opposing winds. Thus, swell usually travels in 

 a direction more than 20° from the direction 

 of the wind although this is not necessarily 

 always the case. Swells decrease in height 

 with travel, and although there may be dif- 

 ficulty in distinguishing between wind waves 

 and swell, the latter usually possesses a more 

 or less smooth, well-rounded profile, has greater 

 wave length and period, and disturbs the water 

 to a greater depth. The HjL ratio for swell 

 normally ranges from 1/35 to 1/200. Under 

 certain conditions, extremely long and high 

 swells will cause a ship to take solid water over 

 its bows regularly in a glassy sea. 



The reporting of a swell is exceedingly im- 

 portant, for its presence in the local area 

 indicates that recently there may have been a 

 very strong wind, possibly even a severe storm, 

 hundreds or thousands of miles away. The 

 direction from which the swell is coming tells 

 in what direction the strong wind was located. 

 In certain instances the onset of a swell is the 

 first indication of an approaching storm. 



10-6 Wave Height. — The vertical distance 

 from the trough to the crest is termed the 

 wave height. In view of the considerable 

 variation in height between waves observed in 

 a 7-minute period, reference is conveniently 

 made to the significant wave height. This 

 wave height is the average of the higher, well- 

 defined waves present during the observation. 

 Statistically, significant waves are defined as 

 the average of the )<i highest waves observed in 

 a given time. As the height is the most im- 

 portant wave characteristic from the operational 

 point of view, care should be taken to observe 

 and report it accurately. While this value for 

 height is about the best that can be expected 

 from visual observation, efTorts are being made 

 to perf?ct the power spectrum analysis of 

 instrument records which will be more valuable 

 for forecasting purposes. 



10-7 Wave Period, Length, Velocity, and 

 Direction. — The wave period is defined as the 

 time interval between successive wave crests 

 as the wave passes a fixed point. Wave length 

 is the horizontal distance between successive 

 crests. The wave velocity is the rate of travel 

 of the wave form through the water. Here 

 again, as in the case of height, there is con- 

 siderable variation in these characteristics in 

 any given wave train. For observational 

 purposes, one should determine the average 

 values for the significant waves. Wave direc- 

 tion is the direction, in degrees true, from which 

 the waves come. 



If wave systems cross each other at a con- 

 siderable angle, the result is a very irregular sea 



H. O. 607 



surface comprised of apparently unrelated 

 peaks and hollows and is termed a cross sea. 

 Waves are said to be shori-creoted when the 

 crests are short compared to the wave length 

 and long-crested when crests are long compared 

 to the wave length. Waves are commonly 

 short crested in cross seas and in the early 

 stages of generation, while swell is generally 

 long crested. In deep water, where the orbital 

 motion of water particles is uninhibited by the 

 bottom while the waves proceed through the 

 water, the wave period is related mathemati- 

 cally to the wave length and wave velocity 

 in such a manner that they travel at different 

 speeds, and are constantly overtaking or pulling 

 away from their neighbors. If the crests of two 

 waves happen to be at the same point at the 

 same time, their combination results in a crest 

 that is higher than either of the component 

 crests. This phenomenon, known as wave inter- 

 ference, accentuates the variability in wave 

 height. Conversely, interference also can cause 

 flat zones when the trough of one wave meets the 

 crest of another. The hydrodynamics of surface 

 wave motion is such that as the period increases, 

 the speed and wave length increases as well. 

 The following approximate formulas show 

 this relation, where the units are knots, feet, 

 and seconds. 



Wave speed=3.0 (Period) 

 Wavelength=5.12 (Period)' 

 Wave Period=0.3 (Wave Speed) 



10-8 Whitecaps. — In deep water the wind 

 may blow strong enough to raise steep and 

 choppy wind waves. When the ratio of height 

 to length becomes too large, the water at the 

 crest moves faster than the crest itself, causing 

 the water to topple forward and form white- 

 caps. The term whitecap is confined to deep 

 water waves while the term breaker is used to 

 describe waves breaking in shoal water or in 

 strong-tidal currents which oppose wave motion. 

 Whitecaps owe their instability to a too rapid 

 addition of energy from the wind to the wave 

 form and breakers to the restrictive effect of 

 the sea bottom or opposing currents upon the 

 water movement in the wave form. 



10-9 Breakers. — A breaker is an ocean wave, 

 either wind wave or swell, which has traveled 

 over a gradually shoaling bottom and reached 

 the point in its transformation wliere it is no 

 longer stable and plunges over or breaks. As a 

 rule, when swell is definitely predominant, the 

 breakers are regular with smooth profiles. When 

 wind waves are predominant, the breakers are 

 choppy and confused. Swell coming into a 

 beach increases in height up to the point of 

 breaking. Wind waves, on the other hand, are 



103 



