AEROGRAPHER'S MATE 3 & 2 



CIRROSTRATUS 



CIRRUS, 



15,000 



10,000 



5,000- 



100 MILES 



Figure 4-11 — Vertical cross section of a fast-moving cold front. 



209.60 



Warm Fronts 



A warm front is the line of discontinuity 

 where the forward edge of an advancing mass 

 of relatively warm air is replacing a retreating 

 relatively colder air mass. As in the case of 

 the cold front, this term is used inexactly 

 when referring to a warm frontal surface. 



Certain characteristics and weather condi- 

 tions are associated with warm fronts. The 

 winds shift from southeast to southwest or west, 

 but the shift is not as pronounced as with the 

 cold front. The temperatures are colder ahead 

 of the front and are warmer after the passage 

 of the front. Not being greatly affected by daily 

 heating and cooling of the earth's surface, the 

 dewpoint is normally more constant than the 

 temperature through the day except with the 

 passage of a front. Therefore, the dewpoint is 

 a good index of frontal passage. The average 

 slope of a warm front is 1:150. 



A characteristic phenomenon of a typical 

 warm front is the sequence of cloud formations. 

 They are noticeable in the following order: 

 Cirrus, cirrostratus, altostratus, nimbostratus, 

 and stratus. The cirrus clouds may appear 700 



to 1,000 miles ahead of the surface front 

 followed by cirrostratus about 600 miles and 

 altostratus about 500 miles ahead of the sur- 

 face front. 



Precipitation in the form of continuous or 

 intermittent rain, snow, or drizzle is frequent 

 as much as 300 miles in advance of the surface 

 front. Surface precipitation is associated with 

 the nimbostratus in the warm air above the 

 frontal surface and from stratus in the cold air. 

 However, when the warm air is convectively 

 unstable, showers and thunderstorms may occur 

 in addition to the steady precipitation. Fog is 

 common in the cold air ahead of a warm front. 



Clearing usually occurs after the passage 

 of a warm front, but under some conditions 

 drizzle and fog may occur within the warm 

 sector. Warm fronts usually move in the 

 direction of the isobars of the warm sector; 

 in the Northern Hemisphere this is usually east 

 to northeast. Their speed of movement is 

 normally less than that of cold fronts; on the 

 average it may be considered to be about 10 

 knots. 



Figure 14-12 summarizes pictorially the 

 pertinent features of warm fronts under average 

 conditions. 



326 



