of the storm are overtaken and outdistanced by the longer waves pro- 

 •duced when the wind is stronger, the separation between them increasing 

 with distance (Barber and Ursell, 1948) (^). The first indication at a 

 ■distant recording station is a narrow band of long low swell ; this is 

 followed b}^ shorter higher swell and the wave-band gradually broadens 

 as the mean period decreases.. The observed wave pattern at any place 

 is generally a combination of waves generated by the local winds with 

 swell components from remote sources. 



To predict the wave pattern or surf breaking on a coast from meteor- 

 ological charts and forecasts the effect of swell from each source must be 

 taken into account and it is necessary to know how the wind energy is 

 shared among the range of wave-lengths that are produced. Recent 

 observations indicate that the amount of energy imparted to waves of 

 a particular period depends on the difference between the geostrophic 

 wind speed and the velocity of the wave and on the time during which 

 the wind acts on the wave ; there is also a factor proportional to the wave 

 period to allow for the increase with wave length of the capacity of a wave 

 for absorbing energ}'' \vithout becoming unstable, and a factor inversely 

 proportional to the wind speed to take account of evidence that there is 

 an optimum wind speed for each wave-length, the growth of each of the 

 shorter wave-lengths being decreased in turn as the rising wind strength 

 ■causes such values to be exceeded. 



Empirical methods based on such conclusions can be used with fair 

 success to predict the wave-spectrum from hour to hour on the west coast 

 of the British Isles, but there are many outstanding problems. 



t{i) Barber, N. F., and Ursell, F. : The Generation and Propagation of Ocean 

 Waves and Swell. Phil. Trans. Roy. Soc, 240, 527 (1948). 



By Authority: R. E. Owen, Government Printer, Wellington. — 1952. 



