24 U. S. COAST AND GEODETIC SURVEY 



The location of the station with respect to the coast Hne also 

 affects the average current velocity due to a given wind velocity. As 

 an example, the results for Diamond Shoal and Cape Lookout Shoals 

 lightships, situated off prominent projections of the coast line, show 

 greater average velocities for a given wind velocity than do any of the 

 other stations. 



From the statements just made, as well as from a study of the values 

 of table 12, it is evident that the average current accompanying a 

 given wind varies with the locality. Average values for the entire 

 area covered by the lightship stations, or any considerable part of it, 

 are, therefore, of limited value as an index to the wind current at a 

 specific location. Results from stations very near or at least similarly 

 situated serve better as a guide to conditions at such a location. 

 For this reason it is not desired to emphasize values obtained by 

 averaging results for all stations. However, for the purpose of com- 

 paring results for one station with those for another, as well as for 

 obtaining averages for all stations for what they may be worth, 

 velocity and direction averages of table 12 for the various stations 

 have been tabulated and averaged in several groupings. The tabula- 

 tions are shown in tables 13, 14, and 15. The values in these tables 

 are on the same basis as those of table 12. It will be seen from the 

 tables that the average ratio of current velocity in knots to wind 

 velocity in statute miles per hour for all wind velocities and all wind 

 directions at all stations is 0.014. The average deviation of the 

 current direction to the right of the wind direction is 14 degrees. 

 The average current velocity produced by a wind of 10 statute miles 

 per hour is about 0.2 knot, for a wind of 20 miles per hour, 0.3 knot; 

 30 miles per hour, 0.4 knot; 40 miles per hour, 0.5 knot; and 50 miles 

 per hour, 0.6 knot. 



As stated previously in this text, the values given in tables 12 to 15 

 are averages of observed currents grouped according to the direction 

 and velocity of the wind. The tidal currents presumably are elim- 

 inated, but residual nontidal effects, whether or not they are wind- 

 produced, are included in these results. A separation of wind current 

 and residual current was not attempted as it is believed that the 

 practical value of the results would not be increased by such a separa- 

 tion. Moreover, the extent to which the residual current itself is a 

 wind current is generally uncertain. 



EXPLANATION OF THE GRAPHIC DATA 



The graphic data contained in figures 8 to 22 seems to require little 

 explanation other than that given on the figures themselves, and in the 

 text explaining the tabular material from which the arrows represent- 

 ing the currents were plotted. The tidal current data represented on 

 figures 8 to 12, were taken from table 5; the nontidal current data on 

 figures 13 to 17, from table 11; and the wind current data on figures 

 18 to 22, from table 12. 



THE GULF STREAM AT DIAMOND SHOAL LIGHTSHIP 



Diamond Shoal Lightship station is situated about 15 miles off Cape 

 Hatteras Light. The approximate inner limit of the Gulf Stream as 

 shown on Coast and Geodetic Survey chart No. 1001 is about 10 miles 

 off Cape Hatteras Light and the station, therefore, lies within the Gulf 



