Chapter 14 — AIR MASSES AND FRONTS 



SLOW-MOVING COLD FRONT. — With the 

 slow-moving cold front there is a general 

 upward motion of warm .air along the entire 

 frontal surface except for pronounced lifting 

 along the lower portion of the front. The average 

 slope of the front is approximately 1:100 miles. 

 The cloud and precipitation area is extensive 

 and is characterized by cumulonimbus and 

 nimbostratus clouds, showers, and thunderstorms 

 at, and immediately to the rear of, the surface 

 front. This area is followed by a region of 

 rain and nimbostratus clouds merging into a 

 region of altostratus clouds and then cirro- 

 stratus clouds, which may extend several 

 hundred miles behind the surface front. 



The development of cumulonimbus clouds, 

 showers, and thunderstorms is largely depen- 

 dent on the original instability characteristics 

 of the warm air mass. Within the cold air mass 

 there may be some stratified clouds in the rain 

 area, but there are no clouds in the cold air 

 beyond this area unless the cold air mass is 

 unstable. In the latter case, some cumulus 

 clouds may develop. Tils type of front is 

 slow moving; 15 knots may be considered 

 average. See figure 14-10 for a cross section 

 through a typical slow-moving cold front. 



FAST-MOVING COLD FRONTS. — With the 

 fast-moving cold front there is descending 

 motion of the warm air along the frontal sur- 

 face at high levels, and the warm air near the 

 surface is pushed vigorously upward. This type 

 of front has a slope of 1:40 to 1:80 miles and 

 usually moves rapidly; 25 to 30 knots may be 

 considered as an average speed of movement. 

 As a result of these factors, there is a 

 relatively narrow but often violent band of 

 weather. If the warm air mass is conditionally 

 unstable and moist, cumulonimbus clouds, 

 showers, and thunderstorms occur just ahead 

 of and at the surface front, and rapid clearing 

 occurs behind the front. Frequently, altostratus 

 and altocumulus cloud layers form and drift 

 ahead of the main cloud bank. Tie more 

 unstable the warm air mass, the more violent 

 the weather. If the warm air is relatively 

 dry, this type of front may not produce pre- 

 cipitation or clouds. It is with the fast-moving 

 cold front that squall lines are associated. 



Figure 14-11 shows a typical cross section 

 through a fast-moving cold front; it also shows 

 the cloud shield, precipitation shield, and frontal 

 slope (exaggerated in the vertical) associated 

 with this type of front. 



15,000- 



1 0,000 - 



5,000 - 



Figure 4-10.— Vertical cross section of a slow-moving cold front. 



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