COASTAL CURRENTS. ATLANTIC COAST 23 



ever, notably Nantucket Shoals and Diamond Shoal Lightships, it 

 differs markedly from the above timing. 



Values for the nontidal current, derived as explained on page 19, 

 are given in table 11. As the nontidal current at many of the stations 

 varies with the seasons, the velocities and directions are tabulated 

 on a monthly basis to bring out this variation. In addition, an 

 average for all months is given in the last column of the table. As in 

 previous tables, the velocities are in knots and the directions are true. 



At most of the lightship stations off the Southern States the seasonal 

 change in the nontidal current is very well defined. The month of 

 July particularly stands out at a number of stations as having a max- 

 imum northward or eastward flow. For Stone Horse Shoal Light- 

 ship, Overfalls Lightship, and a few other stations similarly situated 

 near the entrances to inland waterways, the nontidal current as com- 

 puted sets generally toward the nearby land. At such locations the 

 residual current represented by the values given is in strictness 

 hardly nontidal in character, as it obviously results from the flood and 

 ebb of the tidal current setting in directions that are not opposite, 

 due to local hydrographic conditions. 



The average currents accompanying winds of different velocities 

 blowing from 16 points of the compass are given for a number of 

 lightship stations in table 12. The results were derived from a year 

 or more of observations at each station by the reduction process 

 outlined on pages 19 and 20. In table 12, wind velocities are expressed 

 in statute miles per hour, current velocities in knots, and directions 

 of wind and current are true. The ratio of current to wind was 

 obtained by dividing the current velocity in knots by the wind velocity 

 in statute miles per hour. In interpreting the values presented it 

 should be borne in mind that they represent merely the average 

 observed current for the given wind and station based on the number 

 of observations indicated. It is not to be assumed that each time the 

 given wind blows at the given station the indicated wind current 

 results. For it seems certain from studies of observational data as 

 well as from a general consideration of the problem that a wind current 

 often depends upon factors other than the local wind at the time and 

 place of the current. 



An examination of the values of table 12 reveals that both the 

 direction and the velocity of the average current accompanying a 

 given wind vary considerably with the locality, and the reasons for 

 some of the variations are made clear by reference to figures 18 to 22, 

 which show the locations of the stations. For example, the tendency 

 of the current to follow the direction of the shore line is evidenced 

 at most stations by large deflections of the current direction from 

 the wind direction. These deflections are to the right or left depending 

 generally upon the angle the wind direction makes with the shore line. 

 They are particularly noticeable where the wind is blowing toward 

 the shore or has a considerable shoreward component. The physical 

 conditions existing in the vicinities of the entrances to inland water- 

 ways also are clearly reflected in the directions taken by wind currents 

 in those localities. The general tendency of the wind current to set 

 to the right of the wind direction is evident at most of the stations. 

 This tendency results from the deflective force of the earth's rotation, 

 which in the northern hemisphere tends to deflect all moving bodies 

 to the right. 



