Chapter 11 — WATCH ROUTINES (CONTINUED) 



the upper front, 50 to 100 miles north of the 

 northern tip of the warm sector. 



SECONDARY COLD FRONTS. — In the last 

 stages of an extratropical cyclone, there is a 

 tendency for troughs of a low pressure to form 

 to the rear of the primary trough. A secondary 

 front may occur in this trough. Secondary cold 

 fronts usually occur during outbreaks of very 

 cold air behind the initial outbreak. Secondary 

 cold fronts may follow in intervals of several 

 hundred miles to the rear of the rapidly mov- 

 ing front. Usually, in the case of the forma- 

 tion of a secondary cold front, the primary front 

 tends to dissipate, and the secondary front then 

 becomes the primary front. Secondary fronts 

 do not usually occur during the summer months 

 because it is rare for enough temperature dis- 

 continuity to exist. 



QUASI-STATIONARY FRONTS. — When the 

 past history is accurate, quasi- stationary fronts 

 are easily located. Merely look for them in 

 the same general vicinity as on the previous 

 maps. A previously quasi- stationary front which 

 has moved a considerable distance should not 

 be classified as a quasi- stationary front. In- 

 stead, it should be called either a cold front 

 or a warm front. 



The weather along quasi-stationary fronts 

 is usually a mixture of cold frontal and warm 

 frontal weather. Often there is no weather along 

 the quasi-stationary front. There is usually 

 little difference in temperature across a quasi- 

 stationary front, and the same holds true for 

 the dewpoint. Pressure changes are ordinarily 

 very small across quasi-stationary fronts and 

 the characteristic of the barograph trace is 

 indefinite. Usually the wind shift across a quasi- 

 stationary front is small. Isobars do not often 

 form a well-defined trough; nevertheless, they 

 do kink at the front toward higher pressure, 

 The best indicator of the quasi-stationary front 

 is the cloud deck. A good past history with 

 a band of clouds and little difference other- 

 wise is usually a good indication that the quasi- 

 stationary front is still in existence. 



From the above it can readily be seen that 

 the studying of many station reports, past and 

 curreat, is important in locating and determin- 

 ing the movement of fronts. Examine all re- 

 ports in detail to determine the types of fronts 

 present and their characteristics. 



Once isobars have been drawn for all stand- 

 ard values possible in conformance with the 

 plotted data and the fronts have been located, 

 the weather map is ready for final analysis. 



The final analysis consists primarily of 

 smoothing out any irregularities in the isobars 

 which were drawn during the basic analysis. 



Erase the innermost isobar in a pressure 

 center. Using a freehand stroke, redraw the 

 isobar so that it has a smooth shape or appear- 

 ance. After the isobar has been smoothed, label 

 it with the appropriate millibar value which 

 it represents. (See fig. 11-6.) Erase the next 

 isobar away from the center, smooth it, and 

 label it with the appropriate millibar value 

 which it represents. Continue the smoothing 

 process until all isobars which were sketched 

 during the basic analysis have been erased, re- 

 drawn smoothly, and labeled. 



The isobars near fronts should be drawn 

 so as to bring out the frontal discontinuity of 

 the horizontal pressure gradient. Correctly drawn 

 isobars are kinked without exaggeration at a 

 front, with the kink always pointing toward 

 higher pressure as in figure 11-8. 



After all isobars have been smoothly re- 

 drawn and labeled with their representative 

 pressure value, draw the fronts, using the colors 

 and symbols as shown in figure 11-9. Then 

 label all high-pressure centers with a blue block 



209.301 



Figure 11-8.— Isobars at a front. 



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