Figure 6-11.-Example of final forecast form. followed.  Most  units  involved  in  surface  current forecasts  have  their  own  innovations  and  methods. CURRENTS Aerographer’s  Mates  have  a  knowledge  of  the major ocean currents and the meteorological results of the  interaction  of  sea  and  air.  Oceanic  circulation (currents) plays a major role in the production and distribution  of  weather  phenomena.  Principal  surface current  information  such  as  direction,  speed,  and temperature  distribution  is  relatively  well  known. Tidal and Nontidal currents Currents in the sea are generally produced by wind, tide, differences in density between water masses, sea level differences, or runoff from the land. They maybe roughly classed as tidal or nontidal currents. Tidal currents are usually significant in shallow water only, where they often become the strong or dominant flow. Nontidal currents include the permanent currents in the general circulatory systems of the oceans; geopotential currents, those associated with density difference in water   masses;   and   temporary   currents,   such   as wind-driven   currents   that   are   developed   from meteorological conditions. The system of currents in the oceans of the world keeps the water continually circulating. The positions shift only slightly with the seasons  except  in  the  Southeast  Asia  area  where monsoonal effects actually reverse the direction of flow from summer to winter. Currents appear on most charts as continuous streams defined by clear boundaries and with   gradually   changing   directions. These presentations  usually  are  smoothed  patterns  that  were derived from averages of many observations. Drift The speed of a current is known as its drift. Drift is normally measured in knots. The term velocity is often interchanged  with  the  term  speed  in  dealing  with currents although there is a difference in actual meaning. Set, the direction that the current acts or proceeds, is measured  according  to  compass  points  or  degrees. Observations  of  currents  are  made  directly  by mechanical devices that record speed and direction, or indirectly by water density computations, drift bottles, or  visually  using  slicks  and  watercolor  differences. Ocean  currents  are  usually  strongest  near  the surface and sometimes attain considerable speed, such as 5 knots or more reached by the Florida Current, In the middle latitudes, however, the strongest surface currents  rarely  reach  speeds  above  2  knots. Eddies Eddies, which vary in size from a few miles or more in diameter to 75 miles or more in diameter, branch from the major currents. Large eddies are common on both sides of the Gulf Stream from Cape Hatteras to the Grand Banks. How long such eddies persist and retain their characteristics near the surface is not well known, but large eddies near the Gulf Stream are known to persist longer than a month. The surface speeds of currents  within  these  eddies,  when  first  formed,  may reach  2  knots.  Smaller  eddies  have  much  less momentum and soon die down or lose their surface characteristics  through  wind  stirring. WIND DRIVEN CURRENTS Wind driven currents are, as the name implies, currents that are created by the force of the wind exerting stress on the sea surface. This stress causes the surface water  to  move  and  this  movement  is  transmitted  to  the underlying water to a depth that is dependent mainly on the  strength  and  persistence  of  the  wind.  Most  ocean currents are the result of winds that tend to blow in a given  direction  over  considerable  amounts  of  time. Likewise, local currents, those peculiar to an area, will arise when the wind blows in one direction for some time. In many cases the strength of the wind may be used as a rule of thumb for determining the speed of the 6-17