In fact, there appears to be no way to obtain parameters which 

 completely describe the seaway by visual observations or by the 

 statistical analysis of a wave record or a pressure record. The 

 partial characterization in terms of significant height and average 

 "period" is, however, useful in many aspects if it is interpreted with 

 care in terms of possible wave spectra and the meteorological synoptic 

 situation. 



D. "Wave Lengths" 



1. The Observation of the "Wave Length" 



Photographs of the sea surface, such as figure 1, show that it 

 is composed of short- crested waves. There are medium waves super- 

 imposed on the big waves and short waves superimposed on the 

 medium waves. There are ripples on top of everything else. The waves 

 in a photograph are much more irregular than a corresponding wave 

 record. There appear to be more short waves in a photograph than 

 there are in a wave record. 



Most of the tinrie a dominant direction of travel can be determined 

 for the waves. Then the length of the waves along this direction can 

 be measured. The actual distance between successive crests must be 

 measured. Procedures for measuring the "wave length" are given in 

 Pierson, Neumann, and James (1955). The procedures involve towing 

 a line with floats behind a vessel for use as a scale, and the use of the 

 ship or other ships as a scale factor. 



The average "wave length" cannot be computed from the average 

 "period" in terms of the classical formula. Stated another way, it 

 is not true that the average "wave length" in feet equals 5.12 times the 

 square of the average "period" in seconds. For fully developed seas, 

 the average "wave length", if the theoretical spectrum which is 

 assumed is correct, is given by 



L = - . 5.12 T (37) 



For the theory of the derivation, see Pierson (1954). For nonfully 

 developed seas the formula does not hold, and the derivation of the 

 average "wave length" is more difficult. 



It appears that the p.d.f. of the "wave lengths" cannot be computed 

 from the p.d.f. of the "periods" even if the p.d.f. of the "periods" were 

 known. It would have to be connputed by mapping the wave spectrum 

 as a function of frequency and direction, into a frequency spectrum of 

 the spectral wave lengths. Then, if the theory of the p.d.f. of the 



36 



