84 MANUAL OF CURRENT OBSERVATIONS 
and current generally are used as they appear in Form 270a and the results derived on 
the page-summary sheets are modified when necessary to show the actual relations of 
current to wind. A procedure for accomplishing this modification and at the same 
time placing the results on a true-direction basis is outlined below. 
205. Close approximations to the true directions and velocities of the currents 
resulting from winds blowing from the sixteen true directions—north, north-northeast, 
northeast, ete.—are obtained from the page-summary results as follows. All directions 
should be expressed in degrees when taking the steps indicated. 
(a) Determine the true direction of wind corresponding to each of the sixteen 
wind directions used in-the wind reduction. 
(6) From each true wind direction desired (0° or 360°, 2244°, 45°, ete.) sub- 
tract each true wind direction (a) which is within 11% degrees of it. 
(ec) If only one difference (6) is reckoned from the true wind direction desired, 
apply this difference to the true direction of the resulting current for each 
wind velocity. In this case the velocities of the currents are taken directly 
from the page summary sheets. 5 
(d) If two differences (6) are reckoned from the same true wind direction 
desired, average the two differences and apply the average difference to the 
average of the two corresponding current directions. Average also the two 
current velocities for each wind velocity. 
(e) In case there is no true wind direction (a) within 11% degrees of the true 
wind direction desired, proceed as in (d) using the two nearest true wind 
directions (a). 
206. Having determined the average velocity and true direction of the current for 
the several wind velocity groups for each true direction of wind, the wind velocity 
groups for each wind direction may be combined and average values obtained for the 
ratio of the current velocity to that of the wind and the amount by which the direction 
of the wind current deviates from that of the wind. The average current velocity and 
the ratio of current velocity to wind velocity for each wind velocity group for all the 
wind directions may also be obtained. 
207. In the reduction for wind currents as described above the tidal current is 
presumably eliminated if the series of observations covers a sufficient length of time. 
The results, however, will include any permanent current constituting a part of the 
general circulation of the oceanic waters and also the effects of any permanent local 
condition. A separation of wind current and permanent current usually is not at- 
tempted, as the results of such a separation appear to have little practical value, par- 
ticularly in view of the fact that the extent to which the permanent current itself is a 
wind current generally is uncertain. However, if desired, an approximate separation 
may be accomplished by the following procedure. 
(a) Determine for each of the 16 wind directions the north and east velocity 
components of the average current obtained from all the wind velocity groups. 
(b) Average the 16 north components and the 16 east components obtained 
in (a). These averages are the north and east components respectively of 
the permanent current. : 
(c) Subtract the components obtained in (6) from the corresponding com- 
ponents obtained in (a) for each wind direction. The resulting values are 
approximately the north and east components of the wind currents exclusive 
of permanent current. 
