local current; the speed is figured as 2 percent of the wind’s force. Therefore, if a wind blows 3 or 4 days in a given direction at about 20 knots, it maybe expected that  a  local  current  of  nearly  0.4  knot  is  being experienced. A wind-driven current does not flow in exactly the same direction as the wind, but is deflected by Earth’s rotation. The deflecting force (Coriolis force) is greater at high latitudes and more effective in deep water. It is to  the  right  of  the  wind  direction  in  the  Northern Hemisphere and to the left in the Southern Hemisphere. At latitudes between 10N and 10S the current usually sets downwind. In general the angular difference in direction between the wind and the surface current varies from about 10 degrees in shallow coastal areas to as much as 45 degrees in some open ocean areas. Each layer  of  moving  water  sets  the  layer  below  in  motion. And the layer below is then deflected by the Coriolis effect, causing the below layer to move to the right of the overlying layer. Deeper layers move more slowly because energy is lost in each transfer between layers. We can plot movements of each layer using arrows whose length represents the speed of movement and whose direction corresponds to the direction of the layer’s movements. The idealized pattern for a surface current  set  in  motion  by  the  wind  in  the  Northern Hemisphere is called the EKMAN SPIRAL. Each layer is deflected to the right of the overlying layer, so the direction of water movement shifts with increasing depth. The angle increases with the depth of the current, and  at  certain  depths  the  current  may  flow  in  the opposite  direction  to  that  of  the  surface. Some major wind-driven currents are the West Wind  Drift  in  the  Antarctic,  the  North  and  South Equatorial Currents that lie in the trade wind belts of the ocean,  and  the  seasonal  monsoon  currents  of  the Western Pacific. Chapters 6 and 7 of Oceanography,  Sixth  Edition, by  M.  Grant  Gross,  contain  additional  information  on the subjects of waves, tides, coasts, and the coastal oceans. COASTAL AND TIDAL CURRENTS Coastal  currents  are  caused  mainly  by  river discharge,  tide,  and  wind.  However,  they  may  in  part be produced by the circulation in the open ocean areas. Because  of  tides  or  local  topography,  coastal  currents are generally irregular. Tidal  currents,  a  factor  of  little  importance  in general deepwater circulation, are of great influence in coastal waters. The tides furnish energy through tidal currents, which keep coastal waters relatively well stirred.  Tidal  currents  are  most  pronounced  in  the entrances to large tidal basins that have restricted openings  to  the  sea. This  fact  often  accounts  for steerage problems experienced by vessels. WIND DRIVEN CURRENT PREDICTION Attempts at current prediction in the past have only been  moderately  successful.  There  has  been  a  tendency to consider ocean currents in much the same manner as wind currents in the atmosphere, when in actuality it appears that ocean currents are affected by an even greater number of factors. It therefore requires different techniques  to  be  used. In order to predict current information, it must be understood  that  currents  are  typically  unsteady  in direction and speed. This has been well documented in a number of studies. The reasons for this variability have been attributed to the other forces, besides wind and tides, that affect the currents. Climatological   surface   charts   have   been constructed for nearly all the oceans of the world using data from ship’s drifts. However, this data has been shown to have limitations and should be used as a rough estimate only. Synoptic  Analysis  and  Forecasting  of  Surface Currents, NWRF 36-0667-127, provides a composite method of arriving at current forecasts. This method uses portions of other methods that have been used. Forecasters  should  make  themselves  aware  of  the information  contained  in  this  publication. COASTAL AND TIDAL CURRENT PREDICTION Prediction  of  tidal  currents  must  be  based  on specific  information  for  the  locality  in  question.  Such information  is  contained  in  various  forms  in  many navigational  publications. Tidal Current Tables, issued annually, list daily predictions of the times and strengths of flood and ebb currents and the time of intervening slacks. Due to lack of observational data, coverage is considerably more limited than for tides. The Tidal Current Tables do include supplemental tidal data that can be determined for many places in addition to those for where daily predictions  are  given. 6-18