B-9 Wind Waves (Sea) and Swell.— Table 



B-4, Sea State, presents a word description of 

 the state of the sea and the corresponding lieight 

 of the wave in feet and meters, and is used for 

 recording the wave conditions on tlie A-Sheet. 

 Because of the almost complete lack of quanti- 

 tative reports concerning wave conditions in 

 all parts of the world, it is most important to 

 take observations of wind waves and swell 

 whenever possible while afloat or airborne. 

 These data may be used for the following: 

 planning air-sea rescue or aircraft carrier op- 

 erations; selecting seaplane landing areas; 

 studying local and distant wind systems and 

 their effect on sea conditions; detennining drift 

 and breakup of ice floes; and the movement of 

 supplies and personnel through surf zones. 



Table B-4. Sea state from the WMO Code 3700 for 

 recording sea state 



Height 



Description Feet Meiers 



Calm-glassy 0. 



Calm-rippled to Y, to 0.1. 



Smooth-wavelets H to 1% 0.1 to 0.5. 



Slight IK to 4 0.5 to 1.25. 



Moderate 4 to 8 1.25 to 2.5. 



Rough 8tol3 2.5 to 4. 



Very rough 13 to 20 4 to 6. 



High 20 to 30 6 to 9. 



Very high 30 to 45 9 to 14. 



Phenomenal Over 45 Over 14. 



Every effort should be made to standardize 

 visual observational procedure so that different 

 observers studying similar waves will reach the 

 same conclusions as to what they see, and will 

 record the same data. Unless a standardized 

 procedure is agreed upon, an observer might also 

 have difficulty in comjiaring two of ms own 

 observations made at different times, or in de- 

 ciding if the waves had changed since the last 

 observation. Conscientious attention to the 

 observations will therefore be required. 



A standardized method for recording visual 

 sea and swell observations on a log sheet has 

 been developed. Instructions for making these 

 observations are given in H.O. Pub. No. 60&-e, 

 Sea and Sioell Ohservations. The log sheet used 

 to record these observations is the Shipboard 

 Wave Observation Log, PRNC-NHO-1192 

 (fig. B-28). 



B-10 Sea and Swell Terms.— To enable the 

 obsener to liave a better understanding of the 

 various factors involved in sea and swell obser- 

 vations, definitions of certain oceanographic 

 terms are given below. 



1. Wind Waves. — The character of the sea 

 surface caused by action of the local wind can 

 1)6 described in terms of height, period, length, 

 and direction of the wind waves being fonned. 

 Waves which are still growing under the force 



of the wind are known as wind waves. These 

 waves travel in a direction within about 20° of 

 the local wind, and their dimensions are deter- 

 mined by three factors : 



a. The STRENGTH of the wind. 



b. The DURATION of time the wind has 

 been blowing. 



c. The FETCH or distance of the sea sur- 

 face over which the wind has acted. 



If the wa\-es are newly formed and have not 

 had a cliance to consolidate themselves in a 

 series of regularly connected crests and troughs, 

 the sea surface will be choppy and make descrip- 

 tion difficult. As the waves grow, they form 

 them.selves into a regular series of connected 

 troughs and crests with the H/L ratio (wave 

 height/wave lengtli) customarily ranging from 

 1/12 to 1/35, or 12 to 35 tunes their height. 



2. Swell. — Swell is a system of waves that 

 has moved out of the generating area into a re- 

 gion of weaker or opposing winds, or a calm. 

 Swells decrease in height with travel, and al- 

 though there may be difficulty 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 H/L 

 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 bow regularly in a glassy sea. 



The reporting of a swell is exceedingly im- 

 portant, for its presence in the local area indi- 

 cates that recently there may have been a very 

 strong wind, possibly even a severe storm, hun- 

 clreds or thousands of miles away. The direc- 

 tion from which the swell is coming tells in what 

 direction the strong wind was located. In cer- 

 tain instances the onset of a swell is the first 

 indication of an approaching storm. 



3. 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 i\\& signifi- 

 cant wave height. This wave height is the aver- 

 age of the higher, well-defined waves present 

 during the observation. Statistically, signifi- 

 cant waves are defined as tlie average of the 1/3 

 highest waves obsen-ed in a given time. As the 

 height is the most important wave characteristic 

 from the operational point of view, care should 

 be taken to observe and report it accurately. 

 AVhile this value for height is about the best 

 that can be exjjected from visual observation, 

 efforts are being made to perfect the power 

 spectnim analysis of instrument records which 

 will be more valuable for forecasting purposes. 



•1. Wave Period, Length, Velocity, and Di- 

 rection. — The loave period is defined as the time 

 interval betM-een successive wave crests as the 

 wave passes a fixed point. Wave Length is the 

 horizontal distance oetween successive crests. 



B-9 



