Table II.2. — Summary of additional quantities to be determined for preparing forecasts of swell 

 (Decay is involved but secondary wind is not.) 



Table II. 3. — Summary of additional quantities to be determined for preparing forecasts of swells 



(Secondary wind over decay distance) 



Number 



Term 



Symbol 



Units 



Source 



Page Ref. 



26 



27 

 28 



29 



30 



31 



32 



33 



34 



Wind velocity component in 

 secondary wind area par- 

 allel to direction of wave 

 travel. 



Sign of U' 



Uncorrected wave period at 

 distance D. 



Wave speed at distance D 



Effective distance of decay _. 



Wave period at end of ef- 

 fective decay. 



Reduction factor for wave 

 height. 



Wave height at end of ef- 

 fective decay. 



Travel time to end of ef- 

 fective decay. 



u- 



To (uncorr. 

 Cd 

 De 



Toe 

 Hoe/Hp 



toe 



Knots 



Seconds 



Knots 



Nautical miles 

 Seconds 



Computed from synoptic and 

 prognostic charts. 



Same as No. 26 



Plate VI using Nos. 18 and 



19. 

 Inset of plate VI using No. 



28 

 Equation (II.2) or (II.3), 



using Nos. 19, 26, and 29. 

 Plate VI using Nos. 18 and 



30. 

 Same as No. 31 



Feet _. 

 Hours 



No. 17 multiplied by No. 34^ 



Equation II. 4 using Nos. 19 

 and 3^. 



20 



20 

 21 



20 



20 



21 



21 



21 



20 



If both secondary wind and calm occur, the following additional quantities must be determined : 



35 

 36 



Distance to beginning of 

 secondary wind area. 



Distance to end of secondary 

 wind area. 



D" 



Nautical miles 

 Nautical miles 



Synoptic and prognostic 



charts. 

 Same as No. 35 



21 

 21 



contain reference numbers, terms, symbols, 

 units of measurement, sources from which the 

 value of the terms or quantities are obtained, 

 and page references indicating where the terms 

 are discussed. 



It will also be necessary to determine heights, 

 periods, and travel times at these intermediate 

 distances which are indicated by H, T, and t, 

 respectively, with the appropriate subscript 

 (D' or D"). 



DETERMINATION OF WIND WAVES 



Wind Direction 



Outside of the tropics the wind direction over 

 the ocean can be determined from the isobars 

 by applying the rule that the wind deviates 

 10 degrees to the left of the gradient wind in 



the Northern Hemisphere and 10 degrees to the 

 right in the Southern Hemisphere. Where the 

 isobars are nearly straight (fig. II.l A and B) 

 the winds to be considered in forecasting swell 

 are those with directions within 30 degrees of a 



,15 



