weather naps and cannot be prepared for periods longer than those 

 for which these maps can be considered valid. 



It is conteiapl ated that a more comprehensive edition of this 

 manual will be issued in the near future. This vdll contain meth- 

 ods for determining the transformation of waves in shallow water 

 and for forecasting surf from synoptic weather data or from observa- 

 tions of ViTaves offshore. 



SURFACE WAVES IK WATER 

 General Discussion 



A wave is described by its length , L, i.e. the horizontal 

 distance from crest to crest or trough to trough (see fig. lA) , 

 and by its height , H, i.e. the vertical distance from trough 

 to crest. A wave is furthermore characterized by its period , T, 

 i.e. the time Interval between the appearance of two consecutive 

 crests at a given position. 



B 



Figure 1. Surface v/aves. A. Profile of wave. 

 B. Advance of wave, showing the wave profile at the 

 times t = 0, t = T/4, and t - T/2. In the time T/2 

 the wave has advanced one half wave length, L/2 , 



