the direction of the wind and hence in general Ihey should be observed at a 

 greater distance than the short period components which spread angularly- 

 over a wide area outside of the generating area. These results are thus an- 

 other reason, apart from possible effects of viscosity, why swell has a higher 

 period than the waves in the area of generation and why short period swell is 

 seldom observed. 



It should be noted that |J.v/g is just another way to write v/c where c is 

 the phase velocity of the speclral componentj and it will not be too difficult to 

 write a brief modification of Chapter 3 of H. O. Pub. 60 3 which v/ill employ a 

 family of angular spreading diagrams as a function of v/c and permit better 

 swell forecasts. 



Cox and Muni^ [1954] have fouad that the variance of the slope of the sea 

 surface increases linearly with the wind velocity and that the theoretical spec- 

 trum of Neumann [1954] correctly predicts the total slope variance of the 

 gravity wave part of the spectrum. 



The upwind slope variance is given by equation (11. 47) and the crosswind 



slope variance is giver by equation (11.48). (See Pierso", [1955].) 



.oo tt/2 

 ai.47) ^J=j j [A(m., e)]'^(cose)2dea^x 



4 -it/ 2 " 



oo Tl/2 



(11.48) °"y^"/ J [A(fi, 6)]^^-! (3ine)"dGdia 



-Ti/2 ^ 



When equation (11.4?.) is substituted into equation (11.47) the result can 

 be simplified to the form of equation (11.49) wlere v is in meters/sec. 



14: 



