A STUDY OF WAVE PERSISTENCE IN THE NORTH ATLANTIC OCEAN, 

 NORTH SEA, AND BALTIC SEA 



INTRODUCTION 



Little information has been published on the persistence (duration) 

 of storm waves or time length of continuous spells* of low waves. It is 

 often advantageous to know the expected duration of adverse wave condi- 

 tions, since many nerine operations become ineffective or restricted when 

 waves reach or exceed a certain height. Conversely, for specific marine 

 activities it may be important also to know the duration of waves lower 

 than a particular height category. To serve these needs, two sets of 

 curves have been prepared for selected lightships and ocean station ves- 

 sels for each of the four seasons. The selected stations are grouped 

 regionally for reasons of climatology and textual presentation. Seasonal 

 frequency of occurrence of selected wave height categories is furnished 

 in bar graph form to provide additional information on the wave climatology 

 at each of the selected locations. 



EXPLANATION OF THE GRAPHICS 



Only observations made over an unbroken period of time from stationary 

 ships can be used to provide information on wave persistence. The informa- 

 tional data gathered for this report were obtained from continuous records 

 taken at the lightships and ocean station vessels. The longer the period 

 of continuous record, the more reliable is the graphical mean. In this 

 study, a minimum of three years of continuous record was considered adequate. 



In Figure 3 the set of curves labeled "Persistence of Favorable Waves" 

 indicates the number of times during a season that waves decrease to less 

 than specific height categories. These curves also show how long waves 

 are expected to remain below the specified heights. The other set of 

 curves labeled "Persistence of Unfavorable Waves" indicates the number 

 of times during a season that waves increase to specific height categories 

 or exceed themj they also show how long waves can be expected to exceed 

 the specified heights. 



In each set of curves the number of occurrences of waves of each 

 height category is expressed as the average number of occasions during 

 the season that waves decreased in height below or rose to equal or ex- 

 ceed a specific height. Since percentages are based on the average 

 number of occasions that waves of the lowest height category were observed 

 during a season, it is possible for the number of occasions for the higher 

 height categories during the same season to exceed 100 percent. As an 

 example, in the curves for Prying-pan Shoals Lightship (Figure 3J) for 

 autumn the number of occasions of 6 ft. height category waves exceeds 



^Spells are based upon an analysis of 2-, 3-, I^-, or 6-hourly observations. 



