2. Set index line on wave period T^ on 

 left-hand vertical scale and on given decay 

 distance on vertical decay scale. 



3. Read wave period To or Toe at inter- 

 section of index line with the right-hand ver- 

 tical scale. 



4. Pivot index line about point of inter- 

 section with turning line until index line 

 intersects top horizontal scale at wave height 



5. Read wave height Ho or Hoe at inter- 

 section of index line with bottom horizontal 

 scale. 



6. Reset index line on wave period To on 

 horizontal wave period scale at top of dia- 

 gram and on given decay distance on hori- 

 zontal decay scale. 



7. Read travel time to at intersection of 

 index line with travel time scale. 



ILLUSTRATIVE EXAMPLE 



Forecast of Swell for Brest and Vicinity 



A weather situation in the North Atlantic 

 has been chosen as an example to illustrate the 

 complete technique of swell forecasting. The 

 forecaster should follow through this example 

 in detail before attempting to make independ- 

 ent forecasts. Since reading of the plates is 

 somewhat subjective, the forecaster may not 

 get exactly the results shown, but any discrep- 

 ancies should be minor. 



At 0630Z on 11 March the warm section of 

 a deepening low became effective as a generat- 



Table II. 7. — Calculated quantities for illustrative 

 example 



ing area for Brest, France and vicinity. Fore- 

 casts have been prepared for Brest using the 

 1830Z map of 11 March, the 0630Z and 1830Z 

 maps of 12 March, and the 0630Z map of 13 

 March. The calculated quantities are tabulated 

 in Table II.7, using the symbols of tables II.l, 

 II.2, and II.3. 



When two wave trains arrive at the target 

 simultaneously or nearly so, as shown by those 

 originating from two coexistent fetches on the 

 maps in figures II.5 C and D, the waves will 

 interfere with each other. Therefore, the 

 heights will be more variable than usual since 

 the waves sometimes will be in phase ampli- 

 fying one another and at other times out of 

 phase canceling one another. This fact should 

 be noted on the forecast. 



If subsequent weather maps showed the de- 

 velopment of a secondary wind area over the 

 decay distance, it would be necessary to deter- 

 mine the position of each wave train from 

 plate VI. The method is to enter the plate with 

 Tp, proceed left until the appropriate travel 

 time line is intersected, and then read from the 

 bottom of the plate the distance these waves 

 have traveled. Thus, in the example above, 

 if the 1830Z map of 13 March showed a sec- 

 ondary wind between 400 and 600 miles from 

 the end of the original fetch, the wave trains 

 initiated on 11 March at 1830Z and on 

 12 March at 0630Z will be uninfluenced as they 

 will have traveled 775 and 700 nautical miles,^ 

 respectively, in 48 and 36 hours. However, the 

 wave trains originating later would be in- 



1 Shown by reports on original maps not reproduced. 



' The wave train of 12 March 0630Z will travel only 

 550 nautical miles in 36 hours but its point of origin 

 is 150 miles closer to the target than that of the train 

 of 11 March 1830Z. 



25 



