JP 3-50 National Search and Rescue Manual Vol I - US Navy

or less. The first period should begin at the time of datum
and move backward for eight 6-hour periods.
b. The contribution that winds from each subperiod make
should be determined and then added. The column in Figure
5-6A or 5-6B with the latitude closest to the position where
local wind current is calculated (do not interpolate) is
selected.
c. For each time period in Figure 5-6A, the lower number
shows the relationship between wind speed and current speed,
and the upper number shows the relationship between wind
direction and current direction. The current speed of each
period is found by multiplying wind speed by the lower number.
The current direction for each period is determined by adding
the upper number to the direction from which the wind blew.
These contributions from each time period are added
vectorially to obtain local wind current at the desired place
and time.
D. Tidal Current (TC) is found in coastal waters, and changes
direction and velocity as the tide changes. The effect of the tide on
currents in any area may be found by consulting current tables and
charts, or by seeking local knowledge. Calculating wind and sea
currents close to land masses is normally not possible. Therefore,
drift computations depend on tidal current and leeway.
1. With reversing currents that abruptly change direction
approximately 180 degrees, the effect in one direction is
normally greater than in the other, causing a net drift in one
direction.
2. With rotary tidal currents, an object will move in a generally
elliptical direction.
3. The cumulative effect of tidal current and leeway may move the
target into the influence of different tidal conditions or to
where sea current takes effect. Consideration may shift from
tidal to sea current in the later stages of a SAR case.
Intermediate datums should be computed for small periods of
time to account for different influences.
4. Nearby land masses may also affect tidal current. Inlets will
channel and release a current, often in a different direction
at the inlet mouth. When an object drifts near the mouth of a
bay or inlet, manuals can be used to see whether tidal current
data has changed.
E. Other water currents affecting search objects are usually
difficult to calculate.
1. Lake Current (LC) information usually comes from local
knowledge, charts, tables, or computer models. A current in a