COASTAL CURRENTS; ATLANTIC COAST 19 



was prepared for each set of graphs. Using the last-mentioned scale, 

 the hourly velocities and directions for each hour after the Greenwich 

 transit were read from the graphs. Also, the times of the maximum 

 and minimum phases of the rotary current or of the strengths and 

 slacks of the reversing current referred to the Greenwich transit were 

 similarly obtained directly from the graphs. 



HARMONIC REDUCTION 



A detailed explanation of harmonic analysis as applied to the 

 reduction of tides and tidal currents is given in United States Coast 

 and Geodetic Survey Special Publication No. 98, "Manual of Har- 

 monic Analysis and Prediction of Tides." 



Harmonic analyses have been made of the hourly current velocities 

 observed at many of the lightship stations. For the series for which 

 nonharmonic reductions were made on a reversing basis the velocities 

 were analyzed along the axis of flow, the flood velocity being taken as 

 positive and the ebb velocity as negative. For the weak or rotary 

 currents the north and east component velocities were analyzed 

 separately, a double set of harmonic constants being derived. 



NONTIDAL CURRENT REDUCTION 



The nontidal current for most stations was computed for monthly 

 groups of observations in connection with, or from the results of, the 

 tidal current reductions briefly described above. For those reversing- 

 current series that were plotted and tabulated without being resolved 

 into north and east components, the nontidal current was determined 

 from the average observed velocities and directions of flood and ebb 

 strengths. One-half the vector sum of the two strengths, determined 

 either mathematically or graphically, represented the nontidal current 

 for the period covered by the observations. The method of deter- 

 mining the nontidal current from the north and east components of 

 the observed currents has been described in connection with the 

 reduction of rotary tidal currents. 



WIND CURRENT REDUCTION 



As currents due to winds occur in combination with tidal currents 

 as well as with other nontidal currents, long series of observations are 

 necessary in order that tidal effects may be averaged out in the reduc- 

 tions. Wind effects are relatively important at offshore stations where 

 the tidal current is weak or rotary. At such stations the observed 

 velocities had been resolved into north and east component velocities 

 as a preliminary step to the tidal current reduction. This resolution 

 is also necessary for the wind reduction, and stations at which a year 

 or more of hourly north and east component velocities were available 

 were selected for the wind reductions. 



With some variations in details the wind reductions were made as 

 follows: The hourly component velocities, north and east, of the 

 current were grouped according to the direction and velocity of the 

 wind. Sixteen wind directions beginning with north, north-northeast, 

 northeast, and continuing around the compass were employed. For 

 each wind direction a number of wind velocity groups were tabulated, 

 the first group including the current velocity components correspond- 

 ing to observed winds of 5 to 15 statute miles per hour; the second 



