Chapter 14 — AIR MASSES AND FRONTS 



cannot disappear into the ground, it must move 

 upward. Hence, there is always a net move- 

 ment of air upward in the region of a front. 

 This is another important characteristic of 

 fronts, since the lifting of the air causes con- 

 densation, clouds, and weather. While air motion 

 within an area of high pressure is downward 

 and outward, motion in a frontal zone is inward 

 and upward. (This is the divergence and con- 

 vergence, respectively, mentioned in chapter 13.) 



RELATION OF FRONTS 

 TO CYCLONES 



Every front is associated with a cyclone 

 in a systematic way. The cyclone is a counter- 

 clockwise circulation (in the Northern Hemi- 

 sphere) around a central low-pressure area, 

 about which the fronts move. Cyclones con- 

 tribute to and partly control frontal movements, 

 but the reverse is not true. 



When some temperature, pressure, or wind 

 irregularity occurs along a front, cyclonic 

 circulation may take place. The front will wave 

 forming a cold front and a warm front which 

 will move with the cyclonic circulation. If the 

 fronts are moving at equal speeds, the wave 

 remains stable and moves along the front, either 

 weakening and disappearing, or it becomes 

 absorbed in some large circulation. If the cold 

 front moves faster than the warm front in the 

 cyclonic circulation, it will overtake the warm 

 front and occlude. After it has occluded, the 

 cyclonic circulation and time have changed the 

 original air masses through mixing them and 

 making them so similar that the occluded front 

 loses its identity and dissipates, leaving the 

 remains of the original front well away from 

 the decaying cyclonic circulation. (See fig. 

 14-18.) 



When the polar front moves southward, it 

 is usually associated with the development and 

 movement of cyclones and with outbreaks of 

 cold polar air. The cyclonic circulation 

 associated with the polar front tends to bring 

 polar air southward and warm moist tropical 

 air northward. 



During the winter months, the warm air- 

 flow usually occurs over water, and the cold 

 air moves southward over continental areas. 

 In summer the situation is reversed. 



Large cyclones that form on the polar front 

 are usually followed by smaller cyclones and 

 are referred to as families. These smaller 

 cyclones tend to carry the front farther south- 

 ward. 



In an ideal situation these cyclones come 

 in succession, causing the front (in the Northern 

 Hemisphere) to lie in a southwest to northeast 

 direction. 



Every moving cyclone usually has two 

 significant lines of convergence distinguished 

 by thermal properties. The discontinuity line 

 on the forward side of the cyclone where warm 

 air replaces cold air is the warm front; the 

 discontinuity line in the rear portion of the 

 cyclone where cold air displaces warm air is 

 the cold front. 



In figure 14-18 the wave development and 

 formation of a cyclone shows the frontal 

 systems which are present throughout the 

 different stages. 



Anticyclones are usually coexistent with 

 areas of good weather. This is due mainly to 

 divergence. However, frontal systems may at 

 times penetrate the anticyclones to some extent. 



FRONTAL MOVEMENT 



The weather is greatly affected by the move- 

 ment of frontal systems. From the time the 

 front develops until it passes out of the weather 

 picture, it is watched closely. The speed at 

 which it travels and the modifications which it 

 undergoes are important considerations in 

 analyzing and forecasting the weather. 



Speed 



The speed of the movement of frontal 

 systems is an important determining factor 

 of weather conditions. Rapidly moving fronts 

 usually cause more severe weather than slower 

 moving fronts. For example, fast-moving cold 

 fronts often cause severe prefrontal squall lines 

 which are extremely hazardous to flying. The 

 fast-moving front does have the advantage of 

 moving across the area rapidly, permitting the 

 particular locality to enjoy a quick return of 

 good weather. Slow-moving fronts, on the other 

 hand, may cause extended periods of unfavor- 

 able weather. A stationary front which may 



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