AEROGRAPHER'S MATE 3 & 2 



6. Dewpoint. The dewpoint temperature gen- 

 erally aids in locating fronts. This is espe- 

 cially true in mountainous regions. A drop 

 in the dewpoint is observed with the passage 

 of either type of cold fro.it. 



7. Visibility and ceiling. With the approach 

 and passage of a slow-moving cold front, the 

 visibility and ceilings decrease and remain low 

 after the passage until well within the cold 

 air. Fast-moving cold fronts are preceded by 

 regions of poor visibility and low ceilings due 

 to shower activity. After passage of fast-mov- 

 ing cold fronts, the ceiling rapidly becomes un- 

 limited and the visibility unrestricted. 



WARM FRONTS. — Active warm fronts are 

 generally located in pressure troughs on the 

 surface charts. The troughs are not as pro- 

 nounced as those observed with cold fronts; 

 therefore, other meteorological elements are 

 utilized as follows in locating warm fronts 

 accurately: 



1. Pressure tendencies. Pressure usually 

 falls for an appreciable length of time prior 

 to the frontal passage. Normally, it is steady 

 after passage. The tendencies in advance of 

 the front are therefore \ (steady or unsteady 

 fall). A warm frontal passage is usually indi- 

 cated by aV_tendency. 



2. Wind. Tae wind in advance of warm front 

 in the Northern Hemisphere is usually from 

 the southeast, shifting to southwest after pas- 

 sage. The wind speed normally increases as 

 the front approaches. The wind shift accompany- 

 ing a warm front is seldom as abrupt as with 

 a cold front. 



3. Cloud forms. Warm fronts are nearly 

 always well defined by typical stratified clouds. 

 They are generally cirrus, cirrostratus, alto- 

 stratus, njmbostratus, and stratus, with the 

 cirrus appearing as much as 1,000 miles be- 

 fore the actual surface passage. The cloud 

 types that form after passage of the warm front 

 are typical of the warm air mass. 



4. Precipitation. The precipitation area of 

 warm fronts extends about 300 miles in ad- 

 vance of the surface front. Precipitation occurs 

 mainly in the form of continuous or intermit- 

 tent rain, snow, or drizzle. However, when 

 the warm air is convectively unstable, showers 

 and thunderstorms may occur in addition to the 

 steady precipitation. 



5. Temperature and dewpoint changes. Abrupt 

 temperature changes, like those characteristic 

 of cold fronts, do not accompany the warm frontal 

 passage. Instead, the temperature change is grad- 

 ual. It starts increasing slowly with the ap- 

 proach of the front and increases slightly more 

 rapidly with the passage. The dewpoint is nor- 

 mally observed to rise as the front approaches, 

 and a further increase follows the frontal pas- 

 sage when the air in the warm sector is of 

 maritime origin. 



6. Visibility and ceiling. The visibility and 

 ceilings are normally good until the precipita- 

 tion begins. Then they decrease rapidly. Dense 

 fog frequently occurs in advance of a warm 

 front. An improvement is experienced after 

 passage. 



OCCLUDED FRONTS. — Because the occlu- 

 sion is a combination of a cold front and a warm 

 front, the resulting weather is a combination of 

 conditions which exist with both fronts. Ahead 

 of a cold type occlusion, as the warm air is 

 lifted, all the clouds associated with a warm 

 front are found producing typical prefrontal 

 precipitation extensively for a distance of 250 

 to 300 miles. Typical cold frontal weather is 

 found throughout the narrow belt in the vicinity 

 of the surface front. However, the thunderstorms 

 are less intense than those of a typical cold 

 front, since the source of warm air has been 

 cut off from the surface and the energy re- 

 ceived comes only from the warm air trapped 

 aloft. Instability showers often follow the cold 

 front when the cold air is unstable. The most 

 violent weather occurs on the upper front for 

 a distance of 50 to 100 miles north of the 

 northern tip of the warm sector. After the 

 occlusion has passed, the weather usually clears 

 rapidly. Figures 14-13 and 14-14 of this manual 

 show vertical cross sections of cold type and 

 warm type occlusions. Figure 14-15 shows the 

 occlusions and their associated upper fronts. 



The weather associated with the warm type 

 occlusion (fig. 14-13) is very similar to that 

 of the cold type occlusion. With the warm type 

 occlusion, the high-level thunderstorms asso- 

 ciated with the upper cold front develop quite 

 some distance ahead of the surface front (up 

 to 200 miles), and the weather band, ir _. .*-• 

 eral, is wider (up to 400 miles). The air behind 

 the cold front, flowing up the warm frontal 

 surface, causes cumuliform type clouds to form. 

 In this area, precipitation and severe icing may 

 be found. The most violent weather occurs on 



202 



