AEROGRAPHER'S MATE 3 & 2 



defined and explained in chapter 12, while the 

 remaining terms were defined in chapter 10, 

 under the section on the SKEW-T Dia- 

 gram. 



FRONTS 



Since most major changes of the weather 

 are associated with fronts, it is essential for 

 the Aerographer's Mate to become thoroughly 

 familiar with them. This requires that he 

 understand the relationship of fronts to cyclones 

 and air masses, and the characteristics of, 

 and the weather associated with, the various 

 types of fronts. He must also know the relation- 

 ship between fronts and pressure systems. 

 Finally, he must become adept at following 

 frontal movements and anticipating their speeds 

 and the modifications which they undergo along 

 the way. 



RELATION OF FRONTS 

 TO AIR MASSES 



The descriptive term "front" is defined 

 as a boundary, or line of discontinuity, 

 separating two different air masses. From the 

 definition, the close relationship that exists 

 between air masses and fronts can be readily 

 seen. In fact, without the air masses there 

 would be no fronts. 



The centers of action bring together air 

 masses of different physical properties. The 

 region of transition between two air masses 

 is called a frontal zone. The primary frontal 

 zones of the Northern Hemisphere are the 

 Arctic frontal zone and the polar frontal zone. 

 The most important frontal zone affecting the 

 United States is the polar front. The polar 

 front is the region of transition between the 

 cold polar air and warm tropical air. During 

 the winter months (in the Northern Hemisphere), 

 the polar front pushes far southward due to the 

 greater density of the polar air than during 

 the summer months. During the summer months 

 (in the Northern Hemisphere), the polar front 

 seldom moves farther south than the Central 

 United States. 



On a surface map a front is indicated by 

 a line separating two air masses; this is only 

 a picture of the surface conditions. These air 



209.58 

 Figure 14-9. — Vertical view of a frontal system 

 (without clouds shown) . 



masses also have vertical extent. (See fig. 

 14-9.) 



A cold air mass, being heavier, tends to 

 underrun a warm air mass. Thus, the cold air 

 is below and the warm air is above the sur- 

 face of discontinuity. The slope of a frontal 

 surface is usually between 1 to 50 (1 mile 

 vertical for 50 miles horizontal) for a cold 

 front and 1 to 300 (1 mile vertical for 300 

 miles horizontal) for a warm front. For 

 example, 100 miles from the place where the 

 frontal surface meets the ground, the frontal 

 surface might be somewhere between 2,000 feet 

 and 2 miles above the earth's surface, depending 

 on the slope. The slope of a front is of con- 

 siderable importance in visualizing and under- 

 standing the weather along the front. In general, 

 all the cross sections of fronts as shown in 

 this chapter summarize pictorially all the 

 pertinent features of all types of fronts under 

 average conditions. 



Cold Fronts 



A cold front is the line of discontinuity 

 along which a wedge of cold air is underrunning 

 and displacing a warmer air mass. This term 

 is also used, but inexactly, when referring to 

 a cold frontal surface. 



There are certain weather characteristics 

 and conditions that are typical of cold fronts. 

 In general, the temperature and humidity 

 decrease, the pressure rises, and in the 

 Northern Hemisphere the wind shifts (usually 

 from southwest to northwest) with the passage 

 of a cold front. The distribution and type of 

 cloudiness and the intensity and distribution 

 of precipitation depend primarily on the vertical 

 velocities in the warm air mass. On the basis 

 of this latter factor, cold fronts are classified 

 as slow-moving and fast-moving cold fronts. 



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