CHAPTER 10 

 SEA AND SWELL OBSERVATIONS 



10-1 GENERAL REMARKS.— Because of 

 the almost complete lack of quantitative 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 hs used 

 for the following: Planning air-sea rescue or 

 aircraft carrier operations; selecting seaplane 

 landing areas; studying local and distant wind 

 systems and their effect on sea conditions; de- 

 termining drift and breakup of ice floes; and 

 the movement of supplies and personnel through 

 surf zones. 



Every effort should be made to standardize 

 visual observational procedure so that different 

 observers studying similar waves will reacli 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 comparing two of his own 

 observations made at different times, or in de- 

 ciding if the waves had changed since tlie 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. 606-e, 

 "Sea and Swell Observations." The log sheet 

 used to record these observations is the "Ship- 

 board Wave Observation Log," PRNC-NHO- 

 1192. A sample log sheet is shown in chapter 

 14. 



10-2 Wave Forecasting. — Recent develop- 

 ments in wave forecasting theory indicate that 

 a much more thorough description of the sea 

 surface is needed than is possible by visual 

 observations alone. Consequently, automatic 

 wave recorders of various types have been de- 

 veloped to produce a height-versus-time profile 

 of the sea surface at a given point. One of 

 these, the electric wave staff, is described and 

 instructions for its operation given in this chap- 

 ter. This instrument is intended for use in 

 waters having a depth greater than one-half 

 the wave length. 



Automatic shipboard equipment designed to 

 provide wave data on a continuing and sched- 

 uled basis is now in the developmental stage. 

 This equipment will eventually enable the 

 Oceanographic Forecasting Central of the 

 Hydrographic Office to predict sea conditions 

 over large portions of the ocean on a round- 

 the-clock basis. The immediate need, however, 

 is for accurate observational data, preferably 

 instrument data, to be used in checking and 

 improving forecasting procedui'es. 



10-3 DEFINITIONS OF OCEANO- 

 GRAPHIC TERMS.— To enable the observer to 

 have a better understanding of the various 

 factors involved in sea and swell observations, 

 definitions of certain oceanographic terms are 

 given below. 



10-4 Wind Waves. — The character of the 

 sea surface caused by action of the local wind 

 can be described in terms of height, period, 

 length, and direction of the wind waves being 

 formed. 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 

 determined bv three factors: 



1. The STRENGTH of the wind. 



2. The DURATION of time the wind has 

 been blowing. 



3. The FETCH or distance of the sea surface 

 over which the wind has acted. 



If the waves are newly formed and have not 

 had a chance to consolidate themselves in a 

 a series of regularly connected crests and 

 troughs, the sea surface will be choppy and 

 make description difficult. As the waves 

 grow, they form themselves into a regular 

 series of connected troughs and crests with 

 the H/L ratio (wave height/wave length) 

 customarily ranging from 1/12 to 1/35, or 12 

 to 35 times their height. 



10-5 Swell. — Swell is a system of waves 

 that has moved out of the generating area 

 into a region of weaker winds, a calm, or 



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H. O. 607 



