Of prime importance, of course, is the availability of an accurate 

 prognostic surface weather chart* Assuming such a chart is available, the 

 calculation of wave height and period may not be a simple one since ef- 

 fective wind speed and direction, fetch, and duration must be determined* 

 In many cases it is almost impossible to determine all these quantities 

 with reasonable accuracy, and a sizeable error in any one may result in 

 considerable error in predicted wave heights. Further, the calculation 

 of wave heights for a large number of points would require considerable 

 time and effort, and, in the case of the 2l4-hour wave prognostic chart, 

 could reduce the usable time of the chart to considerably less than 2U 

 hours. In this series, wave heights were calculated for points which 

 would appear to fix the predominant features of the charts. Intermediate 

 points were obtained by interpolation with the weather prognostic charts 

 as a guide. This again is done with the wave prognostic chart used as an 

 overlay on the weather prognostic chart. At present no attempt is made to 

 differentiate between sea and swell. The problem of crossed seas and con- 

 fused seas is handled on the basis of wave height, with wave direction fitting 

 into the predominant wave trains. Where extensive areas of confused sea are 

 reported, these areas are generally indicated as "Variable". 



The actual drawing of the present wave prognostic chart is not done 

 with as straightforward a method as indicated above. The problem of moving 

 fetches is constant, and not an easy one with which to cope. This problem 

 becomes especially important during winter conditions when fast-moving in- 

 tense systems are continually present in the North Atlantic. As a partial 

 solution to this problem, an attempt is made to maintain continuity from 

 chart to chart whenever possible. Consider the case of a system moving 

 at a reasonably constant speed and maintaining a reasonably constant size 

 and intensity. If the 2li-hour prognostic weather chart indicates no ap- 

 preciable departure from the above mentioned conditions, it is reasonable 

 to assume that the wind field will generate waves closely approximating 

 those presently observed. With appropriate corrections for any changes in 

 the characteristics of the system it is possible to extrapolate into the 

 future the wave conditions associated with a particular weather system. 

 This is done whenever possible, even though it is realized that at present 

 considerable subjectivity enters this phase of the prognosis. However, 

 this same subjectivity is quite often met with in determining effective 

 wind speed, fetch, and duration when calculating!; wave neights for selected 

 points, fhe basis for this method is a knowledge of what a system is doing 

 insofar as waves are concerned. and corrections observed wave heights ac- 

 cording to expected changes in the generating system. Much more work needs 

 to be done in this respect. At the present time, such a prognosis is most 

 accurate when made by personnel who have observed a great number of cases of 

 synoptic weather systems and their associated wave systems. 



Although the preparation of the synoptic wave chart involves the cal- 

 culation of wave information, the bulk of the chart can be drawn on the basis 

 of ship observations. The calculated information and the observations can 

 both be corrected to fit the general wave pattern where necessary, result- 

 ing in a fairly accurate wave picture of the ocean area under consideration. 

 During the preparation of the prognostic wave chart, no observational guides 

 are available; every point on the chart must be predicted. 



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