space cross section along the line AB is 

 shown in figure 5.18(b). The warm tropical air that flows into the warm 

 sector of the wave overrides the cool air before the wave to form the 

 transition zone denoted as a warm front. The cold front represents the 

 transition between the generally humid air of the warm sector that has 

 been forced upwards and the advancing cold polar air. Squall lines are 

 drawn to represent belts of vigorous vertical convection, intense thunder 

 showers, and sharp wind shifts that frequently precede fast-moving cold 

 fronts. The fronts shown on a daily weather map represent the ground 

 intersection of the transition zones between various air masses.^ 



2 The reader who wishes a critical appraisal of recent meteorological thinking on 

 fronts, air masses, squall lines, etc., is referred to Dynamic Meteorology and Weather 

 Forecasting, by Godske, Bergeron, Bjerknes, and Bundgaard, American Meteoro- 

 logical Society and Carnegie Institute of Washington, D.C., 1957. 



