52 



THEORY OF SEAKEEPING 



70 



60 



i? 50 



,40 



-S 30 

 > 



.zo 



0030 



e 



1230 



0030 

 7 



20 



1230 0030 1230 0030 1230 0030 1230 



8 9 10 



January 1952 



Legend 



Sverdrup- Munk, Bre+schneider 



(Height and Period) (Height) 



Pierson- Neumann - James 



CHeight) 



I I Pierson - Neumann - James 



(Period. The Spectral Range is 

 indicated by the Vertical Bars) 



Fig. 57 Comparison of hindcast and observed waves at Ocean 

 Station Papa (50°N, 145°W) (from Rattray and Burt, 1956) 



Observed 



Bretschneider 

 (Period) 



may make considerable difference in wave height. Cold 

 air over warm water is unstable and more turbulent. 

 It has, therefore, greater capacity for exciting waves. 

 There is no generally agreed method of allowing for this 

 effect, and it appears that Bracelin (1952) is the only one 

 to have included it in his forecasting instructions. Fig. 

 58 shows the relationship between the wa\-e height and 

 sea-air temperature difference according to Roll (1952). 

 In the case illustrated here the air was 12 deg cooler than 

 the water, and (by extrapolation) a factor of possibl.y 1.3 

 should have been applied to the a^'erage predicted wa\'e 

 heights. Good agreement without this factor indicates 

 that the "averaging" methods of prediction exaggerate 

 the wa^'e height by possilily 30 per cent. 



(b) Walden and Fanner (1957). Walden and Farmer 

 made 47 wave recordings on the MV Atlantis of Woods 

 Hole Oceanographic Institute in November and Decem- 

 ber 1947, on a cruise along the 50.5° W meridian from 

 Newfoundland Banks to the South American Conti- 



130 



120 



110 



100 



90 



Aniahl der Einielwerte 

 402 869 1744 



-6 -5 -4 -3 -2 -I +1 +2 



Degrees C 



Fig. 58 Increase of wave height and steepness plotted 

 versus air-water temperature difference. Air temperature 

 is designated T;,, water temperature Tir. Numbers above 

 diagram indicate number of individual observations included 

 in each plotted point (from Roll, 1952) 



Table 1 Data Corresponding to Walden and Farmer's 



Fig. 59 



" Longitude uniformly 50.5° W. 



' Determined from spectra using Longuet-Higgins' (1952) rela- 

 tionships for a narrow spectrum. 



Table 1 



Pierson, 



Ne\miann, 



James 



-1-19.6 



(10)'' 



Bret- 

 schneider 

 and Wilson 

 -1-52.0 

 (1-1) 



Walden" 

 -1-7.1 



(17) 



Darby- 

 shire 

 -7.0 



(5) 



<■ Walden (1953/54, 1954, 1955, 1955/56a, 1955/566). 



'' Figures in parentheses indicate the number of analyzed cases. 



nental Shelf. Fifteen of these were analyzed by the 

 Hydrographic Office, Washington, D.C., and representa- 

 tive wave spectra for six records are shown in Fig. 59. 

 The wind and wave conditions corresponding to these 

 spectra are given in Table 10. 



The wave records were taken with Tucker's (1956) 

 ship-borne wave recorder. The dotted lines in Fig. 59 

 show the spectra corrected for the pressure-gage depth. 

 Recordings were of 20-min duration, and were taken 

 mostly with the ship drifting and in a few cases at very 

 low speed in head seas. The local wind was measured 



